Gutter & Downspout Sizing
Roof area and design rainfall give the runoff flow, then the gutter capacity by Manning and the downspout capacity are checked against it.
Runoff in litres per second and US gallons per minute, the gutter and downspout capacity beside it, how heavily each is loaded, and how many downspouts the flow actually needs.
Example: 1 200 sq ft of roof at 4 in/h sheds 3.15 L/s (49.9 gpm); a 5 in K-style gutter runs 54 % full and two 2 × 3 downspouts carry 5.77 L/s, so both pass.
Rain falls on a roof
and has to go somewhere.
Where the flow comes from, how the two capacities are worked out, and what the page refuses to assume.
The flow
Runoff from a roof is simply the catchment area times the rainfall intensity, with no infiltration to subtract — a roof sheds essentially everything that lands on it. The area that matters is the plan area, the footprint the rain falls through, not the surface you would walk on: a sloped measurement is corrected by 12 ÷ √(rise² + 144). The design intensity is yours. It is a short-duration rainfall at a return period your local code or meteorological service names, and it differs by a factor of five between regions, so nothing is assumed here.
The two capacities
A gutter is an open channel, so its capacity comes from the Manning equation: Q = (1/n)·A·R^⅔ × √S at the slope and roughness you enter, with an idealised cross-section for the nominal size you chose. A downspout is an outlet, not a channel, so it is limited by free discharge: about 0.6·A·√(2gh) for the head standing in the trough. Outlets are usually what limits a system rather than the trough itself, which is why the page reports how many downspouts the flow actually needs as well as whether the ones you planned are enough.
Limits
Idealised profiles: real K-style and half-round sections, leaf guards, long runs, inside corners, debris and any ponding at the outlet all cut the figures given here, and none of them is modelled. No code table, freeboard rule, overflow provision or safety factor is applied — this page sizes for the storm you name and nothing more. Roof areas are yours, the intensity is yours, and the Manning n for your gutter material is yours. Nothing leaves the browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
- Q = A × i with C = 1 for a roof; gutter capacity by the Manning equation at your slope and roughness; downspout capacity as a free-discharge orifice; the design intensity is your input
Last reviewed 22 September 2026. How results are checked: How we verify.