What is the Rainfall Calculator?
Calculate rainfall volume, runoff, and intensity with this free rainfall calculator. Enter catchment area (m², hectares, km², ft², or acres) and rainfall depth (mm or inches) to get total water volume in liters, gallons, or m³, plus water mass and acre-inches. Use runoff mode with a coefficient (forest, lawn, urban) to estimate storm runoff volume. Use intensity mode to compute mm/hr or in/hr and classify light, moderate, heavy, or violent rain. Metric and imperial units. Runs instantly in your browser.
How to use the Rainfall Calculator
- Choose Rainfall Volume, Runoff Volume, or Rainfall Intensity mode.
- Select metric or imperial units and a catchment area unit.
- Enter catchment area and rainfall depth (mm or inches).
- For runoff, set a coefficient (0–1) or use a land-cover preset.
- For intensity, enter storm duration in hours and review mm/hr classification.
Common use cases
- Sizing a rainwater harvesting tank from roof area and annual rainfall
- Estimating storm runoff from a parking lot or field
- Agronomy homework converting inches of rain on acres to gallons
- Classifying a storm's rainfall intensity for hydrology reports
- Quick volume check for irrigation or flood planning
Frequently asked questions
- How do I calculate rainfall volume?
- Volume = catchment area × rainfall depth. For example, 25 mm on 100 m² gives 0.025 m × 100 m² = 2.5 m³ = 2,500 liters. This calculator handles unit conversions automatically.
- What is a runoff coefficient?
- The runoff coefficient C (0 to 1) is the fraction of rainfall that becomes surface runoff rather than infiltrating or evaporating. Forest might be ~0.15, lawn ~0.25, residential ~0.4, and paved urban areas ~0.85.
- What is rainfall intensity?
- Intensity is rainfall depth divided by duration, usually in mm/hr or inches per hour. It describes how hard it is raining and is used in drainage design and flood risk.
- What is an acre-inch?
- One acre-inch is the volume of water that would cover one acre to a depth of one inch — about 27,154 US gallons. It is commonly used in agriculture and irrigation.
- How is rain intensity classified?
- This tool uses common thresholds: light <2.5 mm/hr, moderate 2.5–10 mm/hr, heavy 10–50 mm/hr, and violent >50 mm/hr. Thresholds vary by country and application.
