Roof Runoff Calculator

Your roof is usually the largest single source of water in your yard. This rainfall runoff calculator works out how much it sheds in a storm — and, more usefully, how much arrives at each downspout, which is what decides whether roof water becomes a drainage problem. Pick a rainfall depth, or use real storm statistics for your city.

Roof footprintUsually your home's footprint — the length × width of the house in feet.sq ft

Rainfall

or enter inches:inches

Or use your city's rainfall statisticsNOAA Atlas 14 storm depths. Pick a city, then the kind of day you want to plan for.

Number of downspoutsHow many points the roof drains through.

Off the whole roof

1,200 gal

in a 1" rain — about 24 bathtubs

At each downspout

300 gal

arriving at each of your 4 downspouts

What this means for your yard

That's 300 gallons delivered at each discharge point in a single storm. If a downspout empties within a few feet of the house, it's pouring that volume into the soil you least want saturated — which is why downspouts discharging at the wall are the most common cause of a wet yard, and the cheapest to fix.

Where downspout water should go →Store one downspout's share in a dry well →Or soak it into a rain garden →

What this doesn't tell you

This is a planning estimate. It assumes rain falls evenly and roughly all of it runs off — real roofs concentrate water through valleys, and local rainfall intensity varies. It can't see where your downspouts actually discharge or whether the ground there can take the water. Use it to grasp the scale of roof runoff, not as an exact figure.

Watch: the shocking math behind roof runoff

A quick look at how much water your roof really sheds in a storm — and why it ends up in your yard.

How the calculator works

The math behind roof runoff is settled and simple. One inch of rain falling on one square foot of surface produces 0.623 gallons of water — a fixed physical constant. So the volume your roof sheds is:

gallons = roof footprint (sq ft) × rainfall (inches) × 0.623 × runoff coefficient

The runoff coefficient accounts for the small fraction of rain a roof doesn't shed — a little clings and evaporates. Roofs shed almost everything, so this calculator uses 0.95. For drainage planning that's deliberately on the generous side: it's safer to slightly over-estimate the water your yard has to handle than to under-estimate it.

A worked example

Take a 2,000 square foot roof in a one-inch storm, draining through four downspouts:

2,000 × 1 × 0.623 × 0.95 = about 1,180 gallons off the whole roof
Split four ways, that's roughly 300 gallons at each downspout
Around 25 bathtubs' worth of water, delivered in a single ordinary storm

A three-inch downpour triples every one of those numbers. Put 300-plus gallons next to a splash block two feet from the wall and the cause of a wet yard explains itself.

Why the per-downspout number is the one that matters

Drainage problems don't happen on the roof — they happen wherever the water comes down. That's why this calculator highlights gallons per downspout rather than just the total. If a downspout discharges within a few feet of the house, it's pouring that volume into the soil you least want saturated, storm after storm. Moving that water further away is the cheapest, highest-leverage drainage fix most homeowners can make.

Once you know the number, the next question is where the water should go — downspout drainage problems covers how far is far enough and what a good discharge point looks like. If your yard is flooding rather than just wet, backyard flooding after rain puts this volume in context.

Roof runoff chart

Gallons off the whole roof by footprint and rainfall depth. Divide by your downspouts.

Roof footprint	1 in of rain	2 in	3 in
1,000 sq ft	590 gal	1,200 gal	1,800 gal
1,500 sq ft	890 gal	1,800 gal	2,650 gal
2,000 sq ft	1,200 gal	2,350 gal	3,550 gal
2,500 sq ft	1,500 gal	2,950 gal	4,450 gal
3,000 sq ft	1,800 gal	3,550 gal	5,350 gal
4,000 sq ft	2,350 gal	4,750 gal	7,100 gal

How much rain to plan for in North Carolina

Storm depths from NOAA Atlas 14 for the largest NC cities: the 24-hour rainfall you can expect about every two years, and the once-a-decade version. The full table, with peak intensities for pipe and basin sizing, is on the NC rainfall data page.

City	2-year, 24-hour	10-year, 24-hour	2,000 sq ft roof, 2-year storm
Asheville	2.6 in	3.8 in	3100 gal
Cary	3.4 in	5 in	4050 gal
Charlotte	3.4 in	4.9 in	4000 gal
Durham	3.5 in	5.1 in	4200 gal
Fayetteville	3.7 in	5.5 in	4350 gal
Greensboro	3.4 in	4.9 in	4000 gal
Raleigh	3.5 in	5.1 in	4150 gal
Wilmington	4.7 in	7.2 in	5500 gal

Frequently asked questions

How much water comes off my roof?

About 0.62 gallons per square foot of roof, per inch of rain. A 2,000 square foot roof sheds roughly 1,200 gallons in a one-inch storm. Because roofs absorb almost nothing, nearly all of it exits through the downspouts, which is why roof water is usually the largest single source of water in a yard.

How do I find my roof area for the calculator?

Use the roof footprint — the horizontal area, which is essentially your home's footprint: length times width of the house in feet. You don't need the sloped surface area. For drainage, the footprint is what matters, since that's the area rain actually falls on from above.

Does roof pitch or shape change the runoff?

Not the total volume — the amount of rain landing on a roof depends on its horizontal footprint, not how steep it is. Pitch and valleys do affect how the water concentrates and which downspouts carry the most, but the overall gallons come from footprint and rainfall.

How much water can one downspout handle?

A standard downspout can carry a lot of water, so the issue is rarely the downspout itself — it's where that water goes after it leaves. The per-downspout figure here shows how much is delivered to each discharge point, which is what determines whether it becomes a drainage problem in the yard.

Why does the per-downspout number matter more than the total?

Because drainage problems happen at the discharge point, not on the roof. A few hundred gallons arriving at one spot next to the house in a single storm is what saturates the soil and causes a wet yard. The total tells you the scale; the per-downspout number tells you what each part of your yard is dealing with.

How much rain should I plan for?

For roof drainage, plan for a hard day of rain that happens every year or two — about 3.5 inches in 24 hours across most of North Carolina, closer to 4.5 on the coast — rather than an average day. The city lookup in the calculator uses NOAA Atlas 14 statistics; if your city isn't listed, 1 inch covers an ordinary soaking and 2 to 3 inches a heavy storm almost anywhere in the eastern US.

Is this calculator for rainwater harvesting?

No — it's built for drainage. Rainwater-harvesting calculators size a storage tank; this one shows how much water your roof is putting into the yard and where, so you can decide whether downspout placement is causing a problem. The underlying math is the same, but the purpose is different.

Downspout drainage problems — where roof water should go
Downspout calculator — how many outlets the roof needs and what size, so the water above is split correctly
Yard slope calculator — does your yard have enough fall to move it
Solution Finder — work through your whole yard