01
Formula
Gallons = square feet × inches × 0.623 × coefficient.
The runoff calculator is the coefficient. Set it to 1 if you want the rainfall calculator volume before losses.
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Rainwater harvesting calculator for a roof
This rainwater harvesting calculator multiplies catchment area by rainfall depth and by a runoff coefficient. It is a rainfall calculator for the gallons in that depth of rain, and a runoff calculator once the coefficient takes out the water that never reaches the tank. One inch on one square foot is 0.623 gallons before the coefficient.
Collectable water
561gal
Before the coefficient623 gal
Coefficient0.90
Rainfall1.00 in
Planning estimate from the numbers you enter. Coverage, density, waste and price are assumptions, not a supplier quote or an engineering specification.
How it works
Gallons equal area times inches times 0.623 times the coefficient.
The 0.623 comes from a cubic foot of water, about 7.4805 gallons, divided by 12 inches. One thousand square feet and 1 inch of rain at a 0.90 coefficient is 561 gallons. The same rain with a coefficient of 1 would be 623 gallons. The difference is the water the roof does not give you. A metal roof is often near 0.9. A green roof or a rough surface is lower. This rainfall calculator does not look up your city's annual rain. Type the storm or the month you are sizing. The tank, the pump, and the filter are not sized here beyond the gallons.
02
Example
1,000 square feet, 1 inch, coefficient 0.90.
Collectable water is 561 gallons. Before the coefficient the storm is 623 gallons. A 2 inch storm at the same roof is twice that, because the formula is linear in rainfall.
03
Tips
Use the flat footprint, not the sloped roof area.
A pitched roof catches the rain that falls on its horizontal shadow. Measuring along the shingles overstates the catchment. Gutters that overflow in a hard storm also cut the coefficient. Lower it if you know the gutter spills.
04
Equipment
The tank and the pump store and move the water.
Dengaro arranges financing for a pump. Gallons are not a loan amount.
Important questions
Know what to expect.
Where does 0.623 come from?+
A cubic foot of water is about 7.48 gallons. One inch is one twelfth of a foot, so each square foot gets about 0.623 gallons per inch of rain.
What coefficient should I use?+
This runoff calculator will not assign one. A smooth metal roof is often near 0.9. Type the factor you are willing to defend. Use 1 only when you want gross rainfall.
Does this size the tank?+
It sizes one storm or one depth of rain. A tank sized for a month needs that month's inches. A tank sized for a single storm needs that storm. The page will not pick the depth.
Is annual rainfall included?+
No. The rainfall calculator uses the inches you type. Look up the local depth, then enter it.
Financing built into the sale