Start with how much water you can collect
Every square metre of roof collects roughly one litre of water for every millimetre of rainfall. Multiply your roof catchment area (the part of the roof that drains to the downpipe you'll connect to) by annual rainfall in millimetres to get the total you could collect in a year.
London rainfall
London averages around 600mm of rainfall a year, but it falls unevenly — more in autumn and winter, less in summer when gardens need it most. The tank is sized to bridge dry spells, not to store the whole year, because water is collected and used continuously.
Then estimate demand
Think about what the water is for: irrigating lawns and borders, filling a hose, or topping up a water feature. A typical garden irrigation system in dry summer might use a few hundred litres a week. Multiply your weekly demand by how many dry weeks you want to bridge — usually two to four — to get a useful storage target.
Balance collection against demand and space
A tank that's too small runs dry in summer; a tank that's too large rarely fills and wastes space and money. A common sweet spot for a typical London garden is in the 1,500–5,000 litre range, but it genuinely depends on your roof, demand and where the tank will sit.
A worked example
A roof with 40m² draining to one downpipe, in a 600mm rainfall area, could collect around 24,000 litres a year. If the garden uses roughly 300 litres a week during dry weather and you want to bridge a three-week dry spell, you'd want around 900 litres of usable storage — so a 1,500–2,000 litre tank gives a comfortable buffer. These are rough figures; we calculate it for your garden on a site visit.
Underground or above-ground?
Underground tanks keep water cool and out of sight but need excavation. Above-ground tanks are simpler and cheaper but should be shaded to limit algae. Your tank choice also affects the maximum practical size you can install.
Add a mains top-up if reliability matters
For irrigation that must never stop, we can fit a mains top-up so the system switches to mains water if the tank runs low. The garden stays watered through long dry spells while still using rainwater first whenever it's available.