How the rainwater harvesting calculator works
One millimetre of rain falling on one square metre is exactly one litre of water. In US units, one inch of rain on one square foot is 0.623 gallons. So the raw yield of a roof is simply its plan area multiplied by the yearly rainfall. Two losses are then taken off: the runoff coefficient, which is the share of rain that actually reaches the gutter rather than evaporating, splashing or soaking into the roof, and a 90% filter efficiency for the leaves and first flush that a downpipe filter sends to the drain.
Yield = roof footprint × annual rainfall × runoff coefficient × 0.9Storage is sized with the simple rule used in UK and German rainwater guidance: a tank holding about 5% of the annual yield, which is roughly 18 days of average collection. Bigger tanks only pay off where rain is highly seasonal and you want to carry winter water into a dry summer.
Worked example
A house with a 100 m² tiled roof in an area receiving 800 mm a year: 100 × 800 = 80,000 litres of raw rainfall. Tiles shed about 85% of it and the filter passes 90%, so 80,000 × 0.85 × 0.9 = 61,200 litres are usable, an average of 5,100 litres a month. A single 10 mm shower fills 765 litres. The suggested tank is 5% of 61,200, about 3,000 litres. For a US home, a 1,500 ft² asphalt-shingle footprint in a 40 inch climate yields 1,500 × 40 × 0.623 × 0.85 × 0.9 ≈ 28,600 gallons a year, 715 gallons per inch of rain, and points to a tank of around 1,400 gallons.
Runoff coefficients and common tank sizes
| Roof surface | Runoff coefficient |
|---|---|
| Smooth metal sheet | 0.90–0.95 |
| Clay or concrete tile, asphalt shingle | 0.80–0.90 |
| Flat roof with membrane | 0.80 |
| Flat roof with gravel ballast | 0.60–0.70 |
| Extensive green roof | 0.30–0.50 |
| Storage | Capacity | Typical use |
|---|---|---|
| Water butt / rain barrel | 200–300 L (55–80 gal) | Watering can for pots and beds |
| IBC tote | 1,000 L (264 gal) | Allotments, sheds, small gardens |
| Above-ground slimline or round tank | 1,500–5,000 L (400–1,300 gal) | Garden irrigation |
| Underground tank | 3,000–10,000 L (800–2,600 gal) | Garden plus toilets or laundry |
Practical points before you buy a tank
- Use the footprint, not the slope. A steep roof catches no more rain than a flat one with the same plan area; measuring the sloping face overstates the yield.
- Count only connected roof. If half the house drains to the street side, only that half feeds your tank.
- Demand limits the value. A garden that uses 300 litres a week in summer does not need a 5,000 litre tank, however big the roof is.
- Fit an overflow. A full tank must discharge to a soakaway or drain, away from foundations.
- Not for drinking. Roof water carries bird droppings, moss and roofing residues. Using it indoors or for drinking needs treatment and, in many places, notification to the water authority; follow local rules.
Frequently asked questions
How much rainwater can I collect from my roof?
Multiply the roof footprint in m² by rainfall in mm to get litres, then take about 75% for losses. A 1,000 ft² roof collects roughly 480 gallons from 1 inch of rain after losses (623 gallons raw).
How big should my rainwater tank be?
A common guideline is 5% of the yearly yield, around three weeks of average collection. For a typical 100 m² roof in a temperate climate that means a tank of 2,500–3,500 litres.
How much water does 1 inch of rain produce on a roof?
Each square foot of roof sheds 0.623 gallons per inch of rain, so a 2,000 ft² roof produces about 1,250 gallons before losses.
How fast will a 200 litre water butt fill?
A 25 m² shed or half-garage roof collects about 19 litres per millimetre of rain after losses, so a 200 L butt fills in roughly 11 mm of rain – a single wet day.
Is it legal to collect rainwater?
It is legal and often encouraged in most of Europe and North America, but a few US states limit the volume stored. Check local rules, especially before using it inside the house.
Last reviewed: 2026-09-27 · How we calculate