The Tank Volume Calculator is a Microsoft Excel spreadsheet that is used (1) to calculate the amount of water in one or more tanks given the water level of the tank(s), (2) to estimate the amount of rainfall needed to fill the tank(s), (3) to estimate the amount of water collected from a given amount of rainfall, and (4) to estimate the number of days calculated volumes will supply household water, given estimated daily use.

The spreadsheet is divided into four panels. The first simply shows the values of two constants: gallons per cubic foot (7.481), and the volume of water per inch of rain that falls on a square foot of roof (7.481/12 = 0.623):

The second panel allows the user to set a series of variables related to the storage tank and roof characteristics of a specific rainwater collection site, as well as the volume of daily water use. Note that values can be entered only for those variables that are bolded; all other (unbolded) variables are locked:

The third panel gives the relationship between water level and volume for the tank dimensions entered in the Site variables panel. Also given are (1) an estimate of the number of days that the calculated volume will last, given the daily water use; and (2) the amount of rainfall needed to fill the tank(s) to capacity for the given site variables:

The last panel gives estimates of the volume of water collected and stored per rainfall amount, as well as an estimate of the number of days of water use resulting from the amount of water saved from this rainfall:

Note that it is assumed that multiple tanks are all the same size and are sited on the same level; if they are not, analyze each tank separately.

PLEASE NOTE: I have done my best to check the accuracy of the calculations in this spreadsheet, but I must assert the usual disclaimer that it is distributed without warranty, and that you use it at your own risk.

Download the Tank Volume Calculator (MS-Excel file)

Updated 20 June 2011