Stormwater Runoff Calculator
Roof and yard runoff gallons from rain depth.
What it does
An inch of rain on a 1,000 ft2 roof is not a puddle but near 600 gallons hunting for gutters. Roofs shed 95% while lawns drink 80%, so surface decides the flood as much as sky.
The tool above takes surface, area, and storm depth in imperial or metric. It returns runoff gallons or liters with a drum count.
The default roof inch gives 592 gal in eleven 55 gal drums. The lawn case of 5,000 ft2 under 2 inches gives 1,247 gal at a thirsty coefficient of 0.2.
EPA stormwater guidance organizes control around impervious cover, and USGS water science publishes the volume arithmetic of rain over area. Published Rational Method coefficients bracket the roof, drive, lawn, and bed values used here.
Volume totals size barrels and dry wells; peak rates size pipes and swales. Pair this page with infiltration trenches and safe overflow paths before the big storm tests the math.
How to use
- Pick feet or meters.
- Pick the surface.
- Type the catchment area.
- Type storm inches or mm.
- Read gallons or liters first.
- Read drums beside it.
Formula
- surface
- Roof, drive, lawn, or bed.
- area
- Catchment area in ft2 or m2.
- rain
- Storm total in inches or mm.
EPA stormwater program frames runoff control around impervious cover with the same coefficient logic, and USGS water science centers publish the rainfall volume arithmetic behind it. Coefficients sit mid published Rational Method ranges for each surface. Metric mirrors convert inches at 25.4 and square feet at 0.092903.
Worked example
Example: inch on a 1,000 ft2 roof
Imperial defaults: roof, 1,000 sq ft, 1 in.
- Roof: 1,000 sq ft, 1 in rain, C 0.95.
- Feet: 1/12 ft x 1000 = 83.33 ft3.
- Gallons: 83.33 x 7.48052 = 623.4, x 0.95 = 592 gal.
- Drums: ceil(592.2 / 55) = 11 x 55 gal drums.
- Metric: 92.9 m2 at 25.4 mm gives 2242 L.
- Tool shows 592 gal, 11 x 55 gal drums, C = 0.95.
Reference table
| Catchment | Math | Volume |
|---|---|---|
| Roof 1000 ft2, 1 in | 0.95 x 83.3 ft3 | 592 gal |
| Lawn 5000 ft2, 2 in | 0.2 x 833 ft3 | 1247 gal |
| Drive 500 ft2, 0.5 in | 0.9 x 20.8 ft3 | 140 gal |
| Roof 92.9 m2, 25.4 mm | 0.95 x 2.36 m3 | 2242 L |
Limits
- Volume totals, not peak rates.
- Coefficients mid range estimates.
- Connected vs drained areas differ.
- Codes rule discharge points.
Common mistakes
- Sizing barrels to averages.
- Using roof math on lawns.
- Forgetting overflow paths.
- Discharging onto neighbors.
- Skipping first flush diverters.
Try it yourself
Quick checks. Answers sit tucked under each question.
Lawn 5,000 ft2, 2 in?
0.2 x 2/12 x 5000 x 7.48052 = 1247 gal, 23 drums.
Driveway 500 ft2, 0.5 in?
0.9 x 0.5/12 x 500 x 7.48052 = 140 gal, 3 drums.
0 inches?
Zero rain runs nothing: enter rainfall above 0.
Questions
Runoff coefficients?
Roof 0.95, driveway 0.9, garden bed 0.3, lawn 0.2. Smooth sheds, soil drinks.
The math?
Volume = C x rain depth x area, then 7.48052 gallons per ft3 or 1000 L per m3.
Use for sizing?
Size gutters, barrels, and dry wells to the storm, not the average drizzle.
Peak flow?
No, this totals volume. Peak flow rates need intensity and time of concentration.
Infiltrate instead?
Yes, through soil, plants, and slow release. Barrels overflow needs a safe path too.
Legal limits?
Local codes rule downspout discharge, grading, and neighbor drainage. Check before digging.