Roof Runoff Calculator
See how many gallons one storm and a full year of rain send off a roof and down toward one side of the foundation. Waterproofing contractors use the numbers to explain wet basements to homeowners who wonder where the water comes from.
Free, no sign-up. Runs in your browser; nothing you type is stored unless you choose to email yourself the results.
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Email me these results. We'll send your numbers, plus how to turn this calculator into a lead magnet on your own website.
Three steps, about a minute
Step 1
Enter the Roof area draining to one side, the section feeding that gutter run.
Step 2
Enter the Rainstorm depth in inches, for example 1 for a one-inch storm.
Step 3
Add the Annual rainfall in inches for your area to see gallons per year.
How the roof runoff calculator works
One inch of rain on one square foot of roof is about 0.623 gallons. That constant comes from a cubic foot holding about 7.48 gallons, divided by 12 for a one-inch depth. The calculator multiplies roof area × storm depth in inches × 0.623 for one storm, divides by 55 to show the total as 55-gallon drums, and repeats the multiplication with annual rainfall for the yearly figure. At the defaults, 1,200 sq ft of roof in a one-inch storm sheds about 748 gallons, roughly 13.6 drums, and 40 inches a year adds up to about 29,904 gallons. It assumes every drop runs off, a slight overstatement since some splashes or evaporates. Gutters, downspout extensions and grading decide where the water goes, and an inspection shows where it's getting in.
| Roof area draining to one side | 1,200 sq ft |
|---|---|
| Rainstorm | 1 in |
| Annual rainfall | 40 in |
| Gallons from this storm | 748 |
|---|---|
| 55-gallon drums' worth | 13.6 |
| Gallons per year | 29,904 |
One inch of rain on one square foot of roof is about 0.623 gallons. Gutters and downspouts decide where it ends up.
Using the numbers like a pro: tips for waterproofing contractors
Run the numbers with the homeowner standing at the downspout. Hundreds of gallons landing a foot from the wall makes extensions and regrading easy to justify before you price interior work.
Offer a low-cost exterior fix alongside your full system quote. Recommending downspout extensions or regrading first earns trust, and if water still gets in, you're the company they call back.
Photograph splash marks, eroded mulch and overflowing gutters at every inspection. Paired with the gallon numbers in your written proposal, they make the problem clear to anyone who reads it.
Put a calculator like this on your own website
Basement leaks show up after heavy rain, and that is when homeowners start searching. A runoff calculator on your waterproofing page gives them a clear reason the water is getting in, then invites them to book an inspection or send photos for a quick opinion. That form can become your rainy-week lead source. We can put one on your site with your service area listed beside it.
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Roof Runoff Calculator questions
How much water comes off a roof in one inch of rain?
About 0.623 gallons per square foot of roof. A 1,000 sq ft roof sheds about 623 gallons in a one-inch storm, and a 2,000 sq ft roof about 1,246 gallons. Nearly all of it ends up wherever the gutters and downspouts send it, which is often right beside the foundation.
How far should downspouts discharge from the foundation?
A common rule of thumb is at least 4 to 6 feet from the foundation, onto ground that slopes away from the house. Some local codes set a minimum distance, especially in expansive soils. Extensions, splash blocks or buried drain lines can carry the water farther when the yard is flat.
Can gutters cause basement leaks?
Yes. Clogged gutters overflow along the wall, and short downspouts dump hundreds of gallons beside the foundation in a single storm. That water saturates the soil, builds pressure against the wall and finds cracks and joints. Cleaning gutters and extending downspouts are often the first fixes to try.
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