One of the most common questions we hear is: how much rainwater can I actually collect? The answer might surprise you.
The Simple Formula
For every square metre of roof area, you can collect roughly 1 litre of water for every 1 mm of rainfall. The formula is:
Roof area (m²) × Rainfall (mm) × 0.8 = Litres collected
The 0.8 accounts for losses — evaporation, splash, and the first flush of dirty water.
A Real-World Example
A typical UK semi-detached house has a roof area of about 80 m². The UK average rainfall is roughly 800 mm per year.
80 × 800 × 0.8 = 51,200 litres per year
That’s 51,200 litres — enough to fill over 250 standard water butts. Even a small 20 m² garage roof collects around 12,800 litres annually.
How Many Butts Do I Need?
While you probably don’t want 250 water butts, the calculation shows how much potential there is. Most gardeners find that a single 200-litre butt per downpipe provides plenty for regular watering.
If you have a large garden, greenhouse, or allotment, consider connecting multiple butts together. A daisy-chain of three 200-litre butts gives you 600 litres of storage — enough to get through a dry spell without resorting to the hose.
Matching Butt Size to Roof Area
A 200-litre butt connected to a single downpipe will fill in less than 20 mm of rainfall on an average house. On a smaller shed or garage, a 100-litre butt is usually sufficient.
The key is to start somewhere. Even one water butt makes a real difference to your water bill and your garden’s resilience.