The calculation starts with rainfall depth, not tank size
Rainfall depth is a convenient volume measure because 1 mm spread over 1 m² is exactly 1 litre. A 10 mm shower over a 50 m² effective roof therefore represents 500 litres before losses. If your planning efficiency is 90%, the estimated collectable quantity is 450 litres.
Use the horizontal catchment, then only the part that drains to this butt
For rainwater yield you need the horizontal or plan-view catchment, not the sloping roof surface. If only one side of a pitched roof feeds the downpipe connected to your water butt, use that effective section rather than the whole building roof. The percentage field lets you reduce a known roof area when only part of it reaches this storage.
Collection efficiency is deliberately editable
Real systems lose some water before it reaches storage. Roof material, gutters, filters, diverters and first-flush arrangements can all change the effective yield. Specialist rainwater tools often apply runoff and filter coefficients, but those values belong to the actual system. This calculator exposes a single editable efficiency so you can use a documented value instead of inheriting a hidden assumption.
Annual yield is not a sensible tank-size answer by itself
A 50 m² roof receiving 800 mm of annual rainfall at 90% efficiency has a theoretical annual collection potential of 36,000 litres. That does not mean you need a 36,000-litre tank. A small tank may fill and empty many times, and real storage need depends on the timing of rain, dry periods, garden demand and how often you actually use the stored water.
How much rainfall fills a water butt?
Turn the collection equation around. A 200-litre water butt supplied by a 25 m² effective roof at 80% efficiency needs about 10 mm of rainfall to collect 200 litres from empty: 200 ÷ (25 × 0.8) = 10 mm. If the tank is already partly full, it will overflow sooner.
Compare storage with garden demand
The same 1 mm × 1 m² relationship helps with watering. If you choose to apply 10 mm of water across 100 m², the arithmetic requirement is 1,000 litres. A full 200-litre water butt represents 20% of that example session. The calculator does not say 10 mm is the correct watering depth; it simply converts the depth you choose into litres.
Where to get rainfall figures
For long-term annual rainfall, use a source that actually publishes climate data rather than a generic UK number. The Met Office location-specific long-term averages include rainfall for official climate stations. The UK-wide 1991–2020 annual average is over 1,100 mm, but rainfall varies substantially by location, so the calculator deliberately asks you to enter the figure you want to use.
Practical water-butt setup matters as much as the arithmetic
RHS guidance recommends collecting from roof gutters/downpipes, using a filter to reduce debris, placing water butts on firm level stands, providing a close-fitting lid and linking multiple butts when more storage is useful. The same guidance says stored rainwater should be used carefully: ordinary water-butt water is for garden use, not drinking water, and gutters and storage should be kept clean.