What this calculator works out
This calculator multiplies a roof area by a rainfall depth and a collection efficiency, giving the litres a single rainfall event would yield.
The relationship underneath is simple and worth remembering: one millimetre of rain on one square metre is one litre of water.
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Before you calculate
- Roof area means the plan area — the footprint the roof covers — not the sloping surface. Rain falls vertically.
- Rainfall is in millimetres, as weather reports give it. A heavy shower might be 5 mm; a wet day 20 mm.
- Collection efficiency accounts for losses: initial wetting of the surface, evaporation, splash and overflow. 80% is a reasonable figure for a clean roof and well-fitted guttering.
- First-flush losses matter more for small events than large ones, so efficiency is lower in light rain.
A millimetre of depth over a square metre is 0.001 m³, which is exactly one litre. That makes rainfall arithmetic unusually easy: a 50 m² roof in 10 mm of rain sheds 500 litres before losses. It also explains why a modest roof fills a water butt so quickly.
How this calculator works
Area times depth times efficiency:
Litres = roof area × rainfall in mm × (efficiency ÷ 100)The efficiency figure is doing real work. At 80% a fifth of the rain never reaches the butt, which for a shallow shower on a dry roof is optimistic — much of the first millimetre or two simply wets the surface and evaporates.
Worked example: a 30 m² roof in 50 mm of rain
Using the default figures — 30 m² of roof, 50 mm of rainfall, 80% efficiency:
- Collected: 1,200 litres
1,200 litres from one substantial spell of rain, which is roughly six standard water butts. That is the reason a single butt on a house downpipe overflows so often — the roof produces far more than most people store. Linking two or three butts together, or fitting a larger tank, captures water that otherwise goes straight to the drain.
Common mistakes
- Using the sloping roof area. Rain falls vertically; use the plan footprint.
- Assuming 100% collection. Wetting, evaporation and overflow all take a share.
- Storing far less than the roof produces. One butt fills in a single wet day.
- Collecting from a roof with unsuitable coverings. Some treatments and materials contaminate run-off.
- Using stored rainwater on edible seedlings without thought. It carries more bacteria than mains water.
Frequently asked questions
How much rain does the UK get?
It varies enormously — roughly 600 mm a year in the driest parts of eastern England to well over 2,000 mm in upland western areas. The Met Office publishes long-term averages by location, which are the right basis for annual planning. A single wet day can deliver 20 mm or more anywhere.
Is rainwater safe for the garden?
For ornamental plants and general watering, yes, and it is better than tap water for acid-loving plants. For edible crops, particularly salads eaten raw, stored rainwater carries more bacteria than mains water — the usual advice is to water the soil rather than the leaves and to use mains water on anything harvested imminently.
Do I need a filter?
A leaf strainer at the downpipe keeps debris out and is worth fitting. A first-flush diverter, which discards the initial run-off carrying dust and bird droppings, improves quality further. Neither makes the water potable — rainwater harvesting for household use requires a properly designed system and is subject to regulation.
How do I stop a butt overflowing?
Link several butts with a connector kit so they fill in sequence, or fit an overflow that returns to the downpipe below the diverter. Simply letting a butt overflow onto the ground can saturate the area next to a wall, which is worth avoiding.
Is what I enter stored?
No. Measurements are processed in your browser and never transmitted or retained.
Related tools
References
- Met Office — UK rainfall averages and regional climate data
- GOV.UK — water efficiency, hosepipe restrictions and rainwater harvesting guidance
Sources are checked at publication and can change — how I choose and check references.