Roof Gutter Calculator
Design flow, what the gutter can carry, and how many downpipes.
Quick answer
With the default inputs: Catchment area 500 m² · Storm intensity q 200 L/(s·100m²) · Runoff coefficient 0.9 etc.Design rainwater flow 9.00 L/s
Edit the inputs below to recalculate.Inputs
Result
Design rainwater flow
9.00L/s
| Gutter flow area | 0.0700m² |
| Hydraulic radius | 0.0933m |
| Gutter capacity | 60.7L/s |
| Capacity ratio | 6.74× |
| Full-flow velocity | 0.87m/s |
| Downpipe capacity | 11.78L/s |
| Downpipes required | 1bars |
| Catchment per downpipe | 500m² |
Design flow Q = runoff coefficient × storm intensity × catchment ÷ 10000, since q is per 100 m². Gutter capacity uses Manning: Q = (1/n)·A·R^(2/3)·√S, with A = b·h and wetted perimeter b + 2h for a rectangular channel. Check two things: capacity above design flow, preferably 1.2×, and full-flow velocity not below 0.6 m/s or silt will settle. Gutter slope must not be under 1‰.
Formula
| Design flow | Q = ψ × q × F ÷ 10000 (L/s) |
| Gutter area | A = width × depth |
| Hydraulic radius | R = A ÷ (width + 2 × depth) |
| Gutter capacity | Q = (1 ÷ n) × A × R^(2/3) × √S × 1000 (L/s) |
| Downpipe capacity | q = pipe area × velocity × 1000 |
| Downpipe count | n = ⌈design flow ÷ pipe capacity⌉ |
500 m² of roof at 200 L/(s·100m²) and 0.9 runoff: 9 L/s design flow; a 350×200 gutter at 3‰ with n = 0.013 carries 60.7 L/s, 6.7 times over; a DN100 pipe at 1.5 m/s handles 11.78 L/s, so one suffices.
FAQ
Why is the calculated capacity so much larger than the flow?
Because storm intensity on a small roof yields little flow, while gutters are sized by construction and cleaning needs rather than hydraulics. A 500 m² roof produces 9 L/s and a single DN100 pipe carries 11.78. The real constraints are downpipe spacing (usually no more than 20 m apart, at least two per catchment) and minimum gutter size for leaf blockage and cleaning. A large margin simply means the section is not the bottleneck.
How many downpipes per catchment?
Code requires at least two outlets or downpipes per catchment so a blockage or failure still leaves a path instead of overflowing. Downpipe spacing should also not exceed 20 m to limit gutter run and flow distance. So even when one is hydraulically sufficient, install two — and add outlets mid-run on gutters longer than 20 m.
What happens with insufficient gutter slope?
Standing water, silt, leaks and eventually damp ceilings and cracked parapet flashings. Below 1‰, velocity is too low to move grit, leaves and droppings, so the section silts up and the gutter overflows in the very storm it was designed for, while the trapped water ages the waterproofing. Code requires at least 1‰, commonly 2-5‰, with the outlet at the low point and no backfalls. Check levels section by section on site.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.