How to Calculate Gallons of Gas Used

Refer to your car's fuel economy, expressed as mpg. This information should be listed in the specifications section of your owner's manual. Two figures are listed: city and highway mileage. For estimation purposes, you an average the two by adding them together and dividing by two.

Divide that figure into one to calculate the number of gallons used per mile. As an example, if you get 20 mpg, then divide 20 into one. This results in 0.05 gallon per mile. This means that for every mile you drive, you are consuming 0.05 gallon of gas.

Look at your odometer and write down your car's total mileage.

Multiply your gallons per mile by your total mileage to compute the number of gallons consumed over the life of your vehicle.To continue the example, if your vehicle has 60,000 miles on it, multiply that figure by 0.05, which tells you your car's total fuel consumption is 3,000 gallons.

Divide this number by the age of your car to calculate average gallons used per year. To continue the example, if your car is 5.5 years old, you would divide that into 3,000, which tells you your car's average fuel consumption is 545.5 gallons per year.