How much rainwater can you collect from a roof?
One millimeter of rain on one square meter of roof gives one liter of water, and in practice you collect about 60–90 % of it. A 100 m² roof with 800 mm of rain a year gives about 64,000 liters at 80 %. Rainwater is not drinkable without filtering, disinfection and a lab test.
Yearly rainwater at 80 % collection
| Roof | 500 mm a year | 800 mm a year | 1,200 mm a year |
|---|---|---|---|
| 50 m² | 20,000 L | 32,000 L | 48,000 L |
| 100 m² | 40,000 L | 64,000 L | 96,000 L |
| 150 m² | 60,000 L | 96,000 L | 144,000 L |
| 200 m² | 80,000 L | 128,000 L | 192,000 L |
Measure the area of the roof as seen from above, not along the slope.
Three things to keep in mind
- Divert the first flush. Let the first 0.5–1 liter per m² of roof run off in every shower. That water carries dust, bird droppings and leaves.
- Size the tank for the longest dry spell, not for the yearly total. The yearly number looks large, but it does not arrive when you need it.
- Plan for the weight. One cubic meter of water weighs one tonne, so the base must carry it.
Safety
Rainwater is not safe to drink without filtering, disinfection and a laboratory test. Never connect rainwater pipes to the mains, and check local rules before you build.
Calculate your roof with the free calculator.
Rainwater harvesting: gutters, cisterns and tanks
How much water your roof gives, how to get it clean into the tank, how big the tank should be and how to install and look after it
- Roof calculator
- Tank calculator
- Tank comparison
- Rainwater system plan
- Tank log
- Rainwater calendar
- Shopping list
Sources: Lancaster, Rainwater Harvesting for Drylands and Beyond; Texas Water Development Board, The Texas Manual on Rainwater Harvesting; WHO, Guidelines for Drinking-water Quality; BS EN 16941-1.
Updated: