Rainwater Harvesting Calculator
How much rainwater a roof collects from your rainfall — per event, per month and per year — and what tank size that suggests.
The arithmetic is pleasingly simple: one millimetre of rain on one square metre is exactly one litre.
How the rainwater harvesting calculator works
The arithmetic is pleasingly simple: one millimetre of rain on one square metre is exactly one litre. So a 150 m² roof in 600 mm of annual rainfall receives 90,000 litres. What you actually capture is less, because some evaporates, some is lost in the first flush and gutters overflow in heavy downpours — a runoff coefficient of 0.8 to 0.9 is realistic for a hard roof.
Roof area means the plan area, the footprint the roof covers, not the sloping surface. A steep roof collects no more than a flat one of the same footprint, because rain falls vertically.
Formula: litres = roof area (m²) × rainfall (mm) × runoff coefficient
Worked examples
| Inputs | Water collected | Note |
|---|---|---|
| A 150 m² roof in 600 mm of annual rain | 76,500 L | 76,500 litres a year |
| A single 20 mm downpour | 2,550 L | 2,550 litres in one storm |
| A small 60 m² roof | 45,900 L | 45,900 litres |
FAQFrequently asked questions
How much rainwater can I collect?
One millimetre of rain on one square metre gives one litre. A 150 m² roof in 600 mm of rain receives 90,000 litres, of which about 85% is capturable.
Does a steep roof collect more?
No. Rain falls vertically, so what matters is the plan area the roof covers, not the sloping surface.
What is a runoff coefficient?
The share actually captured after evaporation, first flush and gutter overflow — about 0.8 to 0.9 for tile or metal.
What size tank do I need?
A common rule is six weeks to two months of average supply, so the answer depends on how evenly your rain falls through the year.
Is rainwater drinkable?
Not without treatment. Roof runoff carries dust, bird droppings and material from the roof itself; most systems use it for gardens, toilets and laundry.
Where these figures come from
- UK DESNZ / DEFRA — Greenhouse gas reporting: conversion factors — the flight, fuel and waste emission factors used here
- Scarborough et al. (2014), Climatic Change — Dietary greenhouse gas emissions — the per-diet daily emission figures
- US EPA — Technical Assistance Document for the Reporting of Daily Air Quality (AQI) — the PM2.5 breakpoints the air quality calculator uses
- UK Department for Energy Security and Net Zero — conversion factors — the official UK emission factors
Last checked: September 2026. Factors are taken from published government and peer-reviewed sources, each named on the page with its year.