What this calculator works out
This calculator estimates the rainwater a roof yields across a year, how much of it you would realistically use, and what that saves at a metered water price.
The last step is the one that matters and the one most often skipped. Collected litres are not saved money — the saving depends on using the water and on being metered in the first place.
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Before you rely on the figure
- Use annual rainfall for your area. UK averages range from around 600 mm in the driest parts to well over 2,000 mm in the wettest.
- The proportion you would actually use is usually well under half the collection. Rain arrives in winter and gardens need water in summer.
- Water prices vary by supplier. Check a recent bill for the combined supply and wastewater rate per cubic metre.
- On an unmetered supply there is no financial saving at all, whatever the volume collected.
The fundamental limit on rainwater harvesting in the UK is timing, not volume. Most rain falls between October and February; most garden watering happens between June and August. Without substantial storage, a large share of what a roof collects simply overflows during the months when nothing needs watering.
How this calculator works
Collection, usage, then money:
Collected = roof area × annual rainfall × (efficiency ÷ 100)
Actually used = collected × (usage proportion ÷ 100)
Saving = used ÷ 1,000 × price per 1,000 litresDividing by 1,000 converts litres to cubic metres, which is how water is billed. The saving applies only to metered supplies — an unmetered household pays a fixed charge regardless of consumption.
The usage proportion is the honest constraint. Storage capacity, garden size and the seasonal mismatch between rainfall and demand all limit it.
Worked example: a 40 m² roof
Using the default figures — 40 m² of roof, 800 mm of annual rainfall, 80% efficiency, using 60% of what is collected, at £2.20 per 1,000 litres:
- Collected a year: 25,600 litres
- Actually used: 15,360 litres
- Saving a year: £33.79
Thirty-four pounds a year, on a metered supply, for a roof shedding over 25,000 litres. That is a real saving and a modest one — a water butt pays for itself in a few years rather than a few months. The stronger arguments are that rainwater suits plants better than chlorinated tap water, and that it remains available during a hosepipe ban.
Common mistakes
- Treating collected litres as saved money. Only the water you actually use saves anything.
- Assuming a saving on an unmetered supply. There is none.
- Using a summer rainfall figure annualised. Use the local annual average.
- Overestimating the usage proportion. Storage and seasonality both limit it.
- Ignoring storage capacity. A single butt captures a small fraction of the annual yield.
Frequently asked questions
Is a water butt worth it?
On a metered supply with a garden that needs watering, yes — it pays back within a few years and keeps working indefinitely. On an unmetered supply the case is environmental rather than financial: less treated water abstracted, and a supply that continues during a hosepipe ban. Many water companies offer subsidised butts, which shortens the payback considerably.
How much storage should I have?
More than you think. A single 200 litre butt on a 40 m² roof fills in about 6 mm of rain — one ordinary wet day. Linking two or three butts, or fitting a larger tank, captures far more of the annual yield. The practical constraint is usually space rather than cost.
Does a hosepipe ban apply to a water butt?
Restrictions apply to mains water use, so a butt filled from a roof can normally be used freely, including with a hosepipe connected to it in most cases. Restrictions vary between water companies and between bans, so it is worth checking the specific wording when one is in force.
Can I use rainwater indoors?
Only through a properly designed system. Harvesting for toilet flushing or washing machines requires separate pipework clearly marked as non-potable, backflow prevention, and compliance with water regulations. It is a plumbing project rather than a garden one, and it must never be connected to drinking water pipework.
Is what I enter stored?
No. Measurements and prices are processed in your browser and never transmitted or retained.
Related tools
References
- Met Office — UK annual rainfall averages by region
- GOV.UK — water efficiency, metering, hosepipe restrictions and water regulations
Sources are checked at publication and can change — how I choose and check references.