Drainage Runoff Calculator
Enter area, rainfall depth and surface type to get the runoff volume in gallons and litres — the rational-method logic for a single storm event.
Last updated: 2026-09-28
How the calculation works
- The runoff coefficient reduces rainfall to what the surface actually sheds: roofs ~95%, grass ~25%.
- Volume is depth × area × C — a single-event number, in gallons and litres.
- 1 inch over 1000 ft² of roof is ~623 gallons — the figure rain barrel sizing starts from.
Formula
V = C × i × A
| Symbol | Meaning | Unit |
|---|---|---|
C | Runoff coefficient by surface | — |
i | Rainfall depth | in |
Worked example
Interpreting the result
Volume and peak flow answer different questions: this volume sizes storage (cisterns, retention), while pipe and gutter sizing need the peak intensity of the design storm. The runoff coefficient is where the physics hides — impervious surfaces shed nearly everything, lawns drink a quarter. For drainage compliance, your jurisdiction's design storm (often the 10- or 25-year event) sets the rainfall input, not a 1 inch default.
Assumptions
- Single storm event, uniform over the catchment.
- Runoff coefficients: roof 0.95, concrete 0.9, gravel 0.5, grass 0.25 — the conservative (high) end of published ranges (roof 0.75–0.95, concrete 0.80–0.95, lawns 0.05–0.25), so volumes err high for typical surfaces.
Limitations
- Peak flow (for pipe sizing) needs storm intensity-duration data, not just depth.
- Storage routing, infiltration and evaporation are not modeled.
Frequently asked questions
How big should a rain barrel be?
A 1000 ft² roof sheds ~600 gallons per inch of rain — barrels fill in minutes. Size storage to your use between storms, or accept that most volume overflows.
What is the runoff coefficient?
The fraction of rainfall that becomes runoff: ~0.95 for roofs, 0.9 concrete, 0.5 gravel, 0.25 lawn. The rest soaks in or evaporates.