Estimate rainwater from roof area
One millimetre of rain over one square metre is one litre before losses.
Back to calculator ↑Use horizontal catchment area
Use the roof’s horizontal projection feeding the tank, not all sloped sheet area. Sum only the sections connected to storage.
Entered catchment should be the horizontal area of roof sections actually piped to the tank. A house may connect only one side of its roof. Measure projected plan area rather than multiplying by roof slope, since rainfall depth is defined over horizontal area.
Allow for collection losses
The capture fraction is your assumption for splash, leaks, first-flush diversion and other losses. It is not a guaranteed rate for every roof.
Changing example capture from 80% to 60% lowers potential water from 8,000 to 6,000 L, yet the 5,000 L initial empty tank space still caps the displayed storage at 5,000 L. During smaller rain periods, gutter losses or first-flush diversion can have a larger practical effect on usable water.
Worked default example
A 100 m² roof with 100 mm rain receives 10,000 L. At 80% capture, potential is 8,000 L; 5,000 L empty tank space caps storage at 5,000 L, leaving 3,000 L excess over the period.
The example 3,000 L beyond initial empty space is a mathematical difference, not an observed overflow. If water is used between separate storms, space may reopen and more water may be stored. A single intense storm, conversely, could overflow before the end of the period.
Beware period totals
This period-total formula does not model a full tank followed by use before another storm. Actual sizing should consider event or monthly rainfall and demand.
Use local monthly or storm-event rainfall and compare it with intended demand, such as garden watering. Roof material, debris screening and maintenance affect collection. Volume alone cannot establish suitability for drinking or food-contact use.
To compare 5,000 L and 10,000 L available storage, change only tank space while holding roof, rainfall and capture fraction constant. The result shows a ceiling for the assumed period, not how often a larger tank would fill. That requires time-series rainfall and household demand.
Common questions
How much is 1 mm on 1 m²?
1 L before losses.
Do I use sloped roof surface area?
Use the horizontal projected catchment feeding the tank.
Is the captured water drinkable?
Volume does not establish potable quality; check treatment and requirements separately.
Sources and calculation guidance
Check these against your chosen product’s label and instructions.