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.

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

Roof footprint square feet

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.

Rainfall intensity inches per hour

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.

How long does it rain? minutes at this rate

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. Choose an on-site drainage assessment or prepare photos for a free online Water MRI demo.

Request a $195 assessment →Try the free Water MRI demo →

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