How the rainwater calculator works
Every inch of rain falling on a square foot of roof is 1/12 of a cubic foot of water, which is about 0.623 US gallons. Multiply by the catchment area and the rainfall, then by an efficiency factor for water lost to splash, evaporation, gutters and first-flush diverters.
gallons = roof area (sq ft) × rainfall (in) × 0.623 × efficiency
liters = roof area (m²) × rainfall (mm) × efficiency
The metric version is neat: 1 mm of rain on 1 m² of roof is exactly 1 liter.
Worked example
A house with a 1,500 sq ft roof footprint gets a 1 inch storm. At 80 percent efficiency: 1,500 × 1 × 0.623 × 0.8 = about 748 gallons, or roughly 2,832 liters. That would fill 14 rain barrels of 55 gallons. Over a year with 40 inches of rain, the same roof could yield about 29,922 gallons, far more than most homeowners store.
Planning by month
For a seasonal plan, enter a monthly rainfall figure from your local weather service. In a month with 3 inches of rain, the same 1,500 sq ft roof at 80 percent yields about 2,244 gallons. Compare that with how much you expect to use in the garden that month to choose a tank size.
Catchment area: measure the footprint
Rain falls vertically, so what counts is the area the roof covers seen from above, including overhangs, not the larger sloped surface. Steep roofs do not collect more water than flat ones with the same footprint. If only part of the roof drains to the downspout that feeds your tank, use just that part.
| Roof footprint | Gallons per 1 in of rain at 80% | At 100% |
|---|---|---|
| 500 sq ft | 249 | 312 |
| 1,000 sq ft | 499 | 623 |
| 1,500 sq ft | 748 | 935 |
| 2,000 sq ft | 997 | 1,247 |
Choosing an efficiency
No roof delivers 100 percent of the rain that falls on it. Smooth, hard roofs such as metal lose the least; rough or porous roofs, gravel-covered flat roofs and overflowing gutters lose more. Planning figures of about 75 to 90 percent are common rules of thumb, and 80 percent is a reasonable default for a typical shingle roof with clean gutters. Light drizzle may produce almost nothing because it wets the roof and evaporates.
| Roof surface | Rule-of-thumb efficiency |
|---|---|
| Metal | about 85 to 95% |
| Asphalt shingle, tile | about 75 to 90% |
| Flat roof with gravel | lower, often well under 75% |
Practical tips
- Size storage for a typical storm, not the whole year. The rain barrel sizing guide walks through it.
- Fit a screen at the inlet and a tight lid to keep out debris, mosquitoes and small children.
- Route the overflow away from the foundation, and set barrels on a firm, level base: a full 55-gallon barrel weighs well over 400 lb.
- Before the first hard freeze, drain barrels and disconnect them so ice does not crack them.
- Use the water for gardens and lawns, like the beds sized with the raised bed soil calculator.
Rules and safety
Untreated roof runoff is not safe to drink. Do not connect a rainwater system to household plumbing without a proper treatment and backflow arrangement installed to code. Rules on collecting rainwater vary: most US states allow rain barrels, but some limit the amount, and some cities offer rebates. Check local and state rules before you install a large tank.
Frequently asked questions
How much water can I collect from 1 inch of rain?
About 0.623 US gallons per square foot of roof footprint. A 1,000 sq ft roof yields about 623 gallons in theory, or 499 gallons at 80 percent efficiency.
How do I calculate rainwater in liters?
Multiply the roof area in square meters by the rainfall in millimeters. Each mm on each m² gives 1 liter, before losses.
Do I use the roof area or the floor area?
Use the roof's footprint seen from above, including overhangs. The slope of the roof does not add catchment.
Is rainwater safe to drink?
Not without proper treatment. Use collected rainwater for gardens, lawns and washing unless a treatment system designed for drinking water is installed.