The price of gas is normally given in dollars per gallon. It is possible to calculate your average gas price for a trip by using a basic algebraic formula where gas price equals dollars paid divided by gallons of gas used. To simplify the calculation you can use your car's average miles per gallon calculation to find the number of gallons used based on your miles driven. This is a good way to help plan and estimate your cost of travel.

Determine the total number of miles driven and your car's average miles per gallon. We'll use the example of a person who drove 500 miles in a car that averages 40 miles to the gallon.

Divide the total number of miles driven by your car's average miles per gallon. In the example, 500 miles divided by 40 miles to the gallon equals 12.5 gallons of gas. This is an estimate of the gallons of gas the person used.

Divide the total cost of the gas you bought by the number of gallons you drove. In the example, let's say the person paid $37.50 for gas. Therefore, $37.50 divided by 12.5 gallons equals an average gas price of $3 per gallon.