Rainfall to Volume Calculator
Enter the rainfall depth and the catchment area to calculate the total volume of water collected.
Last updated: September 5, 2026
Frequently asked questions
How do I convert rainfall in mm to liters?
Multiply the rainfall depth (mm) by the catchment area (m²) - the result is directly in liters, since 1 mm over 1 m² equals exactly 1 liter.
How much water does 10mm of rain produce over a 100m² roof?
1,000 liters - 10mm × 100m² = 1,000, and each 1mm-over-1m² unit equals 1 liter.
Why does 1mm of rain over 1 square meter equal exactly 1 liter?
1mm = 0.001 meters, so 0.001m × 1m² = 0.001 cubic meters, and 1 cubic meter equals 1,000 liters, so 0.001 m³ = 1 liter exactly - the units work out perfectly because a liter is defined as 0.001 cubic meters.
Does this account for runoff losses?
No - it assumes 100% of the rain falling on the area is captured, which is realistic for a smooth, sealed roof feeding a collection system but overestimates yield for permeable surfaces like soil or gravel, where some water soaks in or evaporates instead of running off.
How much water can I collect from a 50m² roof during 1 inch of rain?
About 1,270 liters - 1 inch equals 25.4mm, and 25.4mm × 50m² = 1,270 liters, since each 1mm-over-1m² unit equals 1 liter.
Should I use my roof's footprint area or its sloped surface area?
Use the horizontal footprint (the area as seen from directly above), not the sloped roof surface area - rainfall is measured as vertical depth falling onto a horizontal plane, so a sloped roof collects the same volume as a flat roof of the same footprint, just spread over more physical roof material.
How is this different from a rainwater harvesting or tank sizing calculator?
This tool only converts rainfall depth and area into a volume of water produced - it doesn't account for tank capacity, first-flush diversion, or overflow, which a dedicated rainwater harvesting sizing calculator would factor in for planning a storage system.