Rainwater Harvest Calculator
Enter the catchment area (e.g., the roof size in square meters) in this rainfall harvest calculator, enter rainfall in millimeters for the period you’re analyzing, and collection efficiency as a percentage.
The calculator shows your annual collection in both gallons and liters, plus the daily average in gallons. Results include an 85% collection efficiency factor to account for system losses.
Rainwater Harvest Calculator
| Roof Area (m²) | Rainfall (mm) | Runoff Coefficient | Harvested Water (L) | Calculation |
|---|---|---|---|---|
| 50 | 1000 | 0.8 | 40,000 L | 50 × 1000 × 0.8 = 40,000 |
| 100 | 800 | 0.7 | 56,000 L | 100 × 800 × 0.7 = 56,000 |
| 200 | 1200 | 0.9 | 216,000 L | 200 × 1200 × 0.9 = 216,000 |
| 75 | 600 | 0.6 | 27,000 L | 75 × 600 × 0.6 = 27,000 |
| 150 | 1000 | 0.85 | 127,500 L | 150 × 1000 × 0.85 = 127,500 |
| 500 | 500 | 0.5 | 125,000 L | 500 × 500 × 0.5 = 125,000 |
| 250 | 1500 | 0.75 | 281,250 L | 250 × 1500 × 0.75 = 281,250 |
| 300 | 1000 | 0.8 | 240,000 L | 300 × 1000 × 0.8 = 240,000 |
| 400 | 2000 | 0.9 | 720,000 L | 400 × 2000 × 0.9 = 720,000 |
| 600 | 1200 | 0.85 | 612,000 L | 600 × 1200 × 0.85 = 612,000 |
Harvested Water (L) = Roof Area (m²) × Rainfall (mm) × Runoff Coefficient
Rainwater Harvest Formula
Harvested Water = Area × Rainfall × (Efficiency / 100) × 0.001Where:
- Area: Catchment area (m²)
- Rainfall: Total rainfall (mm)
- Efficiency: Percentage efficiency of the collection system
- 0.001: Conversion factor to cubic meters
Gallons = A × R × C × EDaily Average = Annual Gallons ÷ 365Liters = Gallons × 3.78541Parameters
- A: Roof area in square feet
- R: Annual rainfall in inches
- C: Conversion factor (0.623 gallons/sq ft/inch)
- E: Collection efficiency (0.85 or 85%)
For a roof area of 2,000 sq ft and annual rainfall of 30 inches: Multiply 2,000 × 30 × 0.623 × 0.85 to get 31,773 gallons annually.
The daily average would be 31,773 ÷ 365 = 87 gallons. Converting to liters: 31,773 × 3.78541 = 120,278 liters.
How to Calculate How Much Rainwater I Will Collect?
To calculate your potential rainwater collection, multiply your roof area in square feet by the annual rainfall in inches. Then multiply by 0.623 (the conversion factor for inches to gallons) and 0.85 (system efficiency).
For example, with a 1,500 sq ft roof and 25 inches of rain, calculate 1,500 × 25 × 0.623 × 0.85 to find your annual collection of 19,858 gallons.
Catchment Area: 100 m², Rainfall: 50 mm, Efficiency: 80%
Calculation: Harvested Water = 100 × 50 × (80 / 100) × 0.001 = 4 m³
Result: 4 cubic meters of rainwater can be harvested.
What is Rainwater Harvest?
Rainwater harvesting is the collection and storage of rainwater from rooftop catchment areas. The amount collected depends on the roof area, annual rainfall, and system efficiency. The process captures rainwater that would otherwise become runoff, storing it for later use in activities like irrigation, washing, or with proper treatment, even drinking water.