Water Butt Size Calculator — Rainwater Collection

It depends on roof area and rainfall, not just garden size. The RHS notes a small shed roof in drier south-east England yields roughly 2,500–3,500 litres a year — easily enough to justify a butt. A single 200L or 210L butt suits a small shed or a few pots; a 6m × 3m (18 m²) shed can shed about 16,000 litres a year (RHS), so link two or more butts or fit a 500L+ tank. This tool sizes storage to about one month of average collection (a practical rule of thumb), capped at 1,000 litres for a typical domestic install.

Annual collection (litres) = roof plan area (m²) × annual rainfall (mm) × runoff coefficient. One millimetre of rain on one square metre is one litre. With 800mm rainfall and a 0.8 runoff coefficient (typical for tiles), a 40 m² roof yields about 25,600 litres a year. Flat or smooth roofs run off a little more efficiently than rough or mossy ones.

Not every drop reaches the butt. Splash, evaporation, wetting of the surface, gutter overshoot in heavy downpours, and the first-flush diverter all cost a little. Tiled and slate roofs typically collect 0.8–0.9 of gross rainfall; flat felt or smooth metal can reach 0.9, while rough or moss-covered surfaces are lower. The default 0.8 is a sensible UK figure — adjust it under Advanced Options.

Yes. Link them with a connector kit near the base so that when the first fills, water flows into the next, increasing total storage. This tool reports how many butts of your chosen size meet the recommended capacity. Keep an overflow routed to a drain or a planted area for when every butt is full.

Yes — rainwater is ideal for vegetables and generally better than tap water as it carries no chlorine. Fit a leaf guard or first-flush filter on the downpipe, and avoid collecting from roofs with lead flashing or those recently treated with moss killer or other chemicals.