GardenCalcs.uk
Water Butt Size Calculator — Rainwater Collection
Calculate the ideal water butt size based on your roof area and rainfall.
How We Calculate This
Annual collection (litres) = Roof Area (m²) × Annual Rainfall (mm) × Collection Efficiency. One millimetre of rain on one square metre equals one litre, so the metres-to-litres conversion is built in.
UK long-term average rainfall is roughly 800–900mm per year; the default here is 800mm. The collection (runoff) efficiency defaults to 0.8, typical for tiled roofs.
Recommended butt size is a practical rule of thumb — about one month of average collection, rounded up to the nearest 100 litres and capped at 1,000 litres for a typical domestic install. The butt count is taken from that same capped figure so the recommended capacity and the number of butts always agree.
Frequently Asked Questions
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Last updated: February 2026
All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.