Key takeaways
- One inch of rain on 1,000 square feet is about 623 gallons
- Roofs shed nearly all the rain that lands on them
- Pipes and drains are sized for peak rate, not total volume
- Use our runoff calculator to run your numbers
The magic number
One inch of rain falling on one square foot is about 0.623 gallons. Multiply roof area by rainfall in inches by 0.623 to get gallons. A 2,000 square foot roof in a one-inch storm sheds about 1,246 gallons.
Runoff coefficients
Not all surfaces shed all the water. A roof sheds roughly 95 percent. Pavement a bit less. Lawn on clay perhaps a third to a half in a heavy storm. These coefficients adjust the estimate.
Rate matters too
Drains must handle the peak rate. The rational method multiplies the runoff coefficient by rainfall intensity (inches per hour) by area to estimate peak flow. That is what sizes pipes and basins.
Why it matters
In a burst of rain at three inches per hour, that same 2,000 square foot roof sheds roughly 60 gallons every minute, split between just a few downspouts. That is why where each downspout discharges matters so much.
Try it
Our Rainfall and Runoff Calculator does the math for your roof, driveway and yard and suggests pipe sizes.
Quick answers
How much rain should I design for?
Local design storms vary. We use sensible local intensities and add overflow paths for extremes.
Do gutters limit the flow?
Yes. Gutter and downspout capacity can overflow in heavy bursts.
Want a professional opinion on your property?
We offer a free visit and a clear written plan. Call (615) 393-6985 or use the form below.
