To get an approximate calculation of how much rainwater can be harvested annually from a building, all you need to do is measure the length and width of the building to find the area in square metres, then multiply this by the annual local rainfall in cubic metres. You can divide it by 12 to find the monthly rainwater harvesting potential Use can also use our simple rainwater harvesting calculator which will also suggest the ideal tank size to maximise how much rainwater you can collect Example Calculation: Length 5m x Width 4m = 20m2 20 x Average annual rainfall 0.9 (900m/annum) = 18m cubed of water per year 1 cubic metre of water = 1000L Hence, 18 cubic metres of water = 18000L of harvested rainwater Rainfall varies depending on your location, which can be checked by county throughout the UK.