Rainwater Harvesting Calculator
How much rain a roof collects from its area, the rainfall and a runoff coefficient, per storm, month or year, and the barrels or tank to store it.
Result…
How it's calculated
Collected (L) = catchment area (m²) × rainfall (mm) × runoff coefficient Collected (US gal) = area (ft²) × rainfall (in) × runoff coefficient × 144 ÷ 231 Target per period = water use × share to supply Storage = the target (or the largest period's collection); barrels = storage ÷ barrel size, rounded up
Estimates the rainwater a roof or other hard surface can collect. One millimetre of rain on one square metre is one litre, and one inch on one square foot is 144 cubic inches, or 0.6234 US gallons. The runoff coefficient is the share that actually reaches the tank after splashing, evaporation and wetting the roof; you enter it for your own roof. Enter one rainfall figure or a list, such as twelve monthly totals, and it adds them up and shows each one. Add your water use and the share you want rain to cover to see how much of that target the roof meets and the storage it needs.
Your numbers, step by step
About the Rainwater Harvesting calculator
This rainwater harvesting calculator estimates how much water a roof or other hard surface collects, and the barrels or tank to store it. Enter the catchment area (the roof seen from above, with its overhangs), the rainfall for a storm, a month or a year, and a runoff coefficient for the share of rain that reaches the tank. You can type one rainfall figure or a list, such as twelve monthly totals, and it shows each period. One millimetre on one square metre is one litre, and one inch on one square foot is 144 ÷ 231 US gallons, so the answer comes in both litres and US gallons. Add the water you use and the share you want rain to supply, and it shows how much of that target the roof meets and how much storage it needs. Every step uses your own numbers.
How to use it
- Choose metric or imperial units and enter the roof's length and width, or switch “I know the” to “Area” and type the “Catchment area”.
- Choose whether the “Rainfall is per” storm, month or year, and enter the “Rainfall”: one figure or several separated by commas.
- Enter the “Runoff coefficient” for your roof, from 0 to 1.
- Optionally enter the “Water use per period” and the “Share of that use to supply”.
- Enter the “Barrel or tank size” you would buy, and read the water collected, the storage and the number of barrels.
Worked example
A 12 × 9 m roof (108 m²) in a month with 70 mm of rain, with a runoff coefficient of 0.9. Each millimetre gives 108 × 0.9 = 97.2 L, so the month gives 97.2 × 70 = 6804 L, about 1797 US gallons. If the household uses 3000 L a month and wants rain to supply 50%, the target is 1500 L, which this month's rain covers in full. Storing 1500 L takes 2 tanks of 1000 L.
Updated
Questions people ask
How do I calculate how much rainwater my roof collects?
Multiply the roof's footprint area by the rainfall and the runoff coefficient. In metric, square metres × millimetres gives litres; in imperial, square feet × inches × 0.6234 gives US gallons.
What runoff coefficient should I use for a roof?
It is the share of rain that reaches the tank. As an example, USDA NRCS Texas uses 0.90 for asphalt roofs and says roofs and other hard surfaces can lose 5% to 20% of the rain; gutters that overflow and first-flush diverters lose more.
Should I use the roof's sloping area or its footprint?
Use the footprint, the area seen from directly above, including the overhangs. Rain falls on that area whatever the pitch, so the sloping surface would overstate the catch.
How big a rain barrel or tank do I need?
Enough to hold what you want to use between storms. The calculator sizes storage for one period's share of your water use, or for the largest period's catch if you leave use at 0, and divides by the barrel size you enter. To check the volume of a tank you already have, use the tank volume calculator.
How many gallons is an inch of rain on 1000 square feet?
An inch on one square foot is 144 cubic inches, and a US gallon is 231 cubic inches, so 1000 ft² gives 1000 × 144 ÷ 231 = 623 US gallons before losses. With a coefficient of 0.85 that is about 530 US gallons.
What are the limits of this rainwater estimate?
Rain falls unevenly, a full tank overflows, and dry spells matter more than the yearly total, so treat the answer as a starting point. Rules on collecting and treating rainwater differ by country and region, and drinking water needs proper treatment: check local rules or ask a qualified installer.