Roof and Stormwater Drainage Calculators to AS/NZS 3500.3:2025

Roof and stormwater drainage is one method applied again and again: work out how much water arrives, then check that each gutter, pipe, pit and overflow can carry it. AS/NZS 3500.3:2025 sets that method for Australia and New Zealand, NCC 2025 calls it up for every class of building, and AS/NZS 3500.1:2025 does the same for water supply.

We have built free calculators that run it with Australian design storms. Most follow the Standard's own procedure, and the few engineering checks say so on the page. Each draws the section it is sizing and lists every step of the working with the clause, table or figure it came from. The calculation report for the building surveyor is exported in AEC Assistant, free with an account. They are for architects and building designers laying out downpipes, box gutters and overflows, hydraulic consultants checking a design before they sign it off, plumbers sizing the job in hand, and the surveyors who receive the calculation. This guide explains what each calculator sizes, where NCC 2025 calls up the Standard, how the numbers were checked, and how AEC Assistant can fill the calculators in for you. Several of the calculators run on this page.

In one minute

• Two storms. Eaves gutters and their downpipes are sized for the 5% AEP 5-minute rainfall. Box gutters, valley gutters and every overflow measure are sized for the 1% AEP (Table 3.3.4).

• Rain falls at an angle. The Standard takes wind-driven rain at 2 vertical to 1 horizontal, so a sloping roof or a wall above a gutter collects more than its footprint (Clause 3.4).

• Box gutters are sized by Appendix H. Depth comes from the flow, width and grade, with 30 mm of freeboard, and every box gutter needs an overflow device the Standard recognises.

• Site stormwater is the rational method. Flow is ΣCA × I / 3600 at the 5-minute intensity; pipes are checked for capacity, velocity, grade and minimum size.

• Everything is deterministic. The same inputs always give the same answer, and the working shows how it was reached.

• Australian design storms. The rainfall is Australian: the Standard's Appendix D or the Bureau of Meteorology's design rainfall for the site. The methods are for gravity drainage; the Standard's gutter, downpipe and sump methods do not suit siphonic systems (Section 10).

1. Which calculator for which job

Start with the catchment, then size the element that carries it. Every calculator links its result: the inputs ride in the address bar, so a result can be sent to a colleague and opens exactly as you left it. Where the design carries on, the result links to the calculator that comes next, filled in with the values already settled: the catchment to the eaves or box gutter, the eaves gutter to its overflow and the drain, the box gutter to the stormwater pipe.

The job	Calculator	What it applies
Catchment area of a roof, with walls	Roof catchment area	Clause 3.4, Table 3.4.3.2
Eaves gutter, downpipe and how many	Eaves gutter and downpipe	Figure 3.5.5, Table 3.5.2; Housing Provisions 7.4.3 to 7.4.5
Overflow from an eaves gutter	Eaves gutter overflow	Clause 3.5.3, Appendix F; Housing Provisions 7.4.4 to 7.4.7
A long, straight run to a free outlet	Long eaves gutter	An engineering check beside Figure 3.5.5
Valley gutter	Valley gutter	Clause 3.6, Table 3.6.2 and Figure 3.6.2 (incl. Amd 1:2026); Housing Provisions 7.4.4(4)
An upper roof discharging to a lower one	Downpipe spreader	Clause 3.4.5
A balcony, terrace or flat roof: outlets and overflow	Balcony, terrace and flat roof	Clause 3.8, Figure H.4; NCC 2025 Volume One F1D4
A charged line to a rainwater tank	Charged downpipe	An engineering check of the grade line, with the Section 11 rules
Box gutter to a rainhead	Box gutter and rainhead	Clause 3.7, Figures H.1 to H.3
Box gutter sump with a side overflow	Side overflow sump	Figures H.4 to H.6
Box gutter sump with a high-capacity overflow	High-capacity overflow sump	Figures H.4, H.6(a), H.7, H.8
Box gutter sump with a vertical overflow pipe	Vertical overflow	A Performance Solution check
A stormwater pipe	Stormwater pipe size	Clause 5.4, Figure 5.4.11.2, Table 6.3.4
A branching network of pipes	Stormwater network	The Appendix J method
An overland flow path or swale	Overland flow path	Clause 5.4.9, Table 5.4.9
A grated pit and a surcharge outlet	Grated inlet pit	Equations 5.4.10.1 and 5.4.12.2
On-site detention storage and orifice	On-site detention	Clause 7.10, modified rational method
A basement or courtyard pump-out	Stormwater pump-out	Section 8, Appendix K method
A cold water pipe	Cold water pipe size	AS/NZS 3500.1 Section 3, Appendix C

The calculators. Clause, table and figure numbers are those of AS/NZS 3500.3:2025 unless another document is named. The methods are the ones NCC 2025 calls up: AS/NZS 3500.3 through Volume One F1D3 (Class 2 to 9) and Volume Two H2D2 and H2D6 (Class 1 and 10), the Housing Provisions Part 7.4 route through H2D6(1)(b), and AS/NZS 3500.1 through Volume Three B1D3. The engineering checks and the council detention method say so on their pages.

2. Where NCC 2025 calls up AS/NZS 3500.3

For a Class 2 to 9 building, NCC 2025 Volume One F1D3 requires stormwater drainage to be designed and constructed in accordance with AS/NZS 3500.3. There is no table method for those buildings: anything else is a Performance Solution (A2G2).

For a house or a Class 10 building, NCC 2025 Volume Two gives two Deemed-to-Satisfy routes for gutters and downpipes: AS/NZS 3500.3, or the table method of Part 7.4 of the ABCB Housing Provisions (H2D6(1)). The table method excludes box gutters and may only be used where the roof drainage connects to a stormwater drainage system complying with H2D2 (H2D6(3)). Victoria replaces H2D6(1) with AS/NZS 3500.3 alone, so that is its only Deemed-to-Satisfy route. Site stormwater for these buildings goes to AS/NZS 3500.3 under H2D2(1)(a); Housing Provisions Part 3.3 covers some site drainage only where the system otherwise complies with AS/NZS 3500.3, and it has no sizing method. Our guide to the Housing Provisions gutter tables walks through the table method, and the eaves gutter and downpipe, eaves gutter overflow and valley gutter calculators run either route.

Where roof or surface drainage is plumbing work, the Tasmanian and Victorian Parts E3 and E4 of NCC 2025 Volume Three call up AS/NZS 3500.3 too. In Victoria, E3D2 calls up SA HB 39 beside it for roof drainage, and the Standard prevails where the two conflict. Cold water services in any class of building are designed to AS/NZS 3500.1 under Volume Three B1D3. NCC 2025 references the 2025 edition of both Standards; what changed in AS/NZS 3500:2025 sets out the changes.

3. The two storms and the catchment

A gutter fills in a short burst, so roof drainage is sized for the 5-minute rainfall intensity. The AEP, or annual exceedance probability, is the chance that intensity is reached in any one year. Table 3.3.4 of AS/NZS 3500.3 sets 5% AEP for eaves gutters and their downpipes, and 1% AEP for box gutters, valley gutters and overflow measures, because water that overflows them is more likely to enter the building.

Every roof calculator takes the intensity one of three ways on the AS/NZS 3500.3 route: choose the site's town if the Standard's Table D.1 lists it (Clause 3.3.5.1; Appendix C.2.1(a)), type the figures from the Bureau of Meteorology's design rainfall data for any other site (Appendix C.2.1(b)), or give the site's latitude and longitude and the calculator takes the listed place nearest it as an approximation. Table D.1 differs from Housing Provisions Table 7.4.3d at eight of the places both list (Yorketown, SA: 115 against 155 mm/h at 5% AEP); the Housing Provisions route reads Table 7.4.3d only, as 7.4.3(b) requires, so it offers that table's 68 localities and nothing else.

The catchment is the other half. The Standard takes wind-driven rain at 2 vertical to 1 horizontal (Clause 3.3.3), and the catchment is the greatest for any wind direction (Clause 3.4.1). A sloping roof therefore catches its plan area plus half its vertical rise. For a plain roof that is the multiplier in Table 3.4.3.2: F = 1 + tan(slope)/2, which is 1.29 at 30°.

A wall that rises above a gutter adds half its face, so a box gutter against a parapet or a two-storey wall collects far more than the roof alone suggests. For a roof falling to a gutter against a wall the calculator applies Equation 3.4.4.2 as printed, Ac = Ah + 0.5 (Av1 + Av2), and shows the smaller Appendix I working beside it. For a single wall it counts no more than 10 m of height (Clause 3.4.4.3 Note).

Open the roof catchment area calculator

4. Eaves gutters, downpipes and overflow

Figure 3.5.5 plots the catchment one downpipe can serve against the gutter's effective area, for lines of rainfall intensity, with one chart for gutters laid at 1:500 or steeper and one for flatter gutters. The effective area is the cross-section below a line at least 10 mm under the gutter's overflow (Clause 3.5.5). Table 3.5.2 then gives the downpipe for the gutter's effective area and fall, as an internal size. uPVC downpipes are marked by their outside diameter (AS 1273), so a pipe sold as "100" is smaller inside: check the maker's internal dimension.

The eaves gutter calculator applies both. Enter the whole roof, the part draining to the busiest downpipe, or each downpipe's part, and the gutter you plan to use, or leave the gutter blank to size it. It returns the largest catchment one downpipe can take, the downpipe that suits the gutter, and how many downpipes the whole roof needs. For a Class 1 or 10 building outside Victoria it also runs the other Deemed-to-Satisfy route, the Housing Provisions Part 7.4 tables. In the Standard's own worked example, a 144 m² house with a 24° roof in a 145 mm/h area needs four downpipes with a 7 300 mm² gutter at 1:500 or steeper and five laid flatter, and the calculator reproduces both. For the flatter case the example rounds its chart reading up to 36 m² per downpipe where the printed curve gives 35.55 m², so three of its five downpipes (35.99, 35.62 and 35.99 m²) fail against the exact reading; the calculator says so.

Open the eaves gutter and downpipe calculator

Every eaves gutter also needs overflow measures that shed the 1% AEP flow with the downpipes blocked (Clause 3.5.3). Appendix F (normative) gives a method for catchments up to 400 m². The flow is Q* = Ac × I / 3600, shed along the gutter by one continuous measure: a back gap (F.4.2), slots (F.4.3) or a front bead (F.4.4), or a design by computational methods (F.4.5). Dedicated measures at the outlets, such as weirs, nozzles and rainheads, are Housing Provisions 7.4.7 measures: deemed to satisfy on the Part 7.4 route (NCC 2025 Volume Two H2D6(1)(b)), and on the AS/NZS 3500.3 route a Performance Solution under A2G2 unless a state variation accepts them, as Western Australia's does. The eaves gutter overflow calculator runs Appendix F, or the Housing Provisions 7.4.4 to 7.4.7 capacities for a Class 1 or 10 building outside Victoria.

For a long, straight gutter that discharges freely at its end, the long eaves gutter calculator traces the water surface along the run as the roof feeds it, and sets its capacity beside the Figure 3.5.5 value. It is an engineering check for understanding a run or supporting a Performance Solution under NCC 2025 A2G2, not a replacement for the chart.

5. Valley gutters, spreaders and balconies

Valley gutters carry the 1% AEP flow. For up to 20 m² of catchment, Table 3.6.2 sets the effective width and depth, and the sheet to fold them from, by rainfall intensity. The method applies only where the roof slope is not less than 1:4.5 (12.53°; Clause 3.6.1(a) prints 12.5°) with a nominal side angle of 1:3.4 (16.5°); the Housing Provisions ask for more than 12.5° and a side angle of at least 12.5° (7.4.4(4)). Above 20 m², up to the limit Clause 3.6.1(c) sets, the width is read from AS/NZS 3500.3:2025 (incl. Amd 1:2026) Figure 3.6.2, which runs to 40 m². The valley gutter calculator checks a proposed valley against Table 3.6.2 and, from 20 to 40 m², against the amended Figure 3.6.2, or against the Housing Provisions sizes of Table 7.4.4c.

An upper roof may discharge onto a lower one through a spreader, if the lower roof is protected for 1 800 mm either side of the discharge and the lower gutter and downpipe are sized for both roofs (Clause 3.4.5). The spreader calculator adds the two catchments and sizes the lower gutter for the combined flow. In Victoria, where SA HB 39 applies beside the Standard (Volume Three VIC E3D2), its Clause 5.7.7 also limits a spreader to an upper catchment of 15 m², unless the lower roofing maker's data shows its profile can carry more, and keeps the discharge off ridge tiles, flashings, side laps and sheets that drain to a valley.

A balcony or terrace drains by Clause 3.8: the outlets for the 5% AEP intensity and the overflow for the 1% AEP intensity. The calculator sizes the overflow for the whole 1% AEP flow with every outlet blocked, as the Standard designs its other overflows, and counts half of any wall that rain is driven onto (Clause 3.4.4). For a concrete roof, balcony or podium of a Class 2 to 9 building, NCC 2025 Volume One F1D4 adds the levels: the structural substrate falls at least 1:80, steps down at least 70 mm from the internal floor's structural substrate where one adjoins, and has a monolithic hob at least 70 mm high at its outer edge unless it drains straight to a gutter, and the overflow's invert sits at least 20 mm below the internal floor's structural substrate. The balcony drainage calculator checks both. AS/NZS 3500.3 gives no capacity for a floor outlet or a puddle flange, so the calculator gives the flow each outlet must take, to set against the maker's rated capacity; a sump over a downpipe is sized from Figure H.4.

6. Box gutters

A box gutter sits within the roof, so an overflow that fails puts water straight into the building. Clause 3.7 is correspondingly strict. A box gutter is at least 300 mm wide (200 mm is permitted for domestic work in Australia) and laid at a constant grade between 1:40 and 1:200. The general method takes at most 16 L/s at an outlet: at a rainhead, the design flow of the box gutter it serves (Clause 3.7.3(b)); at a sump, the total design flow through its outlet, every box gutter and any roof that discharges straight into the sump added together (Figures 3.7.4(B) and (C)), with the downpipe within the sizes of Figure H.4 (Clause 3.7.3(c)). A larger flow means more outlets, each with a smaller catchment. The depth comes from Figure H.1 for the flow, width and grade, with 30 mm of freeboard included. The calculators never read past the end of a chart: a flow or width the chart does not reach is refused rather than extrapolated, and a gradient between the printed lines is read on the next flatter one.

Every box gutter discharges through an overflow device the Standard recognises, and each has its own calculator:

• A rainhead (box gutter and rainhead calculator). The gutter runs into a rainhead whose front is open above an overflow weir 25 mm below the gutter sole (Figure H.2). A blocked downpipe overflows without backing water up the gutter, so the gutter depth is its free-flow depth. Figure H.3 gives the rainhead's depth and length.

• A sump with a side overflow, often called a spitter (side overflow sump calculator). When the downpipe blocks, the water rises in the sump and leaves through a channel in the side wall (Figure H.5). The gutter must hold that water plus freeboard, so the overflow usually sets its depth. The sump is at least 400 mm long.

• A sump with a high-capacity overflow (high-capacity overflow sump calculator). When the downpipe blocks, water rises over two weirs, each as long as the adjacent gutter is wide, and flows directly or through an overflow channel into a secondary sump with its own overflow downpipe (Clause 3.7.7.2(b), Figure H.7). The sump is at least 150 mm deep (Clause 3.7.6(e)).

Vertical piped overflows are not among them: Clause 3.7.7.2 Note 3 excludes them. The vertical overflow calculator estimates the water level over the rim of an overflow pipe, and says plainly that the arrangement needs a Performance Solution under NCC 2025 A2G2, assessed against F1P1 or H2P1.

Open the box gutter and rainhead calculator

The calculators are tested against the three box gutter examples in Appendix I. In the first example, 15 L/s in a 450 mm gutter at 1:200 needs at least 141 mm of depth (145 mm rounded up to 5 mm), where the example writes 140: the calculator rounds every minimum up. The Standard disagrees with itself in three places here, and the calculators take the safer reading and say so. Example 2 quotes channel depths of 132 and 102 mm where its own Figure H.6(b) gives 126.1 and 99.8 mm, and the figure governs. Figure 3.7.4(C) and Figure H.7 measure the high-capacity sump's depth from different datums, and the deeper is taken: 253 mm below the gutter sole in Example 3, where the example prints 220. Equation 3.4.4.2 is printed with a plus where Appendix I works it with a minus; the printed form is the default, which lifts Example 1's catchment from 303 to 347 m² and its flow past the 16 L/s a rainhead may take, and the Appendix I working is offered as an option.

7. Site stormwater

Below the downpipes, Section 5 of the Standard sizes the site's drainage by the rational method. NCC 2025 calls this up for every class of building: Volume One F1D3 and Volume Two H2D2(1)(a). The flow is Q = ΣCA × I / 3600, where roofs take C = 1.0, paved surfaces 0.9, and pervious ground a coefficient from Equation 5.4.6 that depends on the soil and the site's 1-hour rainfall (Clause 5.4.6). In Australia the time of concentration is 5 minutes (Clause 5.4.4), so the intensity is again the 5-minute figure. Its AEP depends on what a surcharge would do: Table 5.4.3 runs from 63% AEP where the impact is small to 5% where water would pond against buildings. Where box gutters connect upstream of a surcharge outlet, every drain upstream of that outlet carries the 1% AEP flow instead (Clauses 5.2.3 and 5.4.12.1).

Each pipe is then checked four ways:

• its full-bore capacity, read from the printed lines of Figure 5.4.11.2 for the pipe's roughness, must exceed the flow;

• the full-pipe velocity, 4000 Q / (π DN²), must not exceed 2.0 m/s, and the Standard advises 1.5 m/s for a pipe leaving a pit;

• the grade must be no flatter than Table 6.3.4 allows, 1:100 for DN 90 to DN 150 in Australia;

• the pipe must be at least DN 90 on a single dwelling in a rural area or a residential lot under 1 000 m² (Clause 6.3.3(a)). On other properties a pipe downstream of a stormwater or inlet pit must be at least DN 150 and at least the largest pipe entering the pit (Clause 6.3.3(b)). The Standard's own house-lot example in Appendix J uses DN 150 between pits, so on a house lot the calculator marks a smaller pipe below a pit for checking rather than failing it.

Open the stormwater pipe size calculator

The network calculator does the same for every pipe in a branching system, including the largest pipe entering each pit. Each pipe carries the equivalent impervious area of everything upstream of it, the method of the Standard's own network examples in Appendix J. It reproduces every flow and velocity in the Standard's Tables J.1 and J.2, and it flags the one pipe in Example 3 that is smaller than Clause 6.3.3 allows.

Water that the pipes cannot take has to go somewhere safe. The overland flow path calculator solves Manning's equation for the depth in a swale or channel, with roughness from Table 5.4.9. It checks the water stays 300 mm below adjacent floors (Clause 5.4.9(e)), and reports the flood hazard class that depth and velocity fall in, from Australian Rainfall and Runoff, for information: neither NCC 2025 nor AS/NZS 3500.3 calls it up.

The grated inlet calculator applies Equation 5.4.10.1 to a sag pit, where water ponds over the grate, for ponding up to 0.12 m; the Standard gives no method for deeper ponding. It also sizes the grate of a surcharge outlet by Equation 5.4.12.2 for the drain's whole flow with the drain blocked, which keeps the exit velocity at 0.15 m/s.

8. Charged lines, detention and pump-outs

Charged lines. A charged, or wet, line runs from downpipes underground and rises into a rainwater tank, so it stays full between storms. AS/NZS 3500.3 sets rules for them in Section 11, which applies in New Zealand: DN 100 minimum, a 1:100 minimum grade toward a flush point, and the tank inlet at least 300 mm below the lowest gutter outlet, more for long or busy lines. In Australia AS/NZS 3500.3 sets no charged-line method, so a design that rests on the check is a Performance Solution under NCC 2025 A2G2, assessed against F1P1 or H2P1; in Tasmania, Volume Three TAS B7D3(2) calls up the Director's Guidelines for charged downpipes to drinking-water tanks on Class 1 and 10 buildings. Either way the line only works if the head available exceeds the friction and fitting losses at the gutter design flow (5% AEP for eaves gutters, 1% for box and valley gutters), and the charged downpipe calculator, an engineering check, tests exactly that.

On-site detention. NCC 2025 does not require on-site detention: councils set the permissible site discharge and the storage, and their methods differ. AS/NZS 3500.3 sets how the storage is built (Clause 7.10, which NCC 2025 calls up through F1D3 and H2D2(1)(a)): ponding and overflow levels at least 300 mm below a habitable floor, and for an underground tank an orifice of at least 25 mm in a 3 to 5 mm stainless steel plate, a floor graded to the outlet, access openings and a ladder over 1.2 m. The on-site detention calculator estimates the storage by the modified rational method, minute by minute across the storm durations you enter, and sizes the orifice.

Pump-outs. Section 8 of AS/NZS 3500.3, which NCC 2025 calls up through F1D3 and H2D2(1)(a), covers catchments that cannot drain by gravity, such as basement ramps. The wet well and 30 minutes of pumping together must hold a 10% AEP, 2-hour storm, and the well is never smaller than 1% of the catchment in cubic metres or 3 m³. Each pump must not exceed what the receiving system can take (Clause 8.4(a)). The pump-out calculator reproduces every row of the Standard's Appendix K example: 1 000 m² gives 80 m³ of runoff, and 40, 30, 20 and 10 L/s pumps leave wells of 10, 26, 44 and 62 m³.

9. Water supply pipes

The last calculator moves from AS/NZS 3500.3 to AS/NZS 3500.1:2025, which NCC 2025 Volume Three B1D3 calls up for a cold water service in any class of building. The supply pipe to one or more dwellings takes the demand of Table 3.2.3: at least 0.48 L/s for one dwelling (Clause 3.2.3). Branch piping inside a dwelling takes its fixtures: one fixture's flow from Table 3.2.1, or for several the loading units (a WC 2, a basin 1, a shower 2, a bath 8, a kitchen sink 3) converted by Table 3.2.4.

The size comes from Table C.1 by the Standard's own steps (Appendices C and D): the pressure drop left after the height of the highest outlet and 5 m at the outlet, rounded down to a table, the index length rounded up to a column, and the first size that carries the demand. The water pipe calculator then checks that size: velocity at or below 3.0 m/s (Clause 3.4), at least 50 kPa at the most disadvantaged outlet after friction and any meter or backflow device losses (Clauses 3.3.2 and 3.3.3), static pressure at the lowest outlet at or below 500 kPa (Clause 3.3.4), and 15.0 mm bore for a single dwelling's water service (Clause 3.5.1). It reproduces every pipe size in the Standard's worked examples C.3 and D.3.

10. How the numbers were checked

The calculators are code, not a model: the same inputs always give the same result. Every value they take from AS/NZS 3500.3 and AS/NZS 3500.1 comes from the 2025 editions, with AS/NZS 3500.3 as amended by Amendment 1 (2026), and every value they take from the Code comes from NCC 2025, the Housing Provisions included. Each was checked against its source, the whole of the AS/NZS 3500.1:2025 pipe sizing table (Table C.1) included.

No calculator reads a chart beyond the end of a printed curve. Each was tested against every worked example that reads a chart or applies its method: the eaves gutter examples in Appendix G, the catchment and box gutter examples in Appendix I, every flow and velocity of the two stormwater network examples in Appendix J, every row of the pump-out example in Appendix K, and every pipe size in the water supply examples C.3 and D.3 of AS/NZS 3500.1.

Where the Standard disagrees with itself, as when a worked example rounds a chart reading or an equation is printed one way and worked another, the calculator takes the safer reading and says so in the result. Minimum dimensions are rounded up, never to the nearest.

Some figures come from elsewhere, and each calculator names the source where it uses one:

• makers' published data: gutter effective areas, downpipe internal sizes and roofing drainage tables, each cited to the maker's document;

• the bores of PVC-U pipes from AS/NZS 1260, in the charged line check;

• SA HB 39, which Victoria calls up beside AS/NZS 3500.3 (NCC 2025 Volume Three VIC E3D2); its rule for placing a downpipe in a rainhead or sump is shown in every state as good practice;

• typical loss coefficients for pipe fittings in the charged line check, and the usual discharge coefficient of 0.6 for an orifice;

• the modified rational method that sizes on-site detention storage (councils, not the Standard, set that method), and the flood hazard classes of Australian Rainfall and Runoff;

• two practical limits of our own, where the Standard gives no number: a minimum of 20 mm of clearance added to a downpipe's size where it sits on the floor of a rainhead or in its half of a high-capacity sump, and 25 km as the furthest a site may be from its nearest listed rainfall place before the calculator warns you to use the Bureau's design rainfall for the site, and within it the result still says the nearest place is an approximation.

The long gutter, vertical overflow and charged downpipe calculators are engineering checks rather than the Standard's own method, and each says so on the page.

A calculator is a fast, checkable application of the method, not a design. The designer still decides what drains where, which rainfall applies, and whether the site needs more than the Standard's general method. When a question needs the Code or the Standard read against your project, AEC Assistant answers it with the clause cited: the Code on the free plan, and the Standards, AS/NZS 3500.3:2025 among them, on Plus.

11. Let the assistant fill it in, or do it yourself

Every calculator on this site is yours to run by hand. The same calculators run inside AEC Assistant, so you can describe the job in a chat instead and let the assistant fill them in.

• It asks for what decides the answer. The assistant first takes what it can from the conversation, the project you have open (its state, address and building class) and any drawings you attach. For anything that still decides the answer, such as the site's rainfall, the roof draining to a downpipe or a box gutter's grade, it asks you, one short question at a time, with the choices laid out where there are only a few. It does not present a result built on example values as your answer.

• It runs the calculators. The answer is the calculator's own result, with the clause, table or figure behind each check. Where a design runs through several calculators, from the catchment to the gutter, its overflow and the drain, the assistant carries the settled values on to the next one.

• You can take over at any point. Each result links to the calculator filled in with the inputs the assistant used. Open it to check them, change any input, or carry on by hand.

• The report is ready for the building surveyor. Each result also links to its calculation report: the calculator opens in AEC Assistant with the report ready to save as a PDF, with the design basis, the inputs, every check with its clause, the working and a block to sign.

It works the other way too. On each calculator's page here, Ask AEC Assistant opens your calculation in the app, signed in, ready to hand to the assistant with one press. You sign in first if you are not already. The calculator opens with the inputs you entered, and the page points you to the button under it, Ask AEC Assistant about this result; nothing goes to the assistant until you press it. The assistant then explains the result and asks for any input you left at the calculator's example values.

Get the report, beside it, opens the same calculation in the app for its calculation report. The report is exported there, free with an account; you sign in or create the account first if you have not.