Choose runoff volume, rainfall depth or area and enter the known quantities. This geometric estimate assumes all rainfall becomes runoff; gallons are US liquid gallons.
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Runoff Volume Formula
The runoff volume calculator uses area, rainfall depth, and volume. It converts the entered units to square feet, feet, and gallons before applying the formula.
- V = runoff volume in gallons
- A = total runoff area in square feet
- R = total rainfall depth in feet
- 7.48051948051948 = gallons in 1 cubic foot
To calculate a missing area, the formula is rearranged as:
To calculate a missing rainfall depth, the formula is rearranged as:
The calculator has three functions:
- Calculate runoff volume: enter runoff area and rainfall to find the total volume of water.
- Calculate runoff area: enter rainfall and runoff volume to find the area that would produce that volume.
- Calculate rainfall: enter runoff area and runoff volume to find the rainfall depth needed to produce that volume.
This calculation assumes all rainfall over the entered area becomes runoff. It does not subtract infiltration, evaporation, surface storage, or losses unless you adjust the area or rainfall value yourself.
Common Unit Conversions for Runoff Calculations
| Measurement | Conversion | Used as base in calculator |
|---|---|---|
| Area | 1 acre = 43,560 sq ft | Square feet |
| Area | 1 sq m = 10.7639104167 sq ft | Square feet |
| Rainfall | 1 inch = 1/12 ft | Feet |
| Volume | 1 cubic foot = 7.48051948051948 gallons | Gallons |
| Volume | 1 cubic meter = 264.172052358 gallons | Gallons |
Runoff Volume for 1 Inch of Rain
| Runoff Area | Rainfall | Approximate Runoff Volume |
|---|---|---|
| 100 sq ft | 1 inch | 62.3 gallons |
| 500 sq ft | 1 inch | 311.7 gallons |
| 1,000 sq ft | 1 inch | 623.4 gallons |
| 1 acre | 1 inch | 27,154 gallons |
Example Runoff Volume Calculations
Example 1: Find runoff volume
You have a 1,200 sq ft roof and 2 inches of rain.
The runoff volume is about 1,496.1 gallons.
Example 2: Find rainfall depth
A 500 sq ft area produces 200 gallons of runoff. Find the rainfall depth in inches.
The rainfall depth is about 0.64 inches.
FAQ
Does this calculator include a runoff coefficient?
No. The formula assumes the runoff coefficient is 1.0, meaning all rainfall over the entered area becomes runoff. This is reasonable for a simple roof or paved-area estimate, but it may overestimate runoff from grass, soil, gravel, or landscaped areas.
Why is 7.48051948051948 used in the formula?
The area in square feet multiplied by rainfall in feet gives cubic feet of water. Since 1 cubic foot contains about 7.48051948051948 gallons, multiplying by 7.48051948051948 converts cubic feet to gallons.
Should rainfall be entered as storm total or rainfall intensity?
Enter the total rainfall depth for the storm or time period you are analyzing. For example, use 1.5 inches if the storm dropped 1.5 inches of rain. This calculator finds total volume, not flow rate over time.
