Glad you asked...
Calculating the GPM, or gallons per minute, is not too difficult as long as you are able to gather the necessary figures to input into your equation. For more info please click here http://www.webanswers.com/misc/formula-t...
Then again, this answer might help you...
Please understand that gpm can be either gallons per minute or graduated payment mortgage. One can be calculated by dividing total gallons by the time elapsed, the other requires an Accountant.

