How this calculator works
Rainfall depth and catchment area multiply to give a volume. One millimetre of rain falling on one square metre is exactly one litre, because a cubic metre holds 1,000 litres and a millimetre is one thousandth of a metre. That makes a useful shortcut: depth in millimetres times area in square metres gives litres. The calculator also reports the mass, using the density of fresh water at about 1,000 kilograms per cubic metre. It assumes the catchment is flat, fully exposed and collects every drop, so it is an upper bound for a roof or garden.
Formula
volume = catchment area x rainfall depth. For a rectangle, area = length x width. Mass = volume x 1,000 kg/m3. To reverse, rainfall depth = volume / area. Unit conversions use 1 m3 = 1,000 L, 1 US gallon = 3.785411784 L and 1 imperial gallon = 4.54609 L.
Worked example
A flat roof 10 m by 10 m has an area of 100 m2. A 25 mm rainfall depth gives 0.025 m x 100 m2 = 2.5 m3, or 2,500 litres, weighing about 2,500 kg. A 2,500 litre tank would therefore be filled by that storm if every drop were captured. In the reverse mode, collecting 1,000 litres from that roof corresponds to a depth of 1 m3 / 100 m2 = 0.01 m, or 10 mm.
Advertisement
Frequently asked questions
Is 1 mm of rain really 1 litre per square metre?
Yes, exactly. A millimetre of depth over a square metre is a volume of 0.001 m3, and a cubic metre is 1,000 litres, so the two factors cancel. This only holds for those units, which is why the calculator converts everything to metres first.
How much water can a roof collect?
A sloped roof collects its horizontal footprint, not its sloping surface area. Measure the roof length and the horizontal span it covers. Gutters, evaporation, first-flush losses and overflow reduce the amount that actually reaches a tank, often by 10 to 20 percent.
Should I use the rainfall depth for one storm or for a month?
Use the total depth over the period you care about. Monthly and annual rainfall figures are usually totals, so multiplying by the catchment area gives the total volume for that period. Peak flow, which matters for pipe sizing, needs the intensity over a short interval instead.
Why is the mass not exactly the volume in kilograms?
It is approximately, because one litre of fresh water has a mass of about one kilogram. The exact density changes slightly with temperature and is higher for salt water. The calculator uses 1,000 kg/m3 as a standard approximation.