What size water butt do I need?

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.

How much rainwater can I collect from my roof?

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.

Why is the runoff coefficient 0.8 and not 1.0?

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.

Can I connect multiple water butts?

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.

Can I use rainwater on vegetables?

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.

GardenCalcs

Water Butt Size Calculator — Rainwater Collection

Calculate the ideal water butt size based on your roof area and rainfall.

Roof Length

Plan length of the roof catchment (the footprint, not the sloped surface) — measure along the ground

Roof Width

Plan width of the roof catchment — pitch does not change the plan area that catches rain

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.