The calculator will quickly give you the actual costs of all fuel on hand when you add additional fuel to the existing fuel in your tanks. It will calculate the average cost by taking the number of gallons you already have and the average price you paid for it, combined with the additional fuel you are purchasing and the price. The end result will be the actual cost of all the fuel currently in your tank(s). Gas and Diesel would be calculated separately. The calculator will also give you a proper mark-up based on a percentage of your cost and taxes where applicable.