How Big a Water Butt Do I Need? Roof Area, Rainfall and Litres (UK)
Every millimetre of rain on a square metre of roof is a litre of water. A 20 m² garage roof in a place with 850 mm of rain a year collects about 13,600 litres (80% reaching the butt), and a 10 mm shower gives 160 litres, which fills a 200 litre butt. The water butt calculator does it for your roof, rainfall and butt size.
Rain collected by roof size
At 850 mm of rain a year, with 80% reaching the butt:
| Roof | Area | Litres in a year | Litres in an average month | Litres in a 10 mm shower | Butts of 200 litres |
|---|---|---|---|---|---|
| Greenhouse | 6 m² | 4,080 litres | 340 litres | 48 litres | 1 |
| Shed | 9 m² | 6,120 litres | 510 litres | 72 litres | 1 |
| Large shed or summerhouse | 15 m² | 10,200 litres | 850 litres | 120 litres | 1 |
| Garage | 20 m² | 13,600 litres | 1,133 litres | 160 litres | 1 |
| Large garage | 30 m² | 20,400 litres | 1,700 litres | 240 litres | 2 |
| Half a house roof | 45 m² | 30,600 litres | 2,550 litres | 360 litres | 2 |
| Detached house roof | 90 m² | 61,200 litres | 5,100 litres | 720 litres | 4 |
Rain collected in a year, by region
For a shed, a garage and half a house roof:
| Region | 9 m² (shed) | 20 m² (garage) | 45 m² (half a house) |
|---|---|---|---|
| Dry: the east and south-east of England (600 mm) | 4,320 litres | 9,600 litres | 21,600 litres |
| Average: much of England (850 mm) | 6,120 litres | 13,600 litres | 30,600 litres |
| Wet: the north-west, the west and Wales (1,200 mm) | 8,640 litres | 19,200 litres | 43,200 litres |
| Very wet: the Highlands and the Lake District (1,600 mm) | 11,520 litres | 25,600 litres | 57,600 litres |
Use the average yearly rainfall for your area from the Met Office climate averages if you can: these are typical figures.
Litres in a shower, by size
| Shower | 9 m² roof | 20 m² roof | 45 m² roof |
|---|---|---|---|
| 5 mm | 36 litres | 80 litres | 180 litres |
| 10 mm | 72 litres | 160 litres | 360 litres |
| 20 mm | 144 litres | 320 litres | 720 litres |
Choosing and fitting a butt
- Size. Aim to hold a good shower from the roof. A shed needs 100 to 200 litres, a garage about 200 to 400, and a house roof two or three 200 litre butts or a tank.
- Diverter. A downpipe diverter fills the butt first and sends the overflow to the drain, so the butt does not overflow onto the wall.
- Stand. A full 200 litre butt weighs 200 kg. Put it on a firm, level base, high enough for a watering can under the tap.
- Lid. Keep a tight lid on to keep out light, leaves, insects and children.
- Linking. Link butts with a hose so that they fill together or in turn.
- Winter. Empty a butt that could freeze, or leave room for ice to expand.
Using the water
- Rainwater suits garden plants, pots and greenhouses, and is soft and free of chlorine. Do not use it for drinking.
- Most rain falls in autumn and winter, and most watering is needed in the summer, so store as much as you can by the spring.
- Water in the morning or evening, at the base of plants, to save water.
See also the shed base calculator for the base of a shed, and the turf calculator. These are estimates from typical rainfall figures.
Frequently asked questions
How much rain does a roof collect?
One litre for every square metre for each millimetre of rain. A 20 m² roof in a place with 850 mm of rain a year collects about 13,600 litres, with 80% reaching the butt.
What size water butt do I need for a shed?
A shed roof of 9 m² collects about 72 litres in a 10 mm shower, so a 100 or 200 litre butt fills in one good shower. In a year it collects about 6,120 litres.
How big a water butt do I need for a house?
A house roof is large: half of one (45 m²) gives about 360 litres in a 10 mm shower, so link two or three 200 litre butts, or use a larger tank.
How many watering cans of water is that?
A 20 m² roof collects 1,360 cans of 10 litres in a year, but most of the rain falls in the autumn and winter, so storage matters.
How do I stop a water butt overflowing?
Fit a diverter in the downpipe that sends the overflow to the drain, or link the butt to a second one, and keep the lid on.
More guides
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and check the Building Regulations and any planning rules for anything structural or close to a boundary.