How to use this calculator
- Enter or choose catchment area (plan view).
- Enter or choose area in.
- Enter or choose rainfall.
- Enter or choose rainfall in.
- Enter or choose runoff coefficient.
- The water collected updates as you type. Use Copy result or Share to keep it.
Method
Volume (L) = Area (m²) × Rainfall (mm) × Runoff coefficient
- 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
- Rain falling on the roof = 100 m² × 800 mm = 80,000 L.
- Captured = 80,000 × 0.80 = 64,000 L = 64 m³.
- In US gallons: 64,000 ÷ 3.785 = 16,907 gal.
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.