Rainfall Calculator — Depth, Area and Water Volume

Calculate rainwater volume from depth and horizontal area, with metric or US units, an optional collection fraction and average intensity.

Translate a measured or assumed rainfall depth into the volume falling over a horizontal catchment. Use it for a roof footprint, garden, field or other area where the rainfall depth can reasonably be treated as uniform. A separate collection fraction lets you explore a stated water-harvesting assumption.

How to use this tool

Enter rainfall depth and choose millimeters, centimeters or inches.
Enter the horizontal projected area and its unit.
Use 100% for gross rainfall, or supply a collection fraction for a scenario.
Enter the duration to calculate average intensity, then compare liters, cubic meters and US gallons.

Formula

Gross volume = rainfall depth × horizontal area; collected volume = gross volume × collection fraction; average intensity = depth ÷ duration

Depth is converted to meters and area to square meters before multiplication. One millimeter over one square meter is one liter. One US liquid gallon is exactly 3.785411784 liters; imperial gallons are not used.

Example

A 25 mm rainfall over 100 m² gives 0.025 × 100 = 2.5 m³, or 2,500 L. At an assumed 80% collection fraction, the collected amount is 2,000 L. Over two hours the average intensity is 12.5 mm/h.

Frequently asked questions

Should I enter the sloping roof area?

Use its horizontal footprint for vertical rainfall depth. Roof slope, wind-driven rain, obstructions and gutter arrangements can affect actual collection, but are not modeled by this simple volume calculation.

Is collection fraction the same as a rainfall probability?

No. It is the fraction of the entered gross water volume assumed to be collected. A chance of rain describes an event probability and cannot replace a measured or assumed depth.

Can this size drains or predict flooding?

No. Average depth and duration do not provide peak runoff, storm timing, infiltration or hydraulic capacity. The results are volume bookkeeping, not a drainage design or weather forecast.

Sources

Last updated: 2026-10-30