Rainwater Harvesting Calculator

A roof collects its footprint area times the depth of rain that falls on it, less whatever never reaches the tank. One square foot under one inch of rain is 0.623377 gallons and one square metre under one millimetre is one litre, both of them exact, so the only estimate in the whole calculation is the collection efficiency. Enter the footprint, the rainfall for the period and that efficiency, and this returns what the tank sees, in both unit systems. A second action sizes the storage a dry spell needs and tells you how much rain refills it.

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Measurements In
Every box on this form is read in these units, and every row of the answer comes back in them. The other system is given in brackets beside the main figures.
The footprint, not the slope. Rain falls vertically, so a steep roof catches exactly what its shadow on the ground catches. Measure to the drip edge, which includes the overhang.
In square feet, or in square metres if the measurement choice above is metric. Only used when the option above is set to an area. Add up the sections of an irregular roof, or use the whole building footprint if every downpipe feeds the tank.
In inches, or in millimetres if the measurement choice above is metric. A year, a month or a single storm, whatever you want the answer for. Your local long-run average is the figure to use for sizing.
What actually reaches the tank after the first wetting of the roof, splash off the gutter, evaporation and the first flush diverter. 85 is the usual working figure for metal roofing with good gutters; 75 to 80 for asphalt shingle or tile, which soak up more and shed dirtier.
Per day, in US gallons, or in litres if the measurement choice above is metric.
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Measurements In
Every box on this form is read in these units, and every row of the answer comes back in them. The other system is given in brackets beside the main figures.
Per day, in US gallons, or in litres if the measurement choice above is metric. What you will actually draw out. Garden irrigation in summer is often 0.6 gallons per square foot per week (25 litres per square metre per week) in a dry spell.
The longest run of days with no useful rain that you want the tank to carry you through. Look this up for your own area rather than guessing; it is the number that sets the tank size.
In square feet, or in square metres if the measurement choice above is metric.
In US gallons, or in litres if the measurement choice above is metric.
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