Rainwater Harvesting Calculator
How much rain a roof collects, and the tank to last a dry spell.
Result
Buy 23,844 galEstimate
Rainfall is yours: no climate data is stored.
- Rain falling on the roof
- 28,052 gal (106,188 L)Estimate
- Note
- Rules on collecting rainwater vary by place and must be checked. Untreated rainwater is not drinking water.
https://calcgallery.com/home-systems/rainwater-harvesting-calculator/
Results are estimates for planning only. Actual needs vary with site conditions, products and workmanship. Always confirm quantities with your supplier and follow local building codes and manufacturer instructions. Read the full disclaimer
How to use the Rainwater Harvesting Calculator
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- Set the share you expect to collect after losses.
- Under more options, add your daily use and the longest dry spell to size the tank.
Where 0.623 comes from
An inch of rain on a square foot is 144 cubic inches of water, and a US gallon is 231 cubic inches, so:
gallons = footprint (ft²) × rainfall (in) × 144 ÷ 231 = footprint × rainfall × 0.623litres = footprint (m²) × rainfall (mm)The metric version needs no factor at all: a millimetre of rain on a square metre is exactly one litre.
Losses
Not every drop reaches the tank. A first-flush diverter deliberately sends the dirtiest early runoff away; water splashes off and evaporates from the roof; heavy storms overflow the gutters. The share collected is your own estimate, and it is worth being conservative.
With your numbers
- Gallons = footprint (ft²) × rainfall (in) × 0.623 × share collected
Quick reference
Rainfall is yours: no climate data is stored.
| Job | Water collected |
|---|---|
| 1,500 ft², 30 in, every drop | 28,052 gal |
| 1,500 ft², 30 in, 85% | 23,844 gal |
| One 1 in storm, 85% | 795 gal |
| 140 m², 760 mm, 85% | 90,440 L |
| 50 gal a day for 45 days | 23,844 gal |
| 1,000 ft², 15 in, 80% | 7,481 gal |
Common questions about the Rainwater Harvesting Calculator
- How much water can I collect from my roof?
- Each square foot of roof collects 0.623 gallons for every inch of rain, or each square metre collects a litre for every millimetre. A 1,500 square foot roof in a place with 30 inches of rain a year has about 28,000 gallons falling on it. After losses to first-flush diversion, splash and overflow, you might collect 85% of that, about 23,800 gallons.
- Should I use the roof’s sloped area?
- No, the footprint. Rain falls vertically, so a roof catches the rain falling on the ground area it covers, whatever its pitch. A steep roof has more surface than a flat one over the same house, but it collects exactly the same rain.
- How big a tank do I need?
- That depends on how you use the water, not on how much falls in a year. The tank has to carry you through the longest dry spell: 50 gallons a day for 45 days needs 2,250 gallons of storage, even though the roof may collect ten times that over a year. The tank also needs an overflow for the largest single storm.
- Can I drink collected rainwater?
- Not untreated. Roofs collect dust, droppings and debris, and rainwater for drinking needs filtering and disinfection to a standard. Collection for gardens and toilets is simpler. Rules on rainwater harvesting also vary by place, so check yours.