# Rainfall Harvest Volume Calculator

> Work out how much rainwater a roof or catchment can collect from its area, the rainfall and a runoff coefficient, in litres, cubic metres and US gallons.

Category: Agriculture · Applies to: Worldwide · Last updated: 2026-10-29
Interactive calculator: https://calculators.geekmidst.com/calculators/rainfall-harvest-volume-calculator

Every millimetre of rain on a square metre of catchment is one litre of water. Multiply by the area and by the share that actually reaches your tank.

Also called: Rainwater collection calculator, Roof catchment calculator.

## Inputs
- Catchment area (plan view) (default 100)
- Area in (options: Square metres, Square feet; default m2)
- Rainfall (default 800)
- Rainfall in (options: Millimetres, Inches; default mm)
- Runoff coefficient (unit %; default 80)

## Results
- Water collected
- Water collected
- Water collected (US gallons)
- Catchment area
- Rain not captured

## Formula

```
Volume (L) = Area (m²) × Rainfall (mm) × Runoff coefficient
```

Where:
- Area: horizontal footprint of the catchment (1 ft² = 0.09290304 m²)
- Rainfall: depth of rain over the period (1 in = 25.4 mm)
- Runoff coefficient: share of the rain that reaches storage

## Worked example
1. Rain falling on the roof = 100 m² × 800 mm = 80,000 L.
2. Captured = 80,000 × 0.80 = 64,000 L = 64 m³.
3. In US gallons: 64,000 ÷ 3.785 = 16,907 gal.

**Result:** A 100 m² roof in an area with 800 mm of rain a year can supply about 64,000 litres if 80% of the rain is captured.

## What the result means

Use annual rainfall for the yearly supply and the wettest single event to size gutters and overflow. Tank size depends on how evenly the rain falls and how fast you use the water, so compare monthly supply with monthly demand before buying a tank.

## Assumptions and limits
- The runoff coefficient covers every loss between the catchment and the tank.
- Rain does not arrive evenly; a full tank overflows and wastes later rain.
- Water quality and treatment needs are not assessed.

## Questions

### What runoff coefficient should I use?

Hard, smooth roofs (metal, tile) usually capture most of the rain; gravel, green roofs and ground catchments capture much less. Local rainwater-harvesting guidance gives values for your materials.

### Should I use the sloped roof area?

No. Rain falls vertically, so the plan (footprint) area is what collects it.

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Source: GM¹X CalcVerse — https://calculators.geekmidst.com/calculators/rainfall-harvest-volume-calculator