How this calculator works
One millimetre of rain falling on one square metre is one litre of water. So the water a roof collects in a year is its plan area (in square metres) times the yearly rainfall (in millimetres), and some of it is lost, so we multiply by the share that reaches the butt (about 80%). A shower of a given depth works the same way, and divides into water butts to show how many catch it.
Worked example: a garage roof of 20 m² in a place that gets 850 mm of rain a year collects 13,600 litres (13.6 m³) a year, with 80% reaching the butt, which is 1,133 litres a month on average. A 10 mm shower gives 160 litres, which fills 1 butt of 200 litres, and a 20 mm shower gives 320 litres (2 butts). In a dry area of 600 mm the yearly total is 9,600 litres, and in a wet area of 1,200 mm 19,200 litres. A half house roof of 45 m² collects 30,600 litres a year, and a 6 m² greenhouse roof 4,080 litres.
Rain collected from common roofs (850 mm a year, 80% reaching the butt)
Litres in a year, and in a 10 mm shower.
| Roof | Area | Litres in a year | Litres in a 10 mm shower | Butts of 200 litres |
|---|---|---|---|---|
| Greenhouse | 6 m² | 4,080 | 48 | 1 |
| Shed | 9 m² | 6,120 | 72 | 1 |
| Garage | 20 m² | 13,600 | 160 | 1 |
| Large garage | 30 m² | 20,400 | 240 | 2 |
| Half a house roof | 45 m² | 30,600 | 360 | 2 |
| Detached house roof | 90 m² | 61,200 | 720 | 4 |
Tips before you buy
- Fit the butt to a downpipe with a diverter, so that it fills first and the overflow goes to the drain.
- Link two or three butts together: a downpipe on a house roof fills one butt in a short shower.
- Put the butt on a firm, level base. A full 200 litre butt weighs about 200 kilograms.
- Keep a lid on the butt, to stop light and insects getting in, and add a tap high enough for a can.
- Use rain on the garden, pots and greenhouse plants: it is soft water, and saves mains water in a dry summer.
Tools & materials for this project
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Frequently asked questions
How much rain can a roof collect?
One litre for every square metre for each millimetre of rain. A 20 m² roof with 850 mm of rain a year collects about 13,600 litres, with 80% reaching the butt.
How big a water butt do I need?
Enough to hold a good shower. A 10 mm shower on a 20 m² roof gives 160 litres, so a 200 litre butt fills. Link two or three butts for a larger roof, such as a house.
How much water does a shed roof collect?
A 9 m² shed roof collects 6,120 litres a year at 850 mm of rain, and 72 litres in a 10 mm shower: about a 100 litre butt.
How many watering cans is a year of rain?
1,360 cans of 10 litres for the 20 m² roof, more than enough for a small garden if you can store it.
Is rainwater better for plants than tap water?
Yes: rainwater is soft, with no chlorine, and plants like it, particularly acid-loving ones. It also saves mains water, which costs money if you have a meter.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.