Rainfall Volume Calculator — Rain Catchment
Calculate exactly how much water falls on any rectangular area during a rain event. Enter the catchment dimensions (in metres) and the rainfall depth (in millimetres) to get the volume in litres, cubic metres and US gallons.
m
m
mm
Total water volume falling on the catchment area
- 1
Catchment area
100 × 50 = 5,000 - 2
Rainfall volume
5,000 m² × 25 mm = 125,0001 mm of rain on 1 m² equals 1 litre — area in m² multiplied by depth in mm gives litres exactly.
How does this calculator work?
Volume (litres) = Area (m²) × Rainfall depth (mm). The identity is exact: 1 mm on 1 m² = 1 litre. A 5 000 m² roof receiving 25 mm of rain collects 125 000 litres (125 m³, ~33 000 US gal). Mass of water ≈ 1 kg per litre.
Formula
How this is calculated
The relationship between millimetres of rainfall and litres of water collected is elegantly simple: 1 millimetre of rain falling on 1 square metre deposits exactly 1 litre of water. This follows from unit analysis — 1 mm × 1 m² = 0.001 m × 1 m² = 0.001 m³ = 1 litre. So to find the total volume, multiply the catchment area in square metres by the rainfall depth in millimetres.
This identity makes the calculator useful for rain-harvesting system design, stormwater run-off estimation, irrigation planning and flood modelling. For a 100 m × 50 m field receiving 25 mm of rain, volume = 5 000 m² × 25 mm = 125 000 litres (125 m³), which weighs 125 tonnes. Even modest rain events can deliver enormous volumes over large areas.
The calculator assumes 100% collection efficiency and a flat, uniformly covered surface. In practice, roof catchments lose some water to splash, evaporation and debris; ground catchments lose water to infiltration and run-off. Apply a collection efficiency factor (typically 0.75–0.95 for roofs) to the calculated volume for a more realistic harvest estimate.
Frequently asked questions
Exactly 1 litre. One millimetre of rain on one square metre equals 0.001 m³ = 1 litre. This is why rainfall depth in mm is numerically equal to litres per m² — a very convenient identity for harvesting calculations.
Yes — calculate the volume for your roof area and the typical monthly or annual rainfall in your location. Allow for collection losses (typically 80–90% for a clean roof), and size the tank to balance storage against demand. Local rainfall data is available from weather services.
The calculator shows US gallons automatically (1 litre = 0.264 US gallons). For imperial gallons, divide litres by 4.546. For example, 125 000 litres ≈ 33 000 US gallons ≈ 27 500 imperial gallons.
Also known as
TG we-Calculate Editorial Team. (2026). Rainfall Volume Calculator — Rain Catchment [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/rainfall-volume-calculator
TG we-Calculate Editorial Team. "Rainfall Volume Calculator — Rain Catchment." TG we-Calculate. 2026. https://we-calculate.com/calculator/rainfall-volume-calculator.
TG we-Calculate Editorial Team, "Rainfall Volume Calculator — Rain Catchment," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/rainfall-volume-calculator
@misc{wecalculate_rainfall_volume_calculator, title = {Rainfall Volume Calculator — Rain Catchment}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/rainfall-volume-calculator}}, year = {2026}, note = {TG we-Calculate} }
Did this calculator help you?