Rainfall Volume Calculator
Work out how much water falls on a roof or plot from the rainfall depth and area in litres, gallons and cubic metres, and how much you can harvest after surface and first-flush losses, with tank fill, days of supply and the roof area needed for a target.
How it works
Volume (L) = area (m²) × rainfall (mm) × capture efficiency − first flush · because 1 mm over 1 m² = 1 L · US: gallons = area (ft²) × rainfall (in) × 0.623 × efficiency · area needed = target ÷ (rain × efficiency) · days of supply = stored water ÷ daily use
One millimetre of rain falling on one square metre delivers exactly one litre of water, so rainfall volume is simply area times depth. In US units, one inch on one square foot is 0.623 gallons. Use the flat, horizontal footprint of a roof rather than its sloped surface: a pitched roof catches rain according to the area of its shadow on the ground. Enter the area directly in m², ft², acres or hectares, or as length × width. A capture efficiency below 1 accounts for water lost to evaporation, splashing and absorption. Pick a surface type to use a typical coefficient, from about 0.95 for metal roofs down to 0.1 for lawns. Under More options you can divert a first flush (the dirty first water off the roof, often 1 gallon per 100 ft² per storm), check how full a tank gets and how much overflows, turn the stored water into days of supply, size a tank for a dry spell, and work back from a target volume to the roof area or rainfall you would need.
Worked example
A 25 mm downpour on a 100 m² roof is 100 × 25 = 2,500 litres of rain. At a realistic 90% capture efficiency for a hard roof, about 2,250 litres — roughly 594 US gallons, or 2.25 cubic metres — reaches the tank. With a 0.4 mm first flush, 40 L go to waste, leaving 2,210 L. Into a 1,000 L tank that is a full tank and 1,210 L of overflow. At 100 L a day the full tank lasts 10 days.
Sources: Rainwater harvesting — Wikipedia · Texas Water Development Board — Texas Manual on Rainwater Harvesting
Frequently asked questions
Do I use the sloped roof area or the footprint?
Use the flat footprint — the area of the roof as seen from directly above. Rain falls vertically, so a pitched roof collects water in proportion to its horizontal shadow, not the larger sloped surface area.
Why is 1 mm of rain equal to 1 litre per square metre?
A square metre is 1,000 mm × 1,000 mm. Covering it to a depth of 1 mm gives a volume of 1,000 × 1,000 × 1 = 1,000,000 mm³, which is exactly one litre. The relationship is a direct consequence of the metric definitions.
How many gallons does an inch of rain give?
About 0.623 US gallons per square foot, or 623 gallons per 1,000 ft² of roof. That is the same as 1 L per m² per mm. At 90% capture a 1,500 ft² roof collects roughly 840 gallons from one inch of rain.
What capture efficiency should I assume?
Smooth metal or tiled roofs typically retain 85–95% of the rain that lands on them; rougher or absorbent surfaces and systems that divert a "first flush" retain less. Using 0.8–0.9 is a sensible planning range for a domestic roof.
What is a first-flush diverter?
A pipe or chamber that sends the first water off the roof (carrying dust, leaves and droppings) to waste before the tank starts filling. A common sizing rule is 1 gallon per 100 ft² of roof per storm, about 0.4 mm of rain. Enter it, and the number of storms, under More options.
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This calculator is for educational purposes. Double-check important results before acting on them.