Collectible amount of rain (or Catchment Volume) is calculated from the collection area (in this case, surface area of roof that is to catch rainwater) and average rainwater (or precipitation).
From illustration above, surface area is ‘A’ multiplied by ‘B’. This ‘A X B’ is sometimes referred to as footprint of the collection surface.
From weather stations, one can get precipitation reports. Below is a precipitation report for Nairobi, Kenya.
The precipitation report shows average monthly rainfall over a long period of time –– the longer the period the better.
To measure precipitation for a specific period of time, rainwater for the period is collected, the collected water is poured in a container and the depth of water (in the container) is measured –– measurements of the average depths is what can be seen in the chart above, where, for example, average depth for several January(s) is 50mm, average depth for several February(s) is 40mm….
As illustrated below, ‘the Depths‘ can be renamed in terms of ‘Catchment Volumes‘ per month per unit of ‘Collection Area‘.