Rainfall Volume
Calculator
Results
- Volume (litres)
- 3,000
- Volume (m³)
- 3
- Volume (US gallons)
- 792.516157
Weather results
| Volume (litres) | 3,000 |
| Volume (m³) | 3 |
| Volume (US gallons) | 792.516157 |
formula-map diagram
- Volume (litres)
- 3,000
- Volume (m³)
- 3
- Volume (US gallons)
- 792.516157
Weather relationship
Formula
V (L) = rainfall (mm) × area (m²) [1 mm over 1 m² = 1 L]= 3000
Note
This is a simplified model: it applies the published standard formula to the numbers you entered and ignores local terrain, radiation, precipitation type and instrument error. Wind chill is the 2001 NWS/Environment Canada regression, valid at or below 10 °C with wind at or above 4.8 km/h; the heat index is the Rothfusz regression, reliable above about 27 °C; dew point uses the Magnus approximation (a = 17.27, b = 237.7 °C) over liquid water; wet bulb uses Stull's approximation at standard sea-level pressure; pressure and density altitude assume the International Standard Atmosphere and dry air. Do not use these results for aviation, structural or safety decisions without an official forecast or a qualified professional.
More in Weather and climate
See all →Frequently asked questions
How does this calculator turn a rainfall depth into a volume of water?+
It multiplies the rainfall depth (for example, in millimetres or inches) by the catchment area (like a roof footprint) to get a volume, since a rainfall measurement is really just the depth of water that would accumulate over a flat, non-draining surface of that area.
Why is roof footprint area used rather than roof surface area?+
Rainfall is measured vertically, so the amount collected depends on the horizontal area the roof covers as seen from directly above (its footprint), not the sloped surface area of the roof itself, which is larger than the footprint for any pitched roof.
Why doesn't the calculated volume match what actually ends up in my rain barrel?+
Real collection systems lose water to evaporation, splash-out, gutter overflow during heavy downpours, and diversion for the first-flush (used to flush debris before collection starts), so actual collected volume is typically noticeably less than the theoretical calculation.
How much water can a modest rainfall actually produce over a house-sized roof?+
Because area amplifies even small depths substantially, just 1 mm of rain falling on a 100 square metre roof footprint produces 100 litres of water, which is why rainwater harvesting from ordinary roof areas can add up to meaningful volumes even from routine rainfall.
Can this calculation be used for stormwater or drainage planning too?+
Yes, the same depth-times-area logic is the starting point for estimating stormwater runoff volume from a property, though real drainage planning also factors in surface permeability, since not all of the rain that falls on the ground runs off; some infiltrates into soil rather than becoming collectable or drainable runoff.