GENERAL RULE OF THUMB: You can collect 600 gallons of water per inch of rain falling on 1,000 square feet of catchment surface (e.g. your roof).

OR,

27,000 gallons of water per inch of rain falling on 1 acre of catchment surface.

Example:
Harvestable water (gallons) = catchment area (ft2) X rainfall (ft) X 7.48 gallons/ft3

How to calculate the catchment area (of the footprint) of you roof.

Is your roof square or rectangular? If so, measure the length and width in feet. Length X Width = Sq. Ft. of roof catchment.

In Marin, our annual average rainfall is 48 inches, or 4-ft. The actual rainfall will vary from location to location. You can research your town's rainfall online.