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Calculator · Roof water

How much rain falls on a roof during a storm?

Enter a roof area, a rainfall rate and how long the storm lasts. The calculator shows the total gallons and the average flow in gallons per minute.

Assumes the same rainfall rate over the whole roof for the whole storm. Real storms vary, so treat the result as a teaching estimate.

ROOF RAIN CALCULATOR

Total gallons and gallons per minute from your roof.

Set the roof footprint, rain intensity and how long it rains. See the total gallons and the average flow in gallons per minute (GPM).

TRY IT ON A ROOF

How much water hits your roof?

Enter your roof size, turn up the rain and see the gallons your drainage may need to handle.

Rain on a roof, with runoff collected at a downspout

Illustrative roof and downspout. The drawing does not predict flow speed, gutter capacity or a drainage route.

Estimated runoff0–625+ GPM scale

More blue means a higher estimated runoff rate.

Set your roof and rain

Use the roof footprint viewed from above, including overhangs. Do not use the sloped roof surface or add together the floor areas of a two-story home.

For example, 3 inches per hour means 3 inches of rain if that rate lasts one hour. Choose an example; this is not a forecast or a local design storm.

The rain rate stays constant in this example. Real storms change from minute to minute.

RUNOFF FROM THIS ROOF

62.34 GPM

3,740.3 gallons in 60 minutes

2,000 sq ft at 3.00 inches per hour · 3.00 inches of rain total.

GPM means gallons per minute. This assumes all rain on the roof becomes runoff; it does not include water coming from the yard or neighboring ground.

The calculation: gallons = roof footprint in square feet × total rainfall in inches × 144 ÷ 231. GPM = gallons ÷ minutes. The starting example is 2,000 sq ft × 3 inches × 144 ÷ 231 = 3,740.3 gallons in one hour, or 62.34 GPM.

These are rainfall-volume and average-flow estimates, not a pipe, gutter or pump recommendation. The route, fall, outlet and other contributing areas still need checking. USGS rainfall-volume examples.

Where does that water go at your home?

Show us where it collects. Start with the $89 drainage assessment of your property.

Only need a volume from rainfall depth? Use the roof runoff volume calculator.

Want FAS to look at your property?

Tell FAS what you are noticing. A short chooser suggests a first step and shows its fee before you request anything.

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