A water butt collects rain from a downpipe for the garden, and if the mains goes off it's water for flushing the toilet, though not for drinking. For most gardens, the biggest butt that fits beside a downpipe is the one to get: the 227-litre Harcostar was DIY Garden's best large butt and £59.99 when we checked. For a narrow path, a 100-litre slimline such as the Ward costs about £23.
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Watering plants, and flushing the toilet when the mains is off: the government's Be Ready guidance lists both uses. When the water goes off.
A butt sits beside a downpipe. The RHS suggests firm, level stands that raise it high enough for a watering can to fit under the tap. A full 200-litre butt weighs about 200 kg. Some butts come with a child-safety lid, listed in the specs below.
A diverter fits into the downpipe and sends rain through a short hose into the butt. Once the butt is full, it sends the rest back down the pipe: in Harcostar's words, "When the water butt is full, the rain trap will redirect water back down the downpipe." Garantia adds that its diverters have a winter setting so "no rainwater flows into the connected barrel", to stop the butt freezing. The height of the diverter sets how full the butt gets: Garland says its kit, fitted correctly, fills the butt "to within 10mm of the top".
Each kit fits a stated range of pipe shapes and widths. Common plastic sizes are 68 mm round and 65 mm square, and even "universal" kits have a range. The Garland and Harcostar kits below are for plastic pipes; Garantia says its Speedy also fits cast iron and aluminium.
Some water companies sell subsidised water butt kits. South East Water's customers can buy the 227-litre Harcostar with a diverter and stand for £52.99 through Save Water Save Money; check whether yours does too.
Only on a water meter, and only for water that would otherwise have come from the tap. Without a meter, the bill doesn't depend on how much is used.
An example with Thames Water's metered charges from April 2026 (£2.7346 per 1,000 litres for clean water, £1.4721 for wastewater): using a full 200-litre butt instead of the hose saves about55p on the water charge, or about 84p where the sewerage charge is also based on the water used. Ten full butts in a year would save about £5.47 to £8.41. Each household's rates are on its bill.
So a set-up like the £22.99 100-litre butt with a £9.75 diverter kit (about £32.74) takes many years to pay for itself in water alone. The real value is having water to hand: in a dry spell for the garden, and in an outage for the toilet.
How much rain you can catch isn't the limit: the RHS estimates even a small shed roof in the dry south-east can collect 2,500 to 3,500 litres a year. The saving depends on how often the butt is emptied and the water used. Thames Water's charges.
The RHS suggests clear gutters, regular use and a yearly clean. For winter, some diverters have a setting that stops the butt filling, and makers' instructions cover frost.
Prices checked 4 November 2026, from the shop named on each button. Specs are from the seller's listing.
A slim 100-litre butt, only 38 cm square, for a narrow path or a small garden. Tap and lid included; the diverter and a stand to raise it are extra.
Twice the water in a wide, squat barrel (68 cm across). Comes with a tap and a child-safety lid. Diverter and stand extra.
The biggest here, at 227 litres, 60 cm across, with a tap and a spring lid. DIY Garden named it the best large water butt, finding "the water pressure from the tap is stronger than the others". EvenGreener sells a matching stand for £24.99; the diverter is extra. Garland sells the same butt for £114.99.
The cheapest slimline butt we found, 36 x 32 cm, with a lid and tap. Testers found similar 100-litre slimlines shaky when empty, so it wants level ground and a firm stand.
210 litres with a lockable lid and tap, for the lowest price per litre we found. DIY Garden found the tap on a similar 210-litre set slow to fill a can.
Each fits a stated range of downpipe sizes, listed in its specs.
The basic cut-in kit: you saw a 30 mm section out of the downpipe and fit the diverter in the gap. For standard 68 mm round or 65 mm square plastic pipes.
Fits a wider range of plastic downpipes, round and square, with a flexible hose so the butt can stand a little way off.
Drills into the pipe instead of cutting a section out, with a leaf filter and a winter setting. For round pipes of 70 to 100 mm, so it won't fit the common 68 mm size. Garantia says it fits aluminium, cast iron or plastic downpipes.