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The stair, ramp, occupancy and exits, light and ventilation and sanitary facilities calculators to NCC 2025, and the drainage and water calculators to AS/NZS 3500.3:2025 and AS/NZS 3500.1:2025, run right on this site; the other NCC 2025 instruments run in the app. The same inputs always give the same result. No model in the loop.

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The Tools page in AEC Assistant, with the calculators grouped by topic, from access and egress to roof drainage

Roof drainage, stormwater and water supply

The calculators apply AS/NZS 3500.3:2025 and 3500.1:2025, the Standards NCC 2025 calls up for drainage and water supply, and say so plainly where a check goes beyond them. Each draws the section it sizes and shows the working with the clause, table or figure behind every step.

How they fit together, and which to use when: the guide to roof and stormwater drainage to AS/NZS 3500.3.

Roof drainage

Catchments, eaves gutters, downpipes, overflow, valleys, spreaders, charged downpipes and balconies.

  • Roof catchment area calculator

    Roof catchment area to AS/NZS 3500.3:2025 Clause 3.4: slope factor, walls above the gutter, two roofs and any layout, with the flow at 5% and 1% AEP.

  • Eaves gutter and downpipe calculator

    Size eaves gutters and downpipes to AS/NZS 3500.3:2025 or the NCC 2025 Housing Provisions tables: gutter size, internal downpipe size, downpipes per roof.

  • Eaves gutter overflow calculator

    Check eaves gutter overflow at 1% AEP: Housing Provisions 7.4.4 to 7.4.7 measures, or AS/NZS 3500.3:2025 Appendix F back gaps, slots and front beads.

  • Long eaves gutter calculator

    Capacity of a long straight eaves gutter by spatially varied flow (Beij; Chow), set beside AS/NZS 3500.3 Figure 3.5.5 where the chart covers it.

  • Valley gutter calculator

    Valley gutter width, depth and sheet size to AS/NZS 3500.3:2025 (incl. Amd 1:2026) Table 3.6.2 and Figure 3.6.2, or Housing Provisions Table 7.4.4c.

  • Downpipe spreader calculator

    Discharge an upper roof onto a lower one to AS/NZS 3500.3:2025 Clause 3.4.5: the combined flow, the lower gutter and downpipe, and the sarking rules.

  • Balcony, terrace and flat roof drainage calculator

    Balcony, terrace and flat roof outlets and overflows to AS/NZS 3500.3:2025 Clause 3.8, with the NCC 2025 F1D4 fall, step down, hob and overflow level.

  • Charged downpipe calculator

    Check a charged downpipe line to a rainwater tank: the head each downpipe needs against its height above the tank inlet, DN 65 to 300, Section 11 rules.

Box gutters

Box gutters with rainheads, side overflows and high-capacity overflows, sized by Appendix H.

  • Box gutter calculator

    Box gutter and rainhead sizing to AS/NZS 3500.3:2025 Appendix H: gutter depth by flow, width and grade (Figure H.1), rainhead depth and length (H.3).

  • Box gutter sump and side overflow calculator

    Size a box gutter sump with a side overflow to AS/NZS 3500.3:2025 Figure 3.7.4(B): sump depth, overflow channel and the gutter depth the overflow needs.

  • Box gutter high-capacity overflow calculator

    Size a box gutter sump with a high-capacity overflow to AS/NZS 3500.3:2025 Figure 3.7.4(C): weir height, gutter depth and a sump at least 150 mm deep.

  • Box gutter vertical overflow check

    Vertical piped overflows in box gutter sumps are outside AS/NZS 3500.3:2025 (Clause 3.7.7.2 Note 3). Check one for a Performance Solution.

Site stormwater

Pipes, networks, overland flow, pits, detention and pump-outs.

  • Stormwater pipe size calculator

    Size a site stormwater pipe to AS/NZS 3500.3:2025: design flow, Figure 5.4.11.2 capacity, the 2.0 m/s limit, minimum sizes, grades and cover.

  • Stormwater network calculator

    Size a branching site stormwater network to AS/NZS 3500.3:2025 Appendix J: flow, pipe size, grade, cover, inverts, pits and grates, pipe by pipe.

  • Overland flow path calculator

    Depth, velocity and flood hazard of an overland flow path or open channel by Manning's equation (AS/NZS 3500.3:2025 Eq 5.4.9), with the 300 mm floor check.

  • Grated inlet pit and surcharge outlet calculator

    Sag and on-grade pit inlet capacity (AS/NZS 3500.3:2025 Eq 5.4.10.1, Clause 5.4.10.2), surcharge outlet grate area A = Q/150 and minimum pit size.

  • On-site detention calculator

    On-site detention storage by the modified rational method or the council's rate per hectare, one or two orifices, and the AS/NZS 3500.3 Clause 7.10 rules.

  • Stormwater pump-out calculator

    Size a stormwater pump-out to AS/NZS 3500.3:2025 Section 8: the 10% AEP 2-hour storm, duplicate pumps, the wet well and Figure K.1's start–stop band.

Water supply

Cold water pipe sizing to AS/NZS 3500.1.

  • Water pipe size calculator

    Size a cold water pipe to AS/NZS 3500.1:2025: Table 3.2.3 demand for dwellings, loading units for branches, Table C.1 sizes, 3.0 m/s and 50 kPa.

NCC instruments

Each one answers a question that comes up on most jobs, against the provision that governs it. Every table is checked against NCC 2025 or the Standard it calls up.

How a result reads

The stair calculator showing inputs, a live section drawing, the verdict line and the receipt naming the NCC table it read from

  • Inputs, then a drawing

    Type the riser, the going, the width or the floor area and the diagram redraws as you go. The inputs sit in the address bar too, so a result can be sent as a link and opens exactly as you left it.

  • A verdict line

    One sentence says whether the design complies and, when it does not, which limit it fails and by how much. There is no model in the loop: the same inputs always give the same answer.

  • A receipt that names its table

    Below the verdict, every check and every step of the working names the table, clause or figure it was read from, so the result can be checked against the source. In the app, one button hands the result to the assistant to talk through what it means for your project. On the calculators that run on this site, Ask AEC Assistant opens your calculation in the app, signed in, ready to hand to the assistant with that one press, and Get the report opens it there for its calculation report, exported with a free account.

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Free for the NCC, state legislation, ABCB handbooks and practice notes, with citations, and planning information for any address in Australia. No credit card.

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