There is no honest single answer based on roof area alone. A useful storage size depends on how much rain reaches the butt, how quickly you use that water, how long dry spells last and how much room you have for storage.
A roof can collect far more water over a year than any ordinary water butt can hold at one time. RHS notes that even a small shed roof can collect thousands of litres across a year. That is useful evidence that rainwater is worth collecting — but it is not a tank-sizing formula.
A garden water butt is a store that repeatedly fills and empties. A practical capacity depends on the effective roof area, rainfall timing, how often you draw water, garden demand, available space, downpipe arrangement and where overflow goes. A tank that fills every shower but is never emptied gives no extra useful storage during the next shower.
Rainwater yield should start with the horizontal plan area that actually drains to the connected downpipe. Roof pitch does not create extra rainfall; rain falls through a horizontal footprint. If a pitched roof drains in two directions and your butt is connected to only one side, only that contributing section belongs in the calculation.
A simple projected rectangle can be estimated from length × width. For complex roofs, measure the plan areas that feed the relevant gutter. If only part of a roof reaches the butt, reduce the area accordingly.
One millimetre of rainfall on one square metre equals one litre of water before losses. That makes event calculations straightforward: roof area × rainfall depth gives the gross litres arriving on the catchment. Apply an efficiency factor to estimate what reaches storage.
For annual planning, use a long-term rainfall figure appropriate to your location. The Met Office publishes location-specific 1991–2020 climate averages for official stations. Do not use a generic UK average when a local long-term figure is readily available.
Runoff coefficients, filter efficiency, diverters and gutter behaviour mean actual storage yield is lower than gross roof rainfall. Professional rainwater-harvesting tools often separate runoff and filter coefficients. For a garden water butt, an editable combined efficiency is a transparent way to model those losses without pretending every roof/system is identical.
A useful storage question is: how much rain would fill this tank from empty? A 200-litre butt on a 25 m² catchment at 80% efficiency requires about 10 mm of rain to collect 200 litres. On a larger roof, the same tank fills with less rainfall; on a smaller roof, it takes more.
This is often more practical than annual yield. If a modest storm can refill your tank, adding storage may capture water that would otherwise overflow. If the roof catchment is tiny, adding several large tanks may create capacity that rarely fills.
The same rainfall-depth arithmetic translates a garden watering target into litres. One millimetre over one square metre is one litre. So a chosen 10 mm application over 100 m² equals 1,000 litres. A 200-litre butt contains one-fifth of that example session.
The important word is chosen. Water need varies with plant type, soil, establishment, weather, season and watering method. RHS recommends targeted, efficient watering and makes clear that requirements vary. The sizing tool therefore lets you enter a depth rather than pretending one garden-wide rate is universally correct.
RHS explicitly notes that multiple water butts can be joined with connector kits or siphons. Linking tanks can be useful where a single tall/large tank does not fit the space. Three 210-litre butts provide 630 litres of nominal capacity.
Capacity arithmetic is simple; plumbing behaviour is not. Linked tanks need sensible connections, similar levels and an overflow route. A calculator can add litres, but it cannot guarantee that a particular connector layout will fill or drain evenly.
Suppose a 50 m² effective roof receives 800 mm of annual rainfall and your efficiency assumption is 90%. The theoretical annual collectable volume is 36,000 litres. That number tells you the scale of the potential resource. It does not mean a 36,000-litre garden tank is sensible.
Storage sizing needs rainfall timing and demand timing. A 200-litre butt that is emptied into the garden and refilled many times can capture thousands of litres during a year. A larger tank can bridge longer dry periods, but only if the catchment and rainfall actually replenish it.
RHS guidance recommends a firm, level stand or bricks, a filter to reduce leaves/debris, a close-fitting lid and clean gutters. A full water butt is heavy: 200 litres of water alone is roughly 200 kg before the container and stand, so stable support is a practical necessity.
Plan what happens when storage is full. A rainwater diverter may return excess flow to the downpipe; other systems use an overflow. Larger rainwater-harvesting arrangements can become a different design problem and may need specialist guidance.
Do not treat ordinary stored roof runoff as drinking water. RHS advises using stored rainwater carefully, keeping gutters and tanks clean, using the water frequently and avoiding water from ordinary butts on seedlings because stored water can carry plant pathogens. The guide is about garden storage, not potable-water treatment.
50 m² roof × 10 mm rainfall = 500 litres gross. At 90% efficiency: 450 litres estimated collectable. A single 210-litre butt could theoretically fill, with the remainder needing extra available storage or overflow.
30 m² × 5 mm × 80% = 120 litres.
200 L ÷ (25 m² × 0.8) = 10 mm of rain to collect 200 litres from empty under the stated assumptions.
Three 210-litre butts provide 630 litres nominal capacity. Whether they fill evenly depends on the physical linking arrangement.
50 m² × 800 mm × 90% = 36,000 litres/year theoretical collectable water. That is a yield estimate, not a recommendation for a 36,000-litre tank.
Enter your effective roof area, local rainfall or event depth, collection efficiency and proposed storage in the Water Butt Calculator. Keep the output as a planning estimate and adjust the assumptions to match your real roof, diverter and garden.