Water Butt Calculator

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.

By Gary Hodson · 15+ years UK gardening experience · Rates cross-referenced against RHS, Met Office and UK industry standards · Last reviewed 23 April 2026

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. The instinct is right. A typical UK shed roof — about 6 m by 3 m — sheds roughly 16,000 litres of rain a year on average. A greenhouse catches less but is usually closer to where you'd actually use the water. 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 realistic annual yield, monthly breakdown, and the butt size (or combination) that actually matches how you garden. 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. A single day of heavy rain on a 20 m² shed roof can drop 40 to 80 litres in an afternoon. Three days like that in a row and the butt 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 — the calculator will tell you which.

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 side where the gutter actually runs counts. Most sheds are guttered on one long side only, which means you're catching from half the footprint. A fully-guttered pitched roof catches from the whole thing, minus a small evaporation loss the calculator already accounts for. 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. A close-fitting lid is better than an open top — mosquitoes can't lay in what they can't reach. If algae does show up, it isn't the end of the world: diluted roughly 3:1 with tap water, green butt water is still fine on ornamentals and fruit trees, just not on salad leaves you're about to eat.

When a second water butt actually pays off

Here's the maths nobody explains on the packaging: the first butt is the one that pays for itself — the savings from not using tap water through summer will cover its cost over roughly two dry summers. A second butt saves less per litre in isolation, because the first one rarely overflows often enough to make the second one earn its place. But in a wet April followed by a dry July — which is most of the south-east most years — linked butts mean you carry water from spring rain into mid-summer drought, and that's where they suddenly look like a bargain.

The calculator shows you the crossover point. If it says you need more than 200 L of capacity, the cheapest option is usually two 200-litre butts with a linking kit — about £15 extra for the connector, and it keeps both filling and draining evenly. If you've got the wall space, one 500-litre slimline against the shed is tidier. What rarely makes sense is more than two butts on a single downpipe — the diverter hardware struggles past that, and you're better off fitting a second diverter to a different gutter run. 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 significantly across the UK. Western regions receive considerably more rain than the east.

Getting Started with Rainwater Harvesting

What you need

A water butt (200L standard), a rainwater diverter kit (fits into your downpipe), and a flat, stable base — concrete slabs or a purpose-made stand. Total setup cost: £40–80 for a basic system.

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 stand or bricks so a watering can fits under the tap. Ensure it's on a level, solid surface — a full 200L butt weighs 200kg.

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.

Frequently Asked Questions

Types of Water Butts for UK Gardens

After years of testing different setups across my own garden and allotment, I can tell you that picking the right type of water butt makes a genuine 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.

Which type do I recommend?

For most UK gardens, I recommend starting with a standard 200L round butt with a complete kit (tap, stand, and diverter included). It is the sweet spot between cost, capacity and practicality. A 200L butt holds enough water for roughly 4–6 watering sessions in summer, it fits against any house wall, and at £30–£45 for a full kit it pays for itself within the first season on a water meter.

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. You will still collect thousands of litres per year. And if you have a large veg patch or allotment, look at reconditioned IBC tanks. I picked up a 1,000L cube from a local business for £40, and it stores five times the water of a standard butt at a fraction of the cost per litre.

How to Install a Water Butt — Step by Step

I have installed over a dozen water butts over the years, including my own and for neighbours. The whole job takes about 30–45 minutes with basic tools, and you do not need any plumbing experience. Here is exactly how I do it.

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.

Step 1: Choose your location

Position the butt directly below a downpipe, as close to the area you water most as possible. I keep mine near the vegetable beds because that is where the water goes in summer. The ground must be firm and level — a full 200L butt weighs 200kg, 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.

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. I use three courses of concrete blocks (each block is about 10cm), which gives me 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.

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 — the distance apart 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. In an average UK month, you will collect 1,500–3,000 litres from a 40m² roof section — 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 cheapest garden improvements you can make. I have put together real 2026 prices from UK suppliers so you know exactly what to budget before you start.

Typical project costs

Here is what three common rainwater harvesting setups actually cost in 2026, based on what I have spent and what I recommend to friends.

The annual savings figure (£75–£100) is based on replacing approximately 25,000 litres of mains water per year at the 2026 UK average metered rate of £3.50 per 1,000 litres. If you are on a water meter — and roughly 60% of English households now are — every litre of rainwater you use instead of turning on the tap saves you money. Even if you are on an unmetered supply, you will still benefit from healthier plants, since rainwater is softer, chlorine-free, and at a more natural pH than treated mains water.

5 Water Butt Mistakes I See Every Year

I have made most of these mistakes myself at some point. Here are the five most common problems I see with water butt installations across the UK, along with 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. I see butts sitting directly on the ground all the time, and the owners end up trying to tip the entire 200kg butt forward to get water out. Raise it at least 30cm — 40cm is even better. Three courses of blocks or a proper stand solves this instantly.

2. Placing the butt on soft ground

A full 200-litre water butt weighs 200kg. A 350L butt weighs 350kg. That is the weight of 3–4 adults concentrated on a 60cm diameter circle. 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.

3. Forgetting the overflow

A 40m² roof section in average UK rainfall collects about 2,360 litres per month. A 200L butt fills up in roughly 2.5 days of steady rain. 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. I have seen plastic butts crack cleanly in half after a hard frost in January. 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.

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). 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 — Our Top Picks

We've picked the most popular 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 best-rated options.

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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 — check before buying separately.

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