How much water comes off a roof in a storm?

Plan area times rainfall intensity. One inch an hour on 1,000 square feet of roof is about 10.4 US gallons a minute; in metric, 1 mm an hour on 1 m² is 1 litre an hour, so 100 m² at 36 mm/h is 1 litre a second. Use the plan (bird’s-eye) area the gutter drains, not the sloped surface.

Where do I get the rainfall intensity?

From your local building code, your municipality or a national rainfall-intensity map, for the storm length and return period your code uses. It varies a lot by region, so this calculator does not guess it — type the figure for your area.

How much does one downspout carry?

It depends on its size, shape, the outlet and the gutter feeding it. Gutter and downspout makers publish a capacity (or a roof area per downspout at a given intensity); some codes give one too. Type that figure and the calculator gives the count.

Should the steep part of the roof count for more?

Wind-driven rain on a steep roof can put more water in the gutter than the plan area suggests, and some design guides add a pitch factor for that. If yours does, multiply the plan area by it before typing — or type the adjusted area directly.

Gutter & downspout calculator

Roof length along the gutter

Roof depth draining to it (plan view)

Level distance from the gutter to the ridge (plus any wall or roof above that sheds onto it).

Rainfall intensity (your area)

in/hr

From your local code or rainfall map — it is not a number this site can know.

Capacity of one downspout

gpm

— gpm peak runoff

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Runoff arithmetic on the figures you type. Rainfall intensity and downspout capacity come from your local code and the product maker — this page does not size gutters for you or replace a drainage design.

Type the roof footprint that drains to the gutter, the design rainfall intensity for your area, and — when you have it — what one downspout is rated to carry. The calculator gives the peak flow the gutter has to handle and how many downspouts that takes, spread evenly along its length.

Worked example

One side of a gable roof: 40 ft long, 15 ft from eave to ridge in plan = 600 sq ft. The local design storm is 4 in/hr. Runoff = 600 × 4 × 0.0104 = 24.9 gpm. If one downspout is rated for 12 gpm, that is 24.9 ÷ 12 = 2.08 — round up to 3 downspouts, one about every 13 ft 4 in of gutter (or one at each end and one in the middle).

Metric: 12 m × 4.5 m = 54 m² at 100 mm/h is 54 × 100 ÷ 3600 = 1.5 L/s.

Try one

Quick referencefor roof runoff

Imperial: gpm = sq ft × in/hr × 0.0104 · 1,000 sq ft at 1 in/hr ≈ 10.4 gpm
Metric: L/s = m² × mm/h ÷ 3,600 · 1 mm on 1 m² = 1 L
Downspouts: runoff ÷ capacity of one, rounded UP
Spacing: gutter length ÷ downspouts (keep each in its share of the roof)

The formula

runoff = plan area × intensity · downspouts = ⌈runoff ÷ capacity⌉

Questions

How much water comes off a roof in a storm?: Plan area times rainfall intensity. One inch an hour on 1,000 square feet of roof is about 10.4 US gallons a minute; in metric, 1 mm an hour on 1 m² is 1 litre an hour, so 100 m² at 36 mm/h is 1 litre a second. Use the plan (bird’s-eye) area the gutter drains, not the sloped surface.
Where do I get the rainfall intensity?: From your local building code, your municipality or a national rainfall-intensity map, for the storm length and return period your code uses. It varies a lot by region, so this calculator does not guess it — type the figure for your area.
How much does one downspout carry?: It depends on its size, shape, the outlet and the gutter feeding it. Gutter and downspout makers publish a capacity (or a roof area per downspout at a given intensity); some codes give one too. Type that figure and the calculator gives the count.
Should the steep part of the roof count for more?: Wind-driven rain on a steep roof can put more water in the gutter than the plan area suggests, and some design guides add a pitch factor for that. If yours does, multiply the plan area by it before typing — or type the adjusted area directly.

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