Construction Calculators
Calculators for building and home projects. The Rainwater Harvesting Calculator estimates how much rain your roof can collect in a year.
1 calculator each one also works as a widget
How Much Rain a Roof Collects
Every millimeter of rain that falls on one square meter of roof is one liter of water. Multiply the roof area by the yearly rainfall, then by a runoff coefficient for the roof material, because some water splashes off, evaporates or soaks in. A 100 m² roof in a place with 800 mm of rain a year catches 80,000 liters before losses; with a coefficient of 0.8 that is about 64,000 liters.
In US units, one inch of rain on 1,000 square feet of roof gives about 623 gallons. The calculator takes either set of units and lets you pick the roof material.
Frequently Asked Questions
How do I calculate how much rainwater I can collect?
Multiply the roof area by the rainfall and by the runoff coefficient of the roof. In metric units, square meters times millimeters gives liters: 100 m² × 800 mm × 0.8 = 64,000 liters a year.
How much water does one inch of rain give?
About 623 gallons for every 1,000 square feet of roof, before losses.
Are the construction calculators free?
Yes. They are free and run in your browser, with nothing to download and no account to create.