Rainwater Harvesting Calculator
How much rainwater your roof can collect in a year and in the wettest month, how big a tank to plan for, and how many days of water it gives.
The flat plan area that drains to your pipes, not the sloping surface.
The share of the rain that runs off; the rest soaks in, splashes or evaporates.
Your city's yearly average from your weather service.
Water you can collect in a year
44,593 litres
- In the wettest month
- 14,864 litres
- Days of water at your use
- 89 days
- A year in gallons
- 9,809 UK gal · 11,780 US gal
A 1,000 sq ft roof with 600 mm of rain a year can collect about 44,593 litres, enough for 89 days at 500 litres a day.
How it's worked out
- Area in square metres
1,000 ÷ 10.76492.9 m² - Rule
1 mm of rain on 1 m² gives 1 litrelitres = m² × mm × runoff - In a year
92.9 × 600 × 0.844,593 L - In the wettest month
92.9 × 200 × 0.814,864 L - Days it would supply
44,593 ÷ 500 L a day89 days
- A tank of about 15,000 litres would hold the wettest month's rain; a smaller tank can still catch most of it if you use or recharge the water as it falls.
- Let the first rain of each shower run to waste (a first-flush diverter) and filter the rest. Treat it before drinking.
The formula
Litres = roof area (m²) × rainfall (mm) × runoff coefficient
The runoff coefficient is the share of rain that reaches your pipes: about 0.9 from metal sheets, 0.8 from concrete or tiled roofs, and much less from paving or lawns, where water soaks in.
Finding your rainfall
Use your city's yearly average from the national weather service. Rainfall varies a lot: a dry coastal city can get under 250 mm a year, a hill city over 1,000 mm.
Using the water safely
Fit a first-flush diverter so the dirty first rain washes away, a mesh filter at the inlet, and a covered tank. Rainwater is fine for gardens, washing and flushing; treat it before drinking. Many cities also encourage recharge wells that put the water back into the ground.
Questions people ask
How much water does 1 mm of rain give?
One litre for every square metre of roof, before losses. A 100 m² roof catches 100 litres from 1 mm of rain.
How much rainwater can a 1,000 sq ft roof collect?
With 600 mm of rain a year and a concrete roof (runoff 0.8), about 44,600 litres a year, enough for 89 days at 500 litres a day.
How much water is 1 inch of rain on 1,000 sq ft?
About 2,360 litres (623 US gallons) before losses, or about 1,890 litres from a concrete roof that sheds 80% of it.
How big should my tank be?
A tank that holds the wettest month's collection catches almost everything. A smaller tank still works if you use the water or recharge it into the ground as it falls.
About this calculator
Formula and rules last checked 9 November 2026Written and tested by the CalculatorDex teamReport a problem