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Rainwater Harvesting Calculator

Free rainwater harvesting calculator: enter roof area and rainfall to see collectible water in liters and gallons, adjusted for roof surface efficiency — ideal for tanks and garden planning.

YOUR WORKSPACEFree to use. No sign-up.
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Enter a roof area and rainfall.

Deep dive guides

A LITTLE CONTEXT · 4 MIN READRainwater Harvesting Explained: The Math of Free Water
All 2 related guides

How rainwater harvesting calculator works

The core arithmetic: one millimeter of rain on one square meter of roof yields exactly one liter — the metric system's nicest gift to rain harvesting. In imperial units, one inch of rain on one square foot yields 0.623 gallons. Multiply roof area by rainfall and you have the theoretical maximum capture.

Real roofs do not deliver everything. Splash-out, surface wetting, film loss and first-flush debris mean a fraction of the theoretical total reaches storage, so the calculator applies a runoff coefficient by surface: metal sheeting about 0.95, tile or concrete 0.90, asphalt shingle 0.85, gravel 0.80, and green (planted) roofs around 0.55, since the soil layer absorbs a large share.

Enter roof area (m² or ft²), rainfall depth (mm or inches) and the roof surface. The tool shows the harvestable volume in liters and US gallons, the cubic-meter total for tank sizing, and the loss to surface inefficiency. Use a whole season's rainfall for a seasonal estimate, or a single storm for tank-dynamic planning.

Frequently asked questions

How much water does 1 inch of rain on my roof yield?
About 0.62 gallons per square foot before losses. A 1,000 ft² roof under 1 inch of rain theoretically yields 623 gallons; at a typical asphalt-shingle coefficient of 0.85, roughly 530 gallons actually reaches the gutters. Multiply by your annual rainfall for a year-round picture.
What is a runoff coefficient and why does it matter?
It is the fraction of rain falling on the roof that actually becomes collectible water. Smooth, impermeable surfaces (metal, tile) shed 90–95%; rougher or absorbent surfaces lose more — gravel around 80%, planted green roofs often under 60%. Picking the right coefficient keeps tank math honest.
Can I use my whole annual rainfall in one calculation?
Yes — the formula is linear. Enter, say, 800 mm as the rainfall with your roof area and the result is the theoretical annual yield. Tank reality is more complex (storage limits, seasonal distribution, demand timing), but the annual number is the right first check on whether harvesting is worth plumbing.
How big should my rain tank be?
A common starting point: size storage to capture a typical single storm — often 25–50 mm — from your roof, then grow toward seasonal storage if demand justifies it. A 200 m² roof in a 30 mm storm yields about 5,400 liters with a tile roof, so a 5,000-liter tank is a sensible anchor.
Is roof-harvested rainwater safe to drink?
Not without proper treatment: roofing materials, bird droppings, dust and atmospheric deposits contaminate first-flush water. Harvested rain is excellent for irrigation, toilet flushing and laundry with basic filtering, but potable use requires first-flush diversion, filtration and disinfection per local regulations.

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