Estimate stormwater runoff volume from rainfall depth, drainage area, and runoff coefficient. This calculator can help with rough runoff planning for roofs, driveways, lawns, parking areas, yards, and small drainage areas.
Rainfall depth:
Enter the storm rainfall amount, such as 1 inch or 25 millimeters.
Drainage area:
Enter the area that drains to the point you are estimating, such as a roof, yard, driveway, or lot.
Runoff coefficient:
Use a value from 0 to 1 to estimate how much rainfall becomes runoff.
Storm duration:
Enter the storm length to estimate average runoff flow rate.
A stormwater runoff calculator is useful for drainage planning, rain gardens, downspout estimates, driveway runoff, roof runoff, yard drainage, stormwater capture, erosion checks, and rough site planning.
Actual runoff can vary based on soil type, slope, ground saturation, rainfall intensity, drainage layout, vegetation, surface material, and local stormwater rules.
Your result shows the estimated stormwater runoff volume after applying the runoff coefficient. It also estimates how much rainfall may soak in, evaporate, or remain outside the runoff estimate.
Multiply rainfall depth by drainage area and runoff coefficient, then convert the volume into gallons, cubic feet, liters, or cubic meters.
A runoff coefficient estimates the portion of rainfall that becomes runoff. A value of 0.90 means about 90% of the rainfall runs off the surface.
Roofs often use a high coefficient such as 0.85 to 0.95 for rough estimating because they shed most rainfall.
No. This is a rough estimating calculator. Drainage design should use local rainfall data, codes, site conditions, and professional guidance.