Water butt calculator
Type the size of your roof, shed or greenhouse and where you live. You get the litres it collects in a year, how long a butt takes to fill, and how many butts are worth having.
How much water your roof catches
Measure the roof from the ground: the length and width it covers, not the slope. A 3 m by 2 m shed is filled in to start.
About 3,890 litres a year
6 m² of pitched roof with tiles or slates in the Midlands catches about 3,890 litres a year, or 324 litres in an average month. That is 810 mm of rain a year × 6 m² × 0.8 (the share of the rain that reaches the gutter).
How long to fill a butt
On an average day of rain this roof sends about 11 litres to the gutter. In March, the driest month here for its length, it is about 8 litres a day. Rain comes in showers, not every day, so treat these as the time it takes on average.
| Butt | Average day | In March |
|---|---|---|
| 100 litres | about 9 days | about 12 days |
| 200 litres | about 19 days | about 24 days |
| 250 litres | about 23 days | about 30 days |
How many butts are worth having
About 2 × 100 litres, or 1 × 200 litres, or 1 × 250 litres. That is storage for about 194 litres, or 5% of a year's rain off this roof: the rule of thumb the Environment Agency gives for sizing a rainwater store. 5% of a year is about 18 days of average rain, enough to carry you through a dry fortnight or so.
Month by month
| Month | Rain | Collected |
|---|---|---|
| January | 73.0 mm | 350 litres |
| February | 59.2 mm | 284 litres |
| March | 54.4 mm | 261 litres |
| April | 55.1 mm | 264 litres |
| May | 58.4 mm | 280 litres |
| June | 64.8 mm | 311 litres |
| July | 65.4 mm | 314 litres |
| August | 70.5 mm | 339 litres |
| September | 64.5 mm | 310 litres |
| October | 81.6 mm | 391 litres |
| November | 80.5 mm | 386 litres |
| December | 82.3 mm | 395 litres |
Rain: Met Office 1991–2020 average for the Midlands district, read from this page.
How to use it
Measure the roof from the ground, as if you were looking down on it from above: the length and width it covers, not the length of the slope. A pitched roof catches the rain falling on its footprint, so the slope does not add to it. Only count the part that drains into the gutter you will connect the butt to. Then pick your part of the UK and the kind of roof. The link in your address bar keeps your answer, so you can come back to it.
How to read it
The yearly figure is what an average year brings. Some years are wetter, and dry summers bring a lot less. The days to fill a butt assume the rain is spread evenly, so take them as an average: in real life a butt can fill in one heavy shower and then sit for a fortnight. We also show the driest month for its length, because that is when you need the water most.
The number of butts is a guide to storage worth having, not the most you could fit. It holds about 5% of a year's rain off the roof, which is roughly 18 days' worth.
How it is worked out
One millimetre of rain on one square metre is one litre of water. That is because 1 m² × 1 mm makes a thousandth of a cubic metre, and a cubic metre is 1,000 litres. So the sum is the roof area in m² × the rainfall in mm × the share of the rain that gets to the gutter.
The rainfall is the Met Office average for 1991 to 2020 for your district, from its UK climate averages. Some rain is lost by splashing, evaporation or soaking into the roof. The Designing Buildings Wiki, next to the Environment Agency's sizing rule, gives 0.8 for a pitched roof with tiles and 0.9 for other pitched roofs, and we use those; putting metal, glass, polycarbonate and felt with the 0.9 roofs is our reading. The Environment Agency's rule (from its 2010 guide to harvesting rainwater for the home) sizes a store at 5% of the year's collection, and we use that for the number of butts, leaving out its filter step because a water butt has no filter.
Using the water
Rainwater is best for pots, beds and the greenhouse. To see what to sow and when, try the when to plant tool or the vegetable planting calendar. And the UK frost dates page shows when the last spring frost usually comes where you are.
Garden art prints to go with it
Questions people ask
How much water can I collect from my roof?
Area × rainfall × about 0.8 for a tiled roof. A 3 m by 2 m shed in the Midlands, with 809.77 mm a year (Met Office 1991 to 2020 average), collects about 6 × 809.77 × 0.8 = 3,890 litres a year.
How much rain is 1 mm?
One litre on every square metre. So 10 mm of rain on a 10 m² roof is 100 litres, before any losses.
How many water butts do I need?
A good guide is storage for about 5% of a year's rain off the roof, the Environment Agency's rule of thumb for rainwater stores. For a 3 m by 2 m shed in the Midlands that is about 190 litres: one 200 litre butt.
Should I measure the slope of the roof?
No. Measure the footprint, the area the roof covers seen from above. Rain falls down, so a steep roof catches no more than a flat one over the same ground.