Calculate Your Rainwater Harvest
Enter your roof catchment area and local rainfall — 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?
This free water butt calculator works out how much rainwater you can harvest from your roof each year. Enter your roof dimensions and local rainfall, and get instant results showing annual yield, monthly averages, how many water butts you need, and estimated savings on your water bill.
Rainwater harvesting is one of the simplest and most cost-effective things you can do for your garden. Plants prefer rainwater over tap water — it's free of chlorine and limescale, has a near-neutral pH, and it saves you money every time it rains.
UK Rainfall by Region
Rainfall varies significantly across the UK. Western regions receive considerably more rain than the east.
| Region | Avg Rainfall (mm/yr) | Harvest from 40m² Roof |
|---|---|---|
| East Anglia | 600 | 19,200 L |
| South East England | 700 | 22,400 L |
| East Midlands | 800 | 25,600 L |
| South West England | 850 | 27,200 L |
| UK Average | 885 | 28,320 L |
| North West England | 1,050 | 33,600 L |
| Wales | 1,100 | 35,200 L |
| Scotland (West) | 1,500 | 48,000 L |
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.