Calculate Your Rainwater Harvest
Enter your roof catchment area and local rainfall, and we'll tell you how much water you can collect and how many butts you need.
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How Much Rainwater Can I Collect?
So the water bill's climbing and the shed roof's shedding rain every time it's wet. Before you buy a water butt, here's the real picture.
Every summer in the south-east it's the same conversation. Hosepipe ban starts, vegetables wilt, tap water is chlorinated and expensive, and you stand in the garden looking at the shed or the greenhouse thinking: that's a lot of water just running off the roof into the drain.2 The instinct is right. Take a 6m × 3m roof in the Met Office's south-east and central southern England series, where the 1991–2020 average is 806mm a year: the calculator gives about 11,600 litres a year after its 0.8 allowance.5 Even a small shed roof in dry south-east England can collect 2,500–3,500 litres a year, says the RHS.1 The question isn't whether to catch it. The question is how much capacity you need, where to put it, and what happens in December when it's already full.
That's what the calculator at the top of this page works out. Enter the roof's footprint (not the slope, just the ground area under it), pick your region from the list, and it gives you an annual yield, a monthly average and how many times that would fill the butt size you picked. The rest of this page is the context you need to trust the number it gives you, and to avoid the three mistakes that make most water butts disappointing after the first wet year.
Why rainwater harvesting usually fails
Rainwater harvesting rarely fails for lack of rain. It fails for one of three reasons: the butt was too small and overflowed for eight months of the year, the roof catchment was mis-measured from the start, or the thing turned green by July and the owner gave up on it. All three are fixable with a few minutes of planning before you order anything.
The overflow problem is the most common. A standard 200-litre butt sounds generous until you run the numbers. Every millimetre of rain on a 20 m² roof is about 16 litres after the calculator's 0.8 allowance, so about 12.5mm of rain fills a 200-litre butt. Once it is full, the next week's rain goes straight down the overflow pipe, and you're back to the hose in August. If you're serious about covering summer watering, you either need a bigger butt (500 L and up) or two 200-litre butts linked together, and the calculator will tell you which.1
Roof catchment area — measuring it properly
The single most misunderstood number in rainwater harvesting is roof size. It's not the slope. It's not the length along the ridge. It's the footprint: the ground area the roof covers if you looked straight down at it from above. A 3 m × 4 m shed has a 12 m² footprint whether the roof is flat, single-pitch, or gabled. The pitch changes the speed water runs off, not the total volume. Measure the footprint with a tape (walls included on a shed, drip-line included on anything bigger) and that's the number the calculator needs.
One more catch: only the roof area that drains into the connected gutter counts. On a symmetrical pitched roof with a gutter on one side only, that is about half the footprint. On a single-slope (mono-pitch) roof with the gutter on the low edge, it is the whole footprint. Enter the area that actually drains to your butt; the calculator then applies its 0.8 allowance. If your gutter is leaking, rotted, or missing at one end, measure what's actually working, not what was originally designed to catch.
Water butt leaking, algae, and the maintenance most people skip
A new butt lasts indefinitely if you do one five-minute job a year. Every autumn, before the first hard freeze, open the tap at the bottom and drain the sediment: the silty layer that accumulates from roof tiles, moss, lichen, and whatever the birds left up there. If you never drain it, the tap blocks within two or three years and you get the classic "water butt leaking from the base" problem that always gets blamed on the butt being faulty. It's almost always a blocked tap or a perished washer, both fixable in five minutes with a spanner and a 50p replacement part.
Algae is the other summer problem. A butt in full sun with a bit of roof debris at the bottom will turn green within a fortnight. A dark-coloured butt in shade stays clear for years. Black is better than green or beige. The RHS says a close-fitting lid keeps out rain-borne spores of plant diseases and discourages algae. Use stored water on established plants but not on seedlings, which it may infect with damping-off diseases, and clean the butt once a year with warm water and a little washing-up liquid.1
When a second water butt earns its space
Whether a second butt is useful depends on how often the first overflows and how much stored water you use before the next rain. Compare storage, wall space, overflow frequency and your summer demand rather than assuming a fixed payback period. Linked butts can carry more spring rainfall into a dry spell, but unused or overflowing water does not reduce mains use.1
If the calculator shows collection potential far above one 200-litre butt, compare two linked butts with a larger tank and check current complete-kit prices. More than two butts on one downpipe may be awkward; a second gutter run can be another option. Planning what to grow with all that harvested water? Our planting calendar shows what to sow and plant each month so you don't end up storing water with nothing to use it on.
UK Rainfall by Region
Rainfall varies a lot across the UK, and the west is far wetter than the east. The figures below are GardenCalc averages of the Met Office's annual rainfall totals for 1991–2020, the standard 30-year climate period, rounded to the nearest millimetre. They are our arithmetic from Met Office data, not an average the Met Office publishes itself.5 The harvest column uses the calculator's own formula: 40 m² × rainfall × 0.8.
| Met Office region | Avg Rainfall (mm/yr, 1991–2020) | Harvest from 40m² Roof |
|---|---|---|
| East Anglia | 627 | 20,064 L |
| East and north-east England | 793 | 25,376 L |
| South-east and central southern England | 806 | 25,792 L |
| Midlands | 810 | 25,920 L |
| England | 870 | 27,840 L |
| Northern Ireland | 1,156 | 36,992 L |
| UK (all) | 1,163 | 37,216 L |
| Eastern Scotland | 1,188 | 38,016 L |
| South-west England and south Wales | 1,277 | 40,864 L |
| North-west England and north Wales | 1,338 | 42,816 L |
| Wales | 1,465 | 46,880 L |
| Northern Scotland | 1,701 | 54,432 L |
| Western Scotland | 1,818 | 58,176 L |
Getting Started with Rainwater Harvesting
What you need
A water butt (200L standard), a rainwater diverter kit (fits into your downpipe), and a firm, level base. The RHS suggests a bought stand or bricks, so a watering can fits beneath the tap.1 Check current kit prices before you buy.
Where to place your water butt
Position it directly under a downpipe, ideally near the area you'll water most. Raise it off the ground on a firm, level stand or bricks so a watering can fits under the tap.1 A litre of water weighs about a kilogram, so a full 200L butt holds about 200kg of water.
Connecting multiple butts
If one butt overflows regularly (likely in autumn/winter), add a second butt connected with a linking kit. The overflow from butt 1 fills butt 2 automatically. This is especially worthwhile if you have a vegetable garden that needs regular watering through summer.
Winter care
In freezing weather, open the tap slightly to prevent ice damage. Alternatively, drain the butt before the first hard frost and reconnect in spring. Most modern water butts handle frost well, but a full butt that freezes solid can crack.4
Frequently Asked Questions
Types of Water Butts for UK Gardens
The type of water butt you pick makes a real difference to how much rainwater you actually use. The cheapest option is not always the best value, and the biggest is not always the most practical. Here is how the main types compare. For exact heights, widths and stand sizes by model, see our water butt sizes chart.
| Type | Capacity | Footprint | Price Range | Best For |
|---|---|---|---|---|
| Standard round butt | 200–230L | 60cm diameter | £25–£45 | Most gardens — good capacity, widely available |
| Slimline / space-saver | 100–160L | 35–40cm deep | £30–£55 | Side passages, patios, small gardens |
| Decorative / barrel style | 150–240L | 55–65cm diameter | £60–£120 | Front gardens, visible areas where aesthetics matter |
| Wall-mounted tank | 200–300L | Flush to wall, 25cm deep | £80–£150 | Zero floor space — fixes directly to the wall |
| Large garden tank | 350–500L | 70–90cm diameter | £60–£130 | Allotments, large veg gardens, greenhouses |
| IBC tank (cube) | 1,000L | 120 × 100cm | £30–£80 (reconditioned) | Maximum storage — allotments, serious growers |
| Underground tank | 1,500–5,000L | Buried below ground | £300–£1,200+ | Whole-garden systems, toilet flushing, washing machines |
Where to start
For most UK gardens, a round butt of around 200–240 litres sold as a complete kit (tap, stand and diverter included) is a sensible place to start; the RHS describes 240 litres as a standard water butt.1 It is a practical size that fits against many house walls. Whether it is enough depends on your summer use and how often it refills; compare current complete-kit prices rather than assuming a fixed payback period.
If space is tight (say a terraced house with a narrow side passage), a slimline model at 100–160L is far better than no butt at all. The RHS says slimline models are available for tight spaces, and that even a small shed roof in dry south-east England can collect 2,500–3,500 litres a year.1 For a large veg patch or allotment, a 1,000L IBC tank holds five times as much as a 200L butt.
How to Install a Water Butt — Step by Step
Fitting a water butt to a downpipe needs basic hand tools and no plumbing experience. Here are the steps.
What you need
A water butt with tap, a rainwater diverter kit (check your downpipe diameter: most UK homes use 68mm round), a hacksaw, a spirit level, and either a purpose-made stand or 3–4 concrete blocks.3
Step 1: Choose your location
Position the butt directly below a downpipe, as close to the area you water most as possible. The ground must be firm and level. A full 200L butt holds about 200kg of water, and an unlevel surface will cause it to lean and potentially topple. Paving slabs, concrete, or compacted gravel all work well. Soft lawn does not.1
Step 2: Build a stable base and raise it up
This is the step most people skip, and then regret. Raise the butt at least 30cm off the ground so a standard watering can (which is typically 25cm tall) fits comfortably under the tap. Three courses of 10cm concrete blocks give roughly 30cm of clearance. A proper water butt stand costs £10–£20 and does the same job more neatly. Whatever you use, check it is level with a spirit level before placing the butt on top.1
Step 3: Cut the downpipe
Hold the diverter fitting against the downpipe at the height where it will connect to the butt inlet. Mark two lines, set apart by the distance specified in your diverter instructions (usually 35–50mm). Use a hacksaw to make two clean, straight cuts. Remove the section of pipe. Sand down any rough edges with fine sandpaper.
Step 4: Fit the diverter
Push the diverter into the gap in the downpipe. Most modern diverters like the Floplast Rainwater Diverter simply clip into place with rubber seals (no glue needed). Connect the flexible hose from the diverter to the inlet on your water butt. Make sure the hose slopes downward slightly from the diverter to the butt so water flows by gravity.
Step 5: Fit the tap and overflow
If your butt does not come with a pre-fitted tap, drill a hole near the base using the template provided and screw in the tap with the rubber washer on the inside. For the overflow, connect an overflow pipe to the overflow outlet (usually near the top of the butt) and direct it to a drain, a second water butt, or onto a flower bed. Never let overflow run against the house foundations.
Step 6: Wait for rain
That is genuinely it. The next decent rainfall will start filling your butt. On the Met Office 1991–2020 averages, a 40m² roof section gives about 1,670 litres in an average month in East Anglia and about 3,100 litres on the UK-wide figure, after the calculator's 0.8 allowance.5 Either is far more than a single 200L butt can hold. This is why a good diverter is essential: once the butt is full, excess water flows back down the drainpipe automatically.
If you are also planning raised beds or borders, our soil calculator and compost calculator will help you work out exactly how much growing medium you need.
Water Butt Cost Guide UK 2026
Rainwater harvesting is one of the cheaper garden improvements you can make. The ranges below are a rough budgeting guide; prices change, so check the current price before you buy.
| Item | Price Range (2026) | Notes |
|---|---|---|
| 100L slimline water butt (butt only) | £20–£35 | Basic butt without accessories |
| 200L standard water butt (butt only) | £25–£45 | Common size for UK gardens |
| 200–210L complete kit (butt + stand + tap + diverter) | £40–£70 | Best value — everything included |
| 300–350L large water butt | £55–£100 | Extra capacity for larger gardens |
| Decorative barrel-style butt (190–240L) | £60–£120 | Oak barrel look, suitable for front gardens |
| Rainwater diverter kit (68mm) | £8–£15 | Fits standard UK round downpipes |
| Water butt stand | £10–£22 | Raises butt 25–30cm for watering can access |
| Linking kit (connects 2 butts) | £5–£12 | Doubles your storage capacity |
| Replacement tap | £4–£8 | Standard 3/4″ BSP thread |
| IBC tank 1,000L (reconditioned) | £30–£80 | Buy from industrial suppliers or Facebook Marketplace |
Typical project costs
Rough totals for four common setups, built from the price ranges above.
| Setup | Total Cost | Storage |
|---|---|---|
| Budget single butt (100L slim + diverter) | £35–£50 | 100L |
| Standard setup (200L kit + stand) | £50–£80 | 200L |
| Dual butt system (2 × 200L + linking kit) | £90–£140 | 400L |
| Allotment IBC system (1,000L reconditioned tank) | £40–£90 | 1,000L |
Roof-runoff potential is not the same as water saved. Actual bill savings depend on rainfall timing, storage, overflow, how much stored water you use and your own metered tariff. Use the calculator for collection potential; check current equipment prices and your bill before estimating payback.
5 Common Water Butt Mistakes
Five common problems with water butt set-ups, and how to avoid each one.
1. Not raising the butt high enough
This is by far the most common mistake. If the tap is too close to the ground, you cannot fit a watering can underneath it. A standard 10-litre watering can is about 25cm tall. A butt sitting directly on the ground means tipping a 200kg butt forward to get water out. The RHS advice is a firm, level stand or bricks so a watering can fits beneath the tap. Raise it at least 30cm; 40cm is even better. Three courses of blocks or a proper stand solves this instantly.1
2. Placing the butt on soft ground
A full 200-litre water butt holds about 200kg of water, and a 350L butt about 350kg, all sitting on a small circle of ground. Placed on grass or bare soil, the butt will sink, lean, and eventually topple, especially after heavy rain softens the ground underneath. Always use a solid base: paving slabs, concrete, or a timber platform sitting on compacted MOT Type 1 sub-base.1
3. Forgetting the overflow
A 40m² roof section on the UK-wide Met Office 1991–2020 average gives about 3,100 litres a month after the 0.8 allowance, and about 6mm of rain is enough to fill a 200L butt from it.5 Without a proper overflow, water pours over the top, pools around your foundations, and can cause damp problems. Every water butt must have an overflow connection: either piped to a drain, directed onto a flower bed well away from the house, or connected to a second water butt via a linking kit.
4. Leaving a full butt through winter frosts
Water expands by about 9% when it freezes. A 200L butt that freezes solid effectively tries to hold 218 litres, and something has to give: a butt that freezes solid can crack. The fix is simple: either drain the butt to half-full before the first frost (late November in most of the UK; check our frost date calculator for your area), or leave the tap slightly open so water can escape as it expands. Modern thick-walled butts handle light frosts fine, but a prolonged freeze below -5°C will damage even the best plastics.4
5. Using the wrong diverter for your downpipe
UK homes use two standard downpipe sizes: 68mm round and 65mm square (mostly on pre-1970s properties)3. Buying a 68mm diverter for a 65mm square pipe means a poor fit, constant drips, and water running down the outside of your wall. Before ordering, measure your downpipe. If it is round and roughly the diameter of a tennis ball, it is 68mm. If it is square or rectangular, measure the outer width. Most diverter kits now come in both sizes, or with adaptors. Just make sure you check before cutting your pipe.
More Water Butt Questions
Best Water Butts UK 2026 — What We Link To
These are water butts, diverter kits and accessories available in the UK right now. Whether you need a slim space-saver or a full rainwater harvesting setup, these are the options we link to.
| Product | Size | Best For | Buy |
|---|---|---|---|
| Ward 210L Slimline Water Butt Kit | 210L | Best all-round kit — includes tap, stand & diverter | Not stocked by our partners |
| Harcostar 227L Magnum Water Butt | 227L | Extra capacity, child-safe lid | Calculate above to see it in your kit |
| Strata 100L Slimline Water Butt | 100L | Small gardens & patios — fits tight spaces | Our partner stocks Strata in 210L and 250L, not this 100L |
| Floplast Rainwater Diverter Kit | 68mm | Rainwater diverter — fits 68mm round downpipes; check your size or use an adaptor | ManoMano |
| Water Butt Linking Kit | — | Connect 2+ butts for double storage | Calculate above: shown in your kit when you need two butts |
| Water Butt Stand | — | Raises butt for easy watering can fill | Calculate above to see it in your kit |
Affiliate links: GardenCalc may earn a commission if you buy through the partner links on this page, at no extra cost to you.
Where to Buy Water Butts in the UK
Once you know how many litres of storage you need, compare prices below. Most water butts come with a stand and diverter kit, so check before buying separately.
| Supplier | What They Stock | Best For |
|---|---|---|
| Original Organics (calculate above to see them in your kit) | Harcostar 227L Water Butt Barrel, with the Harcostar rain trap and Strata stand as separate items | Barrel, rain trap and stand — the three items in your kit once you calculate |
| Wickes | Water butts, rain diverters, garden hose connectors | Trade pricing, click & collect |
| B&Q | Strata water butts, slim-line butts, diverter kits | UK-wide stores, same-day collection |
Links marked above are affiliate links, and we may earn a small commission at no extra cost to you. We only link to reputable UK suppliers.
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Sources and checking notes
The rainfall figures (source 5) and the RHS advice (source 1) were re-checked on 1 October 2026; the other sources were last checked on 12 September 2026. Manufacturer instructions supplied with a product always take precedence.
- Royal Horticultural Society, Collecting, storing and reusing water.
- Environment Agency, Why do we have temporary use (hosepipe) bans during a drought?.
- FloPlast, Rainwater Diverter — 68mm Round.
- Chartered Institute of Plumbing and Heating Engineering, FAQs for tackling frozen and burst pipes.
- Met Office National Climate Information Centre, UK and regional series (HadUK-Grid areal rainfall, annual totals, files last updated 1 September 2026). The calculator presets are GardenCalc's average of the 30 annual totals for 1991–2020 for each region, rounded to the nearest millimetre: our arithmetic from Met Office data, not a Met Office published figure.