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 = Roof Area × Annual Rainfall × Collection Efficiency (80%).
UK average rainfall: ~800mm per year. Monthly average: ~67mm.
Recommended butt size is based on one month of average collection, rounded to nearest 100 litres.
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.