Rainwater Catchment

We developed a simple Rainwater Catchment Calculator to estimate the supply of rainwater off of structures, water use, and tank size optimization. The worksheet allows the user to calculate the amount of rainfall that can potentially be collected from an impervious surface such as a roof or asphault parking lot.

The worksheet can also help calculate the sizing of a tank. Insert the tank size in gallons and in the Rainfall Collected table insert estimated water used per month


  1. Click here to access the Rainwater Catchment Calculator.
  2. Click on the above link to use the spreadsheet.  Within the spreadsheet there are two worksheets.  The first one is not editable and the other is editable for use.
  3. Plug in the area of the impervious surface in the green cells to determine how much rainfall could be collected each year.  Green cells can be edited, organge cells should not be edited or done so carefully so as not to mess up the calculation.

General Calculation

  1. Take the dimensions of the footprint of your roof and convert them to inches. (So, a 50′ x 20′ roof is 600″ x 240″.)
  2. Multiply the roof dimensions by the number of inches of rainfall.
  3. Divide by 231 to get the number of gallons (1 gallon = ~231 cubic inches).