How much gas will I use on my trip?

Divide the trip's miles by your vehicle's miles per gallon. The answer is the gallons you will burn. Then check how far a tank gets you while keeping a reserve (tank size minus reserve, times mpg) to see how many fill-ups to plan. The rest of this page shows where to get each number and works through three trips.

Step one: get the distance

Put your start and end points into any map app and note the miles for the route it suggests. If you will stop at several places, add the legs together, and double the total if you are driving home the same way. Side trips to a restaurant or trailhead count too, so round up a little rather than down. The trip gas calculator takes several legs at once if you would rather not add them yourself.

Step two: find your mpg

You have two options, and the first is better.

1. Your own record. Fill the tank, reset the trip meter, drive normally, then fill up again and note the gallons added. Miles divided by gallons is your real mpg for how you actually drive. The fuel consumption calculator does that division and also tells you how many gallons a given distance needs.
2. The EPA rating. If the car is new to you, a rental, or a loan, look it up by year, make and model on fueleconomy.gov. Use the combined figure, which the EPA describes as a weighted average of city and highway values, weighting city by 55% and highway by 45%. The combined figure is a blend, so check the city and highway numbers too. Steady driving at moderate speeds can beat the blend, but fast interstate driving often won't, and hybrids can do better in town than on the highway.

If you are borrowing a vehicle and cannot find a rating, ask the owner what they usually get, or use the rental company's figure. Whatever you use, write it down as an assumption, because the next section is about why it can be wrong.

Step three: gallons = miles ÷ mpg

That is the whole formula. A car that does more miles per gallon needs fewer gallons for the same road. To turn gallons into dollars, multiply by the price per gallon at the stations you expect to use, which the fuel cost calculator does for you.

Why your real number may differ

The rating is a test result, not a promise. The EPA itself says your fuel economy and emissions may be different due to a number of factors. The ones that matter most on a trip:

• Speed. According to the government's fuel economy driving tips, "gas mileage usually decreases rapidly at speeds above 50 mph." Open highway at the limit costs more fuel than the same distance at a slower pace.
• Load and roof cargo. A full car works harder than an empty one, and a roof box or bike rack adds wind resistance as well as weight.
• Terrain and weather. Mountain climbs, headwinds, cold mornings and air conditioning on a hot afternoon all push consumption up.
• Stop-and-go traffic and idling. City-style driving burns more per mile than steady cruising for most cars.

If your trip includes several of these, run the numbers at the rating and again at a lower, pessimistic mpg, and plan for the worse result. Then adjust after your first fill-up, since your own record from a similar loaded trip is the best guide.

Add a reserve, then plan fill-ups

Pick a reserve in gallons, enough that a missed exit or a closed station will not strand you. How much is a personal choice and a planning habit, not a sourced figure. Use a bigger one on sparse stretches of road where stations are far apart, and do not count on the low-fuel light or the gauge as your reserve, because neither tells you exactly what is left. The examples below keep roughly 15% to 20% of the tank in reserve. Your tank size is in the owner's manual or on the manufacturer's spec page; a rental desk can tell you too.

Then work out how far one tank takes you:

usable range = (tank size − reserve) × mpg

Use the same conservative mpg you planned the gallons with, not the rating, so the stop count matches the fuel you expect to burn.

Divide the trip miles by that range and round up. Subtract one, because you leave with a full tank. The result is the number of fill-ups to plan along the way. Treat it as a guide to how often to look for stations, not a schedule; filling up earlier at a good price or a convenient stop is fine.

Three worked examples

These use made-up vehicles and distances to show the method. Swap in your own numbers.

Check the result and share the cost

Put your miles and mpg into the trip gas calculator to confirm the gallons and see tank fill-ups. If you want a second route check, the Department of Energy's trip calculator on fueleconomy.gov lets you "edit vehicles to personalize the MPG and fuel prices." Riding with friends? The gas money calculator splits the fuel bill between riders.