Fuel Economy and Trip Cost Calculator

What this calculator answers

How much the gas or fuel for a trip will cost, how much fuel it will use, and what each person pays when you split it. Enter the distance, your car’s fuel economy and the price you pay. The calculator also gives the cost per mile and per kilometer, and your fuel economy in every common unit. It accepts US and UK (imperial) mpg, L/100 km and km/L, and prices per US gallon, imperial gallon or liter, so it works for trips in the US, the UK, Canada and anywhere prices are per liter.

How to use it

• Distance and its unit: the one-way distance in miles or kilometers, from a map or route planner.
• Trip: choose Round trip to double the distance for the drive back.
• Fuel economy and its unit: the car’s trip computer average, or its official rating. Under the box the calculator shows the same figure in other units, so a number typed in the wrong unit stands out.
• Fuel price and Price unit: what you pay at the pump, and whether that is per US gallon, per imperial gallon or per liter. Use your local price; the example price is only an example.
• Currency: changes the symbol only. No exchange rate is applied.
• People sharing the cost: everyone splitting the bill, including the driver.
• Other costs (optional): tolls, parking or anything else the group shares, as one total for the whole trip. It is added to the fuel and split the same way.

Changing a unit converts the number already in that box, so the answer stays the same until you type your own figure. Choose the unit first, then type. When you switch the fuel economy to L/100 km or km/L, the distance switches to kilometers and the price to per liter. Switching to UK mpg uses miles and turns a price per US gallon into a price per liter, and switching to US mpg brings back miles and US gallons.

The Try buttons load the cases worked through on this page: a metric trip, a UK trip, a shared trip with tolls and a month of driving. To compare two cars or two prices, press Save for comparison, change the fuel economy or price and save again: each saved scenario shows how much more or less it costs than the first.

How to calculate the fuel cost of a trip

Work out how much fuel the trip uses, then multiply by the price. With fuel economy in miles per gallon or kilometers per liter, divide the distance by the economy. With L/100 km, multiply the distance by the consumption and divide by 100.

• $d$ is the distance driven: one way, or twice that for a round trip.
• $\text{mpg}$ is miles per gallon, so $d$ must be in miles and the fuel comes out in the same kind of gallon (US or imperial). With km/L, $d$ is in kilometers and the fuel in liters.
• $p$ is the price per gallon or per liter. The fuel must be in the same unit as the price before you multiply.

Each person’s share is the cost, plus any other shared costs such as tolls and parking, divided by the number of people.

Worked example: 250 miles each way at 30 mpg

Two friends drive 250 miles each way in a car that averages 30 mpg (US), with gas at $3.50 per US gallon.

1. Distance driven: 250 mi × 2 = 500 mi.
2. Fuel used: 500 mi ÷ 30 mpg = 16.667 US gallons, which is 63.09 liters or 13.88 imperial gallons.
3. Fuel cost: 16.667 gal × $3.50 = $58.33.
4. Each person: $58.33 ÷ 2 = $29.17. To the cent, one pays $29.17 and the other $29.16, so the two shares add up to $58.33.
5. Cost per mile: $58.33 ÷ 500 mi = $0.117 (about 11.7 cents), or $11.67 per 100 miles. Per kilometer it is $0.072.

In other units, 30 mpg (US) is 36.03 mpg (UK), 7.84 L/100 km, 12.75 km/L and 3.33 US gallons per 100 miles.

Cost per mile and per kilometer

Divide the price by the fuel economy: $3.50 per gallon ÷ 30 mpg is $0.117 per mile. With L/100 km, multiply the price per liter by the consumption and divide by 100: at 6.5 L/100 km and €1.80 per liter, a kilometer costs 1.80 × 6.5 ÷ 100 = €0.117. The cost per distance does not depend on the trip length, so it also answers “how much will a month of driving cost”. The Metric: 400 km button prices a 400 km drive this way: 26 liters for €46.80. The 1,000 mi a month button shows 1,000 miles at 30 mpg and $3.50: 33.33 gallons for $116.67.

Converting mpg to L/100 km and km/L

L/100 km is 235.215 divided by US mpg, or 282.481 divided by UK mpg. The two run in opposite directions: higher mpg means less fuel, while higher L/100 km means more. km/L is US mpg × 0.425144, and 100 divided by L/100 km.

For your own car, the table under the result converts the figure you enter into all of these units, plus US gallons per 100 miles.

US vs UK (imperial) mpg and gallons

UK mpg is not the same as US mpg. A US gallon is 3.785411784 liters and an imperial gallon 4.54609 liters, about 20% more, so the same car shows about 20% more miles per gallon in UK figures: 30 mpg (UK) is about 25 mpg (US). A price per liter avoids the question altogether, because the calculator turns the fuel into liters before pricing it. The UK mpg: 180 mi button pairs UK mpg with a price per liter: 180 miles at 45 mpg is 4 imperial gallons, or 18.18 liters, which at £1.45 per liter is £26.37.

The calculator converts whatever units you choose. If your fuel economy is in UK mpg and your price per US gallon, or the other way round, it says so above the result, so you can check that the two figures really use different gallons.

Splitting gas money for a road trip

Divide the fuel cost, plus any tolls and parking, by everyone in the car, including the driver. Shares rarely come out to whole cents, so the calculator also lists cent-exact shares that add up to the total. With the 4 people + tolls button, the example trip costs $58.33 of fuel plus $24.00 of tolls and parking entered as other costs, or $82.33, which is $20.58 each: to the cent, one person pays $20.59 and three pay $20.58. The split covers only what you enter. Wear on the car, insurance and depreciation are not included; if the group wants to cover some of the driver’s costs too, agree an amount and add it to the other costs.

Why real fuel use differs from the rating

Official ratings come from standardized tests that make cars easy to compare, but they may not match the mpg you get. The fueleconomy.gov guidance lists the main reasons:

• Aggressive driving (speeding, hard acceleration and braking) can cut mpg by roughly 15% to 30% on the highway and 10% to 40% in stop-and-go traffic.
• Higher speeds, roof boxes, towing and heavy loads add drag or weight.
• Running the air conditioner on “Max” can reduce mpg by roughly 5% to 25%.
• Cold weather, short trips, hills, unpaved roads and four-wheel drive all use more fuel.

The Fuel cost at other fuel economy table under the result shows what a lower real-world figure does to your cost. For the worked example, 25 mpg costs $70.00 and 20 mpg $87.50. The table also shows why mpg can mislead. Going from 25 to 30 mpg saves $11.67 on this trip, but going from 35 to 40 mpg saves only $6.25. fueleconomy.gov calls this the “MPG illusion”: over 1,000 miles, 10 to 15 mpg saves about 33 gallons, but 30 to 35 mpg only about 5. Consumption units such as L/100 km and gallons per 100 miles don’t have this problem, because fuel use grows in proportion to them.

Measuring your own fuel economy

Fill the tank and zero the trip meter. At the next fill-up, divide the distance driven by the fuel it took to fill the tank again. For example, 312 miles on 10.4 US gallons is 30 mpg. In metric, divide the liters by the kilometers and multiply by 100: 40.6 liters over 520 km is 7.81 L/100 km. An average over several tanks is steadier than one fill-up.

Reading the result

• Fuel cost (or Trip cost when you add other costs) is the total for the distance you entered, both ways for a round trip.
• Each person pays is the equal share, with the cent-exact split when the shares don’t come out even. With one person, this tile shows the distance driven in miles and kilometers instead.
• Fuel used is in the unit your price is per, with the other two units under it.
• Fuel cost per mile (or per km) is the fuel cost of each unit of distance, with the other unit and the cost per 100 under it.
• Your fuel economy in other units converts your figure into US and UK mpg, L/100 km, km/L and US gallons per 100 miles.
• Fuel cost at other fuel prices repeats the trip at prices a step or two either side of yours. In the worked example each $0.25 per gallon adds $4.17, so $3.00 costs $50.00 and $4.00 costs $66.67.
• Fuel cost at other fuel economy does the same for the fuel economy, as described above.

Assumptions and limitations

• One fuel economy figure applies to the whole trip. City, highway, hills and weather change it along the way.
• A round trip is the same distance both ways at the same economy.
• All the fuel is bought at one price. Prices vary between stations and over time; enter the price you expect to pay.
• It covers fuel, plus any other costs you enter. Maintenance, tires, insurance and depreciation are not included, so the fuel cost per mile is lower than the full cost of running a car.
• The currency setting only changes the symbol. Enter the price in the currency you want the result in.
• The result is an estimate for planning, not a quote.

Common mistakes

• Mixing US and UK gallons. A UK mpg figure used as US mpg understates the fuel by about 17%, and a price per imperial gallon used as per US gallon overstates the cost by about 20%. Check the units against where the numbers came from.
• Swapping L/100 km and km/L. 8 L/100 km is 12.5 km/L. Typed as 8 km/L, the fuel comes out more than half again too high. The line under the fuel economy box shows the figure in mpg, which makes the slip easy to spot.
• Forgetting the drive back. If the distance you entered covers the outward trip only, choose Round trip to count the return.
• Using the official rating for a hard trip. A loaded car at highway speed with the air conditioning on often uses more fuel than its rating. The “other fuel economy” table shows the cost at lower figures.
• Leaving the example price in. The price of $3.50 is only an example. The calculator reminds you if you change other inputs and leave it.