What is a water butt, and is the water OK for plants?
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Part of our garden and rainwater guide
The butt at the bottom of the drainpipe is full again. What is it, how does it work, and what is actually in the water? You can check this at your own tap with a 20-in-1 test strip.
Short answer: A water butt is a container that collects rainwater from a roof downpipe for the garden. The RSPB says a standard one holds over 200 litres, and the RHS counts standard butts as 240 litres. The RHS says the water suits established plants but not seedlings, and government Be Ready advice says not to drink it.
By Andrew Tomlinson, WaterKit · Last reviewed 29 October 2026
What is a water butt?
A water butt is a container that catches rain running off a roof so you can use it on the garden later. It usually has a tap near the base, and the RSPB says a standard one can hold over 200 litres. The Royal Horticultural Society (RHS) says you can collect from the roof of a house, garage, greenhouse or shed, and that a roof with a decent pitch and a fairly clean surface gives the cleanest water. Rainwater is not mains water, so what is in your butt can differ from what comes out of your tap. You can compare the two with a 20-in-1 test strip. The RHS also says collecting roof rainwater is legal in the UK, and our guide to whether it is illegal to collect rainwater goes into that.
How does a water butt work?
Rain runs off the roof into the gutter and down the downpipe, a fitting on the pipe sends it into the butt, and a tap near the bottom lets you fill a watering can. The RHS lists what you need.
- A gutter with a downpipe, or a rainwater downpipe kit.
- A filter to keep leaves and debris out.
- A firm, level stand or bricks, so a watering can fits under the tap.
- A close-fitting lid, to keep out rain-borne spores of plant diseases and discourage algae.
You can attach a hose to the tap, but the RHS says good pressure needs a pump. The RSPB says to find a flat, firm surface beside a downpipe, and to lay an old slab down if the butt will stand on grass or soil.
On a full butt, the sources we read do not say where the overflow goes, so follow the fitting instructions that came with yours. The RHS says you can join several butts with connector kits or siphons to store more, and that collecting rainwater in butts can help reduce flooding during heavy rainfall. The Northern Ireland government's nidirect site also describes connector kits for linking two or more. Our guide to how rainwater harvesting works covers larger systems.
What size water butt do I need?
In the sources we read (RHS, RSPB, nidirect, Be Ready) there is no official size rule, and they give different figures. The RSPB suggests starting with one butt and adding more depending on how much water your garden needs.
| Source | What it says |
|---|---|
| RSPB | A standard water butt can hold over 200 litres |
| RHS | In dry south-east England, 2,500 to 3,500 litres a year can be collected from a small shed roof, enough to fill around 10 to 15 standard 240-litre butts |
Is water butt water OK for plants?
The RHS says yes for established plants and no for seedlings. It calls rainwater the best option for garden watering, because it is naturally soft, low in minerals and has the right pH for most plants, including acid-lovers such as rhododendrons and camellias. It adds that minerals in mains water, especially in hard water areas, can raise the pH of soil. You can see how hard your own tap water is in our guide How hard is my water?.
The RHS gives the reason for the seedling caution as damping-off diseases that stored water may contain. It says rainwater suits houseplants, particularly orchids and carnivorous plants, and its pond algae advice says a pond is better topped up with rainwater than tap water, because nutrients in tap water encourage algae (see our guide to keeping pond water clear). The RSPB says plants and pond wildlife prefer rainwater.
For vegetables and fruit, the RHS watering page recommends collecting rainwater in butts to cut mains use. In the sources we read there is no official guidance on butt water and crops eaten raw. The RHS garden health advice says to wash your hands after gardening, especially before eating. Grey water is a different thing, covered in our grey water guide.
Why can the water in your butt differ from what you expect?
A general description of rainwater is an average. Your butt is shaped by your own roof, gutters, lid and how long the water sits. The RHS names several things that change it.
- Roof and gutters. The RHS says a roof with a decent pitch and a relatively clean surface gives the cleanest water, and that keeping gutters clear stops organic debris entering stored water. It links that debris to plant diseases such as Phytophthora and Pythium.
- Lid and light. The RHS says a close-fitting lid keeps out rain-borne plant disease spores and discourages algae. It says the lack of a cover on dipping tanks, which are open troughs, means uncovered water can stagnate and harbour harmful micro-organisms.
- Time and warmth. The RHS says to use stored water frequently to keep it fresh. Its health advice says Legionella bacteria can multiply in standing water between 20°C and 45°C, and that stored water above 20°C should not be used for mist irrigation or sprinklers.
“If your butt has been sitting a while, I would not assume it matches what you read about rainwater in general, because your roof, gutters, lid and how long the water stays put all shape it. A simple strip check on the butt and the tap side by side is a sensible way to see your own set-up rather than relying on the average.”
Two butts on the same street can hold different water, and one butt can change between a cool spring and a hot August. A 20-in-1 strip dipped in the butt and at your tap shows how your set-up compares on the pads it reads.
How do I keep a water butt clean, and why does it smell?
Empty and scrub it out once a year, says the RHS, which says annual cleaning prevents disease build-up and smells.
- Clean with warm water and a little washing-up liquid.
- If the water smells, empty and clean the butt rather than adding chemicals.
- Insulate the butt or paint it a light colour, to limit warming in hot weather.
- Fit a lid. nidirect says it stops leaves collecting and small animals falling in and drowning.
On hoses, the RHS says to use a coarse spray setting on a hose connected to a butt, because breathing in droplets of stagnant, warm water can pose a health risk, and to disconnect hoses and let them drain after use.
What one reader found
One poster in a 2021 Mumsnet gardening thread said they inherited a water butt when they moved house a year earlier. They wrote "I haven't had any issues with stagnant water" and said that when they emptied the butt a few weeks earlier to move it, the inside was clean and had no smell. The water that came out into their watering can was clear, and they called it great for the garden. It is one person's account, not a test result. Shared on Mumsnet
Can you drink water from a water butt?
No, according to the government's Be Ready advice. In its page on water supply outages it says not to drink water from a water or rain butt because it could contain bacteria or disease, and that it can be used for flushing toilets or watering plants. Our guide to drinking rainwater in the UK looks at the question more widely.
How can I check my butt water against my tap?
Put a strip in each and compare. The WaterKit Water Test Kit is made for drinking water, and we have not validated it on butt water, so a reading on a butt sample is an indication only. Fill a clean glass or cup, dip one strip, wait 30 to 60 seconds and match the pads to the chart, or enter the colours in our free results checker. See also how to test your water at home. The last column is our own reading of the chart steps.
| Check | What the sources say | Our strip chart |
|---|---|---|
| Hardness | The RHS says rainwater is naturally soft and low in minerals, and that minerals in mains water can raise soil pH in hard water areas. | Steps at 0, 25, 50, 100, 250 and 425. The unit basis is unconfirmed, so a butt reading beside your tap's is a rough comparison |
| pH | The RHS says rainwater has the right pH for most plants. In the sources we read there is no official pH range for water used on plants. | Steps at 6.2, 6.8, 7.2, 7.6, 7.8 and 8.4. We give no verdict on a reading |
| Nitrate | In the sources we read there is no official figure for nitrate in water used on home garden plants. | Steps at 0, 10, 25, 50, 100, 250 and 500 |
A reading cannot tell you whether the water suits a particular plant. Several things are also outside what the kit shows.
- The optional rapid bacteria test is a presence screen for coliforms. We do not claim it for butt water, and it does not look for the plant disease organisms the RHS names or for Legionella.
- The lead and iron pads start above UK drinking-water limits, so a blank pad on a butt sample indicates very little about them. For lead, see our guide to lead pipes.
- In the sources we read there is no UK official water-quality standard for watering a home garden, so a reading has nothing official to be set against.
The RHS description of rainwater as soft, low in minerals and the right pH for most plants is an average, and your own butt may differ because of your roof, gutters, lid and how long the water sits. The hardness and pH pads are the ones that matter here, and a butt reading that sits away from your tap's would indicate the two waters differ on that pad, while a change between seasons would indicate the butt itself has changed. Best pack: 20 strips, enough to read butt and tap water side by side a few times through a season, £5.99 (or £10.99 with a bacteria test), with free 2nd Class delivery on every order. See the kit
Your water butt questions
What is a water butt?
A container that collects rainwater running off a roof for use on the garden. The RSPB says a standard one can hold over 200 litres.
How does a water butt work?
A fitting on the downpipe sends rain into the butt, and a tap near the bottom fills a watering can.
What size water butt?
In the sources we read there is no official size rule. The RSPB suggests starting with one and adding more if your garden needs it.
How many litres in a water butt?
The RSPB says a standard water butt can hold over 200 litres, and the RHS refers to standard butts of 240 litres.
How to use a water butt?
The RHS says to use the water on established plants, not seedlings, and to use it often so it stays fresh.
How to clean a water butt?
The RHS says to empty and scrub it once a year with warm water and a little washing-up liquid.
What happens when a water butt is full?
The sources we read do not say. The RHS says you can link butts with connector kits or siphons to store more.
Why does my water butt smell?
The RHS says to empty and clean the butt rather than add chemicals.
Can you drink water from a water butt?
The government's Be Ready advice says not to, as it could contain bacteria or disease, and to use it for flushing toilets or watering plants.
Sources
- Royal Horticultural Society, Collecting, storing and reusing water (undated page; rainwater and pH, what you need, 240-litre butts and the shed example, seedlings and damping-off, dipping tanks, annual cleaning, smell, hoses, gutters, legality; read 29 October 2026).
- Royal Horticultural Society, Garden Safety: How to Reduce Health Risks (undated page; Legionella and temperature, cleaning and insulating butts, 20°C for mist irrigation, hand washing; read 29 October 2026), Watering vegetables and fruit (undated page; butts to reduce mains use; read 29 October 2026) and Pond algae and blanket weed (undated page; rainwater to top up ponds; read 29 October 2026).
- RSPB, Waterwise gardening: installing a water butt (undated; over 200 litres, start with one butt and add more, firm flat surface, plants and pond wildlife prefer rainwater; read 29 October 2026).
- UK Government, Be Ready, Water supply interruptions or outages (undated page; do not drink butt water, use for flushing or watering plants; read 29 October 2026).
- nidirect, Saving water in the garden (undated page; lid and connector kits; read 29 October 2026).
- Mumsnet, Water Butts. A good thing? (Gardening board thread from June 2021; one poster's account, paraphrased; read 29 October 2026).
- WaterKit, Water Test Kit (20-in-1 strip, 30 to 60 second read, optional rapid coliform test read after 24 to 48 hours, 20 strips £5.99 or £10.99 with a bacteria test, free 2nd Class delivery on every order, 30-day money-back guarantee) and results portal. The strip-chart column and the list of limits are WaterKit's own.
Contains public sector information licensed under the Open Government Licence v3.0.