Option 1

Click on the link with the compatible excel for your computer and enable editing on the bar at the top.  Choose meter type, type in Field Name and click on Make Field.  At the bottom of the screen is a tab with the Field Name you just entered.  Click on that and it will take you into the calculator sheet. 

  spreadsheet Calculator for Excel 2003                     default Calculator for Excel 2010

Option 2

A link to UNL's Irrigation Meter Calculator:  http://cropwatch.unl.edu/irrigationmetercalculator

Option 3

If you prefer a paper method, you can use this  pdf Printable Formula .