Rainwater harvesting calculator: tank size
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| What is the water for? | House and garden (tank) |
| Roof area | rectangle 10 m × 8 m |
| Average rainfall a year | 600 mm |
| Roof drainage coefficient | 0.75 × |
| Filter efficiency | 0.9 × |
| Water you would use from the tank | 100 L/day |
| Tank size as a share of the yearly figure | 5% |
Tank: 1,620 L
| Result | Value |
|---|---|
| Tank size | 1,620 L |
| Rainwater you can collect a year | 32,400 L |
| Water you would use a year | 36,500 L |
| Roof area | 80 m² |
Show the working
- Yield: 80 m² × 600 mm × 0.75 × 0.9 = 32,400 L a year
- Demand: 100 L a day × 365 = 36,500 L a year
- Tank: 5% × 32,400 L (the lesser figure) = 1,620 L
How it works
- Yield (L a year) = roof plan area (m²) × rainfall (mm) × drainage coefficient × filter efficiency
- Demand (L a year) = litres a day × 365
- Tank (L) = 5% (the BS 8515 simplified figure) × the lesser of yield and demand
- Water butts = tank ÷ butt size, rounded up
Without a demand figure the tank is sized on the yield alone, which may be bigger than you need.
A system feeding WCs or washing machines needs a separate non-drinking water supply and backflow protection: a plumber designs it.
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Roof drainage coefficient | 0.75 × | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 L7 RH1 (0.75). Researcher to verify against BS 8515 for the roof type. |
| Filter efficiency | 0.9 × | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 L7 RH1 (0.9). Researcher to verify against BS 8515 / filter maker. |
| Tank size as a share of the yearly figure | 5 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 L7: tank = 5% of the lesser of yield and demand (BS 8515 simplified method). Researcher to verify. |
Regulations and standards
- BS 8515 (rainwater harvesting systems): simplified sizing approach
Explained: BS 8515 (rainwater harvesting).
Our checks use the guidance for England unless a badge says otherwise. Wales, Scotland and Northern Ireland have their own rules: check the source for where you live.