How much water does a roof collect?

About 1 litre for every square metre of roof (measured from above) for every millimetre of rain, less 10–20% that never reaches the butt. A 25 m² roof in London, at 615 mm a year, sends about 12,450 litres a year to its downpipe.

What size water butt should I get?

For a garden of pots and baskets, a standard 200–250 litre butt usually covers most of the summer. Veg patches and greenhouses need much more, often 500–1,000 litres or several butts linked together. Put your garden in the calculator to see where more size stops helping.

How often will my water butt overflow?

Often, and mostly in winter: a typical 210 litre butt on half a house roof is full and overflowing on around 100 days a year. That's normal. A diverter sends the extra down the drain instead of over the lid.

Does a water butt save money?

Only on a water meter, and not much: a few pounds to about £20 a year for most gardens at 2026/27 prices. The bigger benefits are water during a hosepipe ban and rainwater for plants that dislike hard tap water.

Should I measure the sloping roof or the footprint?

The footprint, the area seen from above. Rain falls roughly straight down, so the slope doesn't change how much a roof catches.

Water Butt Calculator

How much rain your roof can catch, what size water butt to buy, how often it would overflow or run dry, and what it saves on a water meter.

Your roof and garden

What the rain falls on

A house roof. Some rain soaks into tiles or blows off before it reaches the gutter.

Roof size, seen from above

Length

Depth

Measure the footprint, not the slope: rain falls straight down, so a steep roof catches no more than a flat one the same size.

Downpipes this roof drains into

The butt only gets its downpipe's share: 25 m² of roof.

Rainfall where you live

Nearest place

Rain a year

Met Office average for London, 1991–2020. Hills and coasts nearby can be much wetter: type your own figure if you know it.

Garden watering

In a dry summer week

About 9 watering cans of 9 L. A patio full of pots, watered most days. Spring and autumn need less, winter none, and a rainy day none at all.

Your water butt

Size of each

Slimline butts start at about 100 L, the usual round one is 200–250 L, and an IBC tote holds 1,000 L.

Butts linked together

Water bill

Price per 1,000 L

On a meter, water and sewerage together: about £4.20 (Thames) to £5.50 (United Utilities) in 2026/27. Leave blank if you're not on a meter.

Water butt to get

210 L

The smallest common size that gets 90% of what 2,000 L of butts would.

Rain off the roof

12,454 L

A year, from 25 m² at 615 mm

Garden needs

1,134 L

A year, on dry days

From your 210 L

1,057 L

93% of the garden's water

Saving a year

£4.76

On a water meter

Overflows

106 days

A year: 11,098 L goes down the drain

Runs dry

9 days

A year, when the garden wanted water

Bigger butt, more water?

Garden needs

From the butt

Water the garden gets from the butt each year, by size. The curve flattens once the butt holds enough to bridge a dry spell.

Overflowing is normal, mostly in winter. A downpipe diverter stops taking water once the butt is full and sends the rest down the drain, so nothing spills over the lid.

A year in your water butt

15 Jun

76 L of 210 L

A dry day.

Water in the butt, day by day

JanFebMarAprMayJunJulAugSepOctNovDec

OverflowingEmpty when neededBars along the top: rain

A typical year of simulated weather for London, built from its monthly averages: real years bunch rain and dry spells differently. Drag across the chart to see any day.

Month by month, on average

JFMAMJJASOND

Rain into the buttGarden water from the buttShort: from the tap

The roof catches far more than the garden uses, so the rain bars are cut off at the top.

Get · 210 L£4.76/yr

This water butt calculator works out how much rain your roof sends down a downpipe, what size of water butt suits your garden, how often it would overflow or run dry, and what it saves on a water meter.

Pick the roof, measure it from above, choose the nearest of 13 UK places for its rainfall and say how much water the garden takes in a dry summer week. The calculator runs a butt through 30 years of simulated daily weather and shows a typical year day by day.

How much rain does a roof collect?

One millimetre of rain on one square metre is one litre. So the rain off a roof is its area times the year's rainfall, less what never reaches the butt: rain that soaks into the roof, evaporates or blows off, and what the downpipe diverter or filter lets go past.

Measure the roof's plan area, its footprint as seen from above, not the sloping surface. Rain falls more or less straight down, so a steep roof catches no more than a flat one of the same footprint. The British Standard for rainwater harvesting (BS 8515, now BS EN 16941-1) works the same way.

Most house roofs drain into two or more downpipes, and a butt only gets its own downpipe's share. A 10 × 5 m roof split front and back between two downpipes gives each one 25 m².

Example: 25 m² of tiled roof in London, where the Met Office average is 615 mm a year. Tiles shed about 90% of the rain and a diverter passes about 90% of that: 25 × 615 × 0.9 × 0.9 is about 12,450 litres a year. That's far more than most gardens use: the catch is that most of it falls in autumn and winter.

Roof types and runoff

The runoff coefficient is the share of rain that reaches the gutter. The calculator uses the BS 8515 and CIRIA C753 figures:

Metal sheet: 0.95
Tiles or slate: 0.9 (the Environment Agency uses 0.8, so a tiled roof may give a little less)
Flat, smooth membrane or bitumen: 0.8
Flat with gravel: 0.6
Green (sedum) roof: 0.5
Greenhouse glass and shed felt: no standard gives these, so glass is taken as metal (0.95) and felt as a smooth flat roof (0.8)

What size water butt do you need?

A bigger butt catches more of the rain that would otherwise overflow, so it carries the garden through longer dry spells. But each extra litre of size helps less than the last: once the butt holds enough to bridge a typical dry spell, the rest sits full and unused.

The calculator tries every size from nothing to 2,000 litres in the same weather, and recommends the smallest common size (100, 160, 210, 250, 300, 350, 500 or 1,000 litres, or several linked) that gets 90% of the water 2,000 litres would. The chart under the result shows that curve, so you can see where it flattens.

Example: the London roof above, with a patio of pots and baskets that takes 80 litres in a dry summer week. A 210 litre butt covers about 93% of the garden's water; going bigger adds almost nothing. A veg patch at 300 litres a week needs far more: a 210 litre butt covers under 60% of it, and it takes about 1,500 litres of butts to get close to what's possible.

BS 8515's quick rule sizes a tank at 5% of the yearly yield or the yearly use, whichever is less (about 18 days' worth). It's meant for toilets and washing machines that use water every day, so it undersizes a garden butt, whose water is all needed in summer.

How the simulation works

A yearly total can't say how often a butt overflows or runs dry: that depends on how the rain falls, day by day. So the calculator builds 30 years of daily weather from the Met Office 1991–2020 averages for your place: how much rain falls each month and on how many days it rains at least 1 mm.

Wet days cluster into spells, and each month is wetter or drier from year to year, so some summers have long dry runs and some are washouts. Each day, rain runs into the butt first and anything over the brim overflows; then, on a dry day, the garden takes what it needs. The butt starts full on 1 January.

Garden use follows the season: your dry summer week in July, about 90% of it in June and August, 60% in May, half in September, less in spring and autumn and nothing from November to February. A day with 1 mm of rain or more needs no watering. These shares are a rule of thumb, not measured.

The weather is made up from averages, not real records, so treat overflow and dry days as a guide. Real droughts like 2018 and 2022 can be longer than anything the simulation throws up.

How much does a water butt save?

Only if you're on a water meter. Metered households in England and Wales pay for the water and for sewerage on most of it, garden water included: in 2026/27 that's about £4.21 per 1,000 litres with Thames Water and £5.51 with United Utilities. Some companies charge sewerage on 90–95% of the water, so the true saving is a little less.

Example: the 210 litre butt above gives the pots about 1,060 litres a year, worth about £4.80 at £4.50 per 1,000 litres. Even the big veg garden saves only £10 to £20 a year, so a butt takes a few years to pay for itself. The bigger wins are water in a hosepipe ban and plants that prefer rainwater, especially acid lovers like blueberries and camellias.

Households in Scotland and Northern Ireland don't pay by the litre, so a butt saves water but not money there.

Getting the most from a water butt

Fit a downpipe diverter: it fills the butt and sends rain down the drain once it's full, so nothing spills over the lid.
Put a butt on every downpipe you can reach, including the shed and greenhouse. Two small butts on two roofs beat one big butt on one.
Link butts with a connector kit to add capacity on one downpipe.
Stand the butt on a stand or blocks so a watering can fits under the tap.
Keep the lid on: it keeps out light (no algae), leaves, mosquitoes and children.

Frequently asked questions

How much water does a roof collect?: About 1 litre for every square metre of roof (measured from above) for every millimetre of rain, less 10–20% that never reaches the butt. A 25 m² roof in London, at 615 mm a year, sends about 12,450 litres a year to its downpipe.
What size water butt should I get?: For a garden of pots and baskets, a standard 200–250 litre butt usually covers most of the summer. Veg patches and greenhouses need much more, often 500–1,000 litres or several butts linked together. Put your garden in the calculator to see where more size stops helping.
How often will my water butt overflow?: Often, and mostly in winter: a typical 210 litre butt on half a house roof is full and overflowing on around 100 days a year. That's normal. A diverter sends the extra down the drain instead of over the lid.
Does a water butt save money?: Only on a water meter, and not much: a few pounds to about £20 a year for most gardens at 2026/27 prices. The bigger benefits are water during a hosepipe ban and rainwater for plants that dislike hard tap water.
Should I measure the sloping roof or the footprint?: The footprint, the area seen from above. Rain falls roughly straight down, so the slope doesn't change how much a roof catches.