Gutter and Downspout Calculator

DIY GUTTERS HOME IMPROVEMENT ROOFING
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By Checked against sources by , Founder & Architect, RECATOOLS Published Updated Last reviewed

Gutter and downpipe calculator. Enter the eaves length and downpipe spacing for the downpipe count (12 m by default, the NCC 2022 limit), and the roof’s plan area, pitch and a design rainfall for the flow the gutter must carry in litres per second. Arithmetic only, not a design check. Runs in your browser.

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Gutter and Downspout Calculator

Downpipes by gutter length

The NCC 2022 length rule: one downpipe for at most 12 m of gutter.

Downpipes by length
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Rain the gutter must carry

Flow = design rainfall × effective roof area ÷ 3,600, in litres per second, with all the rain reaching the gutter.

Flow to the gutter
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This tool does not size a gutter or downpipe for purchase and is not a design check. The count follows the spacing you enter and ignores rainfall; the flow follows the rainfall you choose and ignores the spacing.

📅 Research current as of 19 Oct 2026 · Sources: NCC 2022 Housing Provisions Part 7.4 (7.4.3, 7.4.4, 7.4.5, Table 7.4.3d); PUB Code of Practice on Surface Water Drainage, 7th ed. with Addendum No. 3 (April 2025), Appendix 4; Approved Document H, 2015 edition, Table 1; IPM Group, How to Determine Roof Gutter Size? (2024), MSMA 2nd ed. constants for Hulu Jabor.
Standards, published formulas, and reference data are revised over time — check against a current authoritative source before relying on these figures.
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How to Use the Gutter Calculator

Enter the gutter run

Type the eaves length in metres. The spacing starts at NCC’s 12 m.

Describe the roof

Enter the area it covers in plan and its pitch in degrees.

Pick the rain

Choose a labelled preset or enter the figure your code gives.

Read both results

The downpipe count and the flow in L/s sit side by side.

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Two ways to count what a gutter faces

A gutter has two questions put to it: how far its water travels before it reaches an outlet, and how much water arrives. This calculator answers each one separately and shows the answers next to each other. Neither answer names a gutter profile or a downpipe size, and neither is a design check.

The first answer is a count. The Australian National Construction Code, NCC 2022 Housing Provisions 7.4.5(a), says a downpipe must not serve more than 12 m of gutter length; its explanatory note gives the reason as effective fall and adequate capacity at the code’s 5% annual exceedance probability. NCC 2025, published on 1 May 2026, keeps the clause. The tool divides your gutter length by the spacing you enter, which starts at that 12 m, and rounds up. The count takes no notice of rain. NCC itself does not stop at length: the same clause has downpipes selected from Table 7.4.3a, which tabulates 30 to 70 m² of roof per downpipe against design rainfall, and this tool does not apply that table.

The second answer is a flow, by the rational method that Singapore’s PUB Code of Practice on Surface Water Drainage uses for drains: runoff is the runoff coefficient times rainfall intensity times area. The tool takes the coefficient as 1, the value PUB gives paved areas, so every drop of rain is assumed to reach the gutter. In litres per second that is intensity in mm/h times area in m², divided by 3,600, since 1 mm of rain on 1 m² is 1 litre. The area is the roof’s plan area plus half the area it shows in elevation. England’s Approved Document H (2015) prints that as factors of 1.29 at 30°, 1.50 at 45° and 1.87 at 60°, and IPM Group’s summary of the Malaysian MSMA manual gives the same rule, explaining it as wind-driven rain at a maximum slope of 2 vertical to 1 horizontal. Above 70° Approved Document H changes method, so the tool stops there.

The design rainfall is where codes differ most, and the presets say exactly what each one is. NCC 2022 Table 7.4.3d lists 5-minute intensities by city at 5% and 1% annual exceedance probability; the tool carries Darwin, Brisbane, Sydney, Melbourne and Canberra as printed. PUB prints its Singapore curves only as a chart, so the two Singapore presets, about 214 mm/h at 10 years and about 300 mm/h at 100 years at 5 minutes, are our readings of that chart, good to about 3%, and the chart is for drains rather than roof gutters. Singapore’s roof-drainage standard is SS 525:2024, which we have not read, so it supplies nothing here. The Malaysian presets are computed from the MSMA constants for a single station, Hulu Jabor in Kemaman, Terengganu, as IPM Group gives them; they describe that station, not the country. For anywhere else, enter the figure your own code gives, in mm/h or in/h.

Two things sit outside both answers. Codes state the fall differently: NCC 7.4.4 asks for not less than 1:500, IPM Group’s summary of MSMA also gives 1:500, and Approved Document H asks only for any fall towards the nearest outlet. And in Singapore, NEA names roof gutters, with drains and gully traps, among the areas where water cannot be completely removed and where its officers use larvicide. The calculation runs entirely in your browser.

Gutter length gives one answer and the rain gives another. This page shows both and chooses neither.

Where the 12-metre rule, the 1:500 fall and the pitch factor come from

01

NCC 2022 Housing Provisions 7.4.5(a): a downpipe must “not serve more than 12 m of gutter length”. NCC Part 7.4 carries this unchanged into NCC 2025, published 1 May 2026.

02

The same clause has downpipes selected from Table 7.4.3a, which tabulates roof catchment of 30 to 70 m² per downpipe against design rainfall. This calculator does not apply that table.

03

NCC 7.4.3(2) sizes eaves gutters on the 5-minute rainfall at 5% annual exceedance probability, listed by city in Table 7.4.3d.

04

NCC 7.4.4 sets a gutter fall of “not less than 1:500”. England’s Approved Document H (2015) asks for “any fall towards the nearest outlet”.

05

Approved Document H Table 1 multiplies plan area by 1.29 at 30°, 1.50 at 45° and 1.87 at 60°: plan area plus half the area seen in elevation, the rule this tool uses.

06

Over 70°, Approved Document H counts “elevational area x 0.5” instead, so this tool stops at 70°.

07

PUB’s Code of Practice on Surface Water Drainage computes runoff as Qr = CIA/360 and gives paved areas a runoff coefficient of 1.00, the value this tool assumes for a roof.

08

PUB’s code sets a 10-year return period for drains serving catchments under 100 ha. Its rainfall curves are for surface drains; Singapore’s roof-drainage standard is SS 525:2024, which we have not read.

09

IPM Group’s summary of MSMA (2nd edition) gives a 5-minute storm for all roofs, 20 years for eaves gutters and 50 years for valley and box gutters.

10

NEA names roof gutters, with drains and gully traps, among “areas where water cannot be completely removed”, the places its officers treat with larvicide.

Frequently Asked Questions

  • It divides the gutter length by the spacing you enter, rounds up, and so never gives fewer than one. The spacing starts at 12 m, the most gutter one downpipe may serve under NCC 2022 Housing Provisions 7.4.5(a). A blank, zero or negative spacing asks for a spacing instead of returning a count.
  • From the Australian National Construction Code: NCC 2022 Housing Provisions 7.4.5(a) says a downpipe must not serve more than 12 m of gutter length, and NCC 2025 keeps the rule. The code’s own explanatory note gives the reason as effective fall and adequate capacity. The same clause also has downpipes selected from Table 7.4.3a, which tabulates roof area per downpipe (30 to 70 m²); the tool does not apply that table.
  • It is the rain the gutter must carry, in litres per second: design intensity in mm/h times effective roof area in m², divided by 3,600, because 1 mm of rain on 1 m² is 1 litre. Every drop is assumed to reach the gutter, a runoff coefficient of 1. For 100 m² in plan at 30° under 214 mm/h the tool gives 7.66 L/s.
  • The tool adds half the area the roof shows in elevation to its plan area: plan area times 1 + tan(pitch) ÷ 2. That reproduces Approved Document H Table 1 (1.29 at 30°, 1.50 at 45°, 1.87 at 60°), which gives no reason for it. IPM Group attributes the same rule to MSMA and gives the reason as wind-driven rain, taking a maximum rainfall slope of 2 vertical to 1 horizontal.
  • The one the code for your building names. The tool offers NCC 2022 Table 7.4.3d for five Australian cities, two readings of PUB’s Singapore chart, one Terengganu station computed from MSMA constants, and your own figure in mm/h or in/h. Each preset says its source, return period or annual exceedance probability, and duration. The tool does not choose for you.
  • PUB’s Code of Practice on Surface Water Drainage prints its rainfall curves only as a chart (Appendix 4), with no table. About 214 mm/h at 10 years and about 300 mm/h at 100 years, both at 5 minutes, are our readings of that chart, ±3%. Those curves are for surface drains, not roof gutters. Singapore’s roof-drainage standard is SS 525:2024, which we have not read, so it supplies no preset here.
  • The Malaysian figures come from MSMA 2nd edition constants for one station, Hulu Jabor in Kemaman, Terengganu, as given in IPM Group’s 2024 summary; we could not open MSMA itself. The tool computes 232 mm/h at 20 years and 286 mm/h at 50 years from those constants. Other stations have their own constants, so use “Your own figure” for anywhere else.
  • Codes state it differently. NCC 2022 Housing Provisions 7.4.4 says not less than 1:500, and IPM Group’s summary of MSMA also gives 1:500 for eaves gutters. England’s Approved Document H asks for any fall towards the nearest outlet. The calculator does not use or check a fall.
  • No. The tool does not size a gutter or downpipe for purchase and is not a design check. It does not read any gutter-capacity table, such as NCC Table 7.4.3a or Approved Document H Table 2. The per-downpipe flow assumes the roof drains evenly to every outlet, which a real roof may not.
  • Yes, with no account or limit. The calculation runs in your browser.

Method & sources

How it computes

Gutter length → downpipe count (NCC 2022 7.4.5(a), at most 12 m of gutter per downpipe; ceil, ≥1; spacing ≤0 refused). Flow Q (L/s) = I (mm/h) × A × (1 + tan θ/2) / 3600, C = 1, plan area A, pitch θ ≤ 70° (AD H 2015 Table 1). Presets: NCC Table 7.4.3d (5 cities, 5%/1% AEP), PUB chart readings ~214/~300 mm/h (10/100-yr, ±3%), Hulu Jabor MSMA constants via IPM (20/50-yr). GutterKernel, tested. Not a design check.

What this tool implements

  • gutter length = eaves; downspouts = max(1, ceil(eaves/spacing)); ~1 per 10–12 m
  • Corrected 2026-10-19: a blank or 0 spacing printed "Infinity" downspouts and a negative one returned 1; both now ask for a spacing. The spacing tile is shown to 0.1 m (22 m at 10.6 m had read "One per 11 m" beside 3). The 12 m default is credited to NCC 2022 Housing Provisions 7.4.5(a) ("not serve more than 12 m of gutter length", unchanged in NCC 2025), not to folk practice; NCC Table 7.4.3a (30-70 m2 per downpipe) is named and not applied. Added: flow Q = I x A x (1 + tan(pitch)/2) / 3600 L/s with C = 1, pitch 0-70 deg (AD H 2015 Table 1 reproduced; over 70 deg AD H uses elevational area x 0.5). Presets: NCC 2022 Table 7.4.3d Darwin 233/274, Brisbane 236/306, Sydney 201/262, Melbourne 132/187, Canberra 143/192 mm/h (5%/1% AEP, 5 min); PUB Appendix 4 chart readings about 214 (10-yr) and about 300 (100-yr) mm/h at 5 min, labelled as drain curves, +/-3%; Hulu Jabor 3933001 20-yr 232 and 50-yr 286 mm/h computed from MSMA constants via IPM. No England, US or SS 525 preset (none read). Removed: the unsourced "about 1:600" fall (NCC 7.4.4 is 1:500; AD H "any fall"), "reliable scoping estimate", temperate autumn/spring cleaning advice (replaced by NEA naming roof gutters among areas where water cannot be completely removed), unsourced damp/subsidence and K-style claims, "collects no data".

Sources

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