Quick answer
The rational method estimates peak runoff as Q = C × i × A, in cubic feet per second when i is in inches per hour and A in acres (with a small correction of 43,560 ÷ 43,200). C = 0.35, i = 2 in/hr and A = 5 acres give 3.53 cfs, about 1,584 gallons per minute.
Enter the runoff coefficient, rainfall intensity and drainage area. The calculator returns peak flow in cfs and gallons per minute.
Related tools: Water Viscosity Calculator: Dynamic viscosity of liquid water at a temperature. Water Velocity Calculator: Water velocity in a pipe from flow and inside diameter.
How to use the Water Runoff Calculator
- Enter runoff coefficient (between 0 and 1).
- Enter rainfall intensity (in/h) (above 0).
- Enter drainage area (acres) (above 0).
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Runoff coefficient | A number | > 0, ≤ 1 |
| Rainfall intensity | A number, in in/h | > 0 |
| Drainage area | A number, in acres | > 0 |
Formula and method
Formula
Q = C i A × (43,560 ÷ 43,200) cfs
IEEE-754 double precision; shown to the decimals indicated.
The Rational Method applies to small catchments; pick C from a design manual for your surface.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Runoff coefficient = 0.35; Rainfall intensity = 2; Drainage area = 5.
| Result | Value |
|---|---|
| Peak runoff | 3.5292 cfs |
| Gallons per minute | 1,584 |
Example 2
Inputs: Runoff coefficient = 0.9; Rainfall intensity = 1.5; Drainage area = 0.75.
| Result | Value |
|---|---|
| Peak runoff | 1.0209 cfs |
| Gallons per minute | 458.2 |
Example 3
Inputs: Runoff coefficient = 0.2; Rainfall intensity = 3; Drainage area = 12.
| Result | Value |
|---|---|
| Peak runoff | 7.26 cfs |
| Gallons per minute | 3,258.5 |
The first line is the headline result (peak runoff). Below it the water Runoff Calculator lists gallons per minute, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
Choosing C
C is the share of rainfall that becomes runoff: roughly 0.05 to 0.35 for lawns and 0.7 to 0.95 for pavement and roofs, depending on slope and soil. Larger values mean more runoff.
Limits of the method
The rational method is for small catchments (often under about 200 acres) with a design storm lasting at least the time of concentration. Use engineering standards for design work.
How the result changes with runoff coefficient
Holding every other input at the first worked example, this table changes only runoff coefficient. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Runoff coefficient | Peak runoff | Gallons per minute |
|---|---|---|
| 0.175 | 1.7646 cfs | 792 |
| 0.35 | 3.5292 cfs | 1,584 |
| 0.7 | 7.0583 cfs | 3,168 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Runoff coefficient: entering "1.5" shows "Runoff coefficient must be at most 1." beside that field and no result.
- Rainfall intensity: entering "0" shows "Rainfall intensity must be greater than 0." beside that field and no result.
Choosing between the Water Runoff Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Rainwater Harvesting Calculator](/science/rainwater-harvesting-calculator/) | roof catchment area, rainfall and collection efficiency | Water collected |
| [Water Viscosity Calculator](/science/water-viscosity/) | temperature | Dynamic viscosity |
| [Water Velocity Calculator](/science/water-velocity-calculator/) | flow and inside diameter | Velocity |
| [Snow Water Equivalent Calculator](/science/snow-water-equivalent-calculator/) | snow depth and snow density | Snow water equivalent |
| [Water Weight Calculator](/health/water-weight-calculator/) | volume, volume unit and water temperature | Weight |
Assumptions and limitations
- Small catchments only
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for runoff coefficient and rainfall intensity to confirm this.
Frequently asked questions
What is the time of concentration?
The time for runoff from the farthest point to reach the outlet.
What is cfs?
Cubic feet per second.
Why 43,560?
Square feet per acre.
What does the Water Runoff Calculator calculate?
Peak runoff by the Rational Method. It returns Peak runoff and Gallons per minute.
What formula does the Water Runoff Calculator use?
Q = C i A × (43,560 ÷ 43,200) cfs. The Rational Method applies to small catchments; pick C from a design manual for your surface.
What do I need to enter in the water Runoff Calculator?
You enter Runoff coefficient, Rainfall intensity (in/h) and Drainage area (acres).
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Small catchments only.
What is an example of a water Runoff result?
With runoff coefficient 0.35, rainfall intensity 2 and drainage area 5, the peak runoff is 3.5292 cfs.
Where does the method come from?
The method follows: Rational Method (e.g. FHWA Hydraulic Engineering Circular 22); 1 acre·in/h = 1.00833 cfs.
Related calculators
- Water Viscosity Calculator — Dynamic viscosity of liquid water at a temperature.
- Water Velocity Calculator — Water velocity in a pipe from flow and inside diameter.
- Snow Water Equivalent Calculator — Depth of water in a snowpack from snow depth and density.
Sources
- Rational Method (e.g. FHWA Hydraulic Engineering Circular 22); 1 acre·in/h = 1.00833 cfs
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.