Work out rainwater barrel sizing instantly with clear inputs, formula shown and shareable results.
One millimetre of rain on one square metre is one litre, so collection is roof area times rainfall in millimetres times the collection efficiency. The litres-per-millimetre figure is the useful planning number: a 40 square metre roof yields 34 litres from every millimetre, so a single shower fills a small barrel.
Water collected
litres = roof area m2 x rainfall mm x collection efficiency
litres = roof area m2 x rainfall mm x collection efficiency. One millimetre of rain on one square metre is one litre, so collection is roof area times rainfall in millimetres times the collection efficiency.
The litres-per-millimetre figure is the useful planning number: a 40 square metre roof yields 34 litres from every millimetre, so a single shower fills a small barrel.
Enter roof catchment area, rainfall in the period, collection efficiency, barrel size you have. The defaults shown are a realistic worked example — swap in your own figures to get a result you can use.