How to Calculate Gas Cost for a Road Trip

Published October 11, 2026

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Fuel is often the biggest cost of a road trip, and the easiest one to estimate in advance. All you need is the distance, your car's fuel economy and the price of gas. This guide shows the formula, how to split the cost fairly, how to handle round trips, commutes and metric units, and how much a more efficient car would save, using the free Gas Cost Calculator.

The gas cost formula

• Gallons used = miles ÷ miles per gallon (mpg)
• Gas cost = gallons used × price per gallon

Say you're driving 300 miles each way, so 600 miles in total, in a car that gets 28 mpg, with gas at $3.50 a gallon:

• Gallons used: 600 ÷ 28 = 21.4 gallons
• Gas cost: 21.4 × $3.50 = $75.00

That works out to about 12.5 cents a mile.

Finding your car's fuel economy

• The official rating is listed in your car's manual and, for US cars, on the EPA's site fueleconomy.gov. Use the highway figure for road trips and the city figure for stop-and-go driving.
• Your real-world figure is more accurate. Fill the tank, reset the trip meter, drive normally, then fill up again: divide the miles driven by the gallons it took to refill.

Real-world economy is usually somewhat lower than the official rating, especially at high speeds, with a heavily loaded car, a roof box or the air conditioning running hard. For a cautious estimate, knock a few mpg off.

How price changes the total

Gas prices vary by region and season. For the same 600-mile trip at 28 mpg:

Each 50-cent change in price moves the cost of this trip by about $10.71.

Splitting the cost with passengers

Divide the total by the number of people sharing it. The $75 trip costs $37.50 each for two or $25 each for three. Agree before you set off whether tolls and parking are shared too, and some groups add a little for wear and tear on the driver's car.

Round trips and regular drives

For a round trip, double the one-way distance before you calculate. For regular drives, multiply the cost of one trip by how often you make it.

A 20-mile commute each way at 28 mpg and $3.50 a gallon costs $5.00 a day. Over 21 workdays a month that's $105 a month, or $1,260 a year.

Kilometres and litres

Outside the US, fuel economy is usually quoted in litres per 100 kilometres (L/100 km), where a lower number means a more efficient car:

Litres used = kilometres × (L/100 km) ÷ 100

A 500 km trip in a car using 6.5 L/100 km takes 500 × 6.5 ÷ 100 = 32.5 litres. At €1.80 a litre, that's €58.50.

To convert between the two systems:

• L/100 km = 235.2 ÷ US mpg, so 28 mpg is about 8.4 L/100 km.
• US mpg = 235.2 ÷ L/100 km, so 6.5 L/100 km is about 36 mpg.
• UK mpg uses the larger imperial gallon: L/100 km = 282.5 ÷ UK mpg.

How much would a more efficient car save?

On the 600-mile trip, a car that gets 40 mpg instead of 28 uses 15 gallons, costing $52.50. That's $22.50 less per trip, or $270 a year if you make the trip once a month. Savings like this add up quickly on a daily commute, which is worth knowing when you compare cars; the Car Affordability Calculator covers the rest of the cost of owning one.

How to use the Gas Cost Calculator

1. Open the Gas Cost Calculator.
2. Choose the units: Miles and mpg, or Kilometres and L/100 km.
3. Enter the one-way distance and keep Round trip ticked if you're coming back.
4. Enter your fuel efficiency and the fuel price, and choose your currency ($, €, £ or ₹).
5. Enter the number of people sharing the cost, and how many times a month you make the trip if you want the monthly and yearly cost.
6. Optionally, enter another car's fuel efficiency to compare.
7. Read the results: the trip's fuel cost, the fuel used, the cost per person and per mile (or kilometre), the yearly cost and, if you entered one, how much the other car would save. A chart and a table show the trip's cost at higher and lower fuel prices.

Tips for spending less on gas

• Drive at a steady speed. Hard acceleration and high speeds burn noticeably more fuel.
• Check your tire pressure. Underinflated tires increase rolling resistance.
• Travel light. Remove roof boxes and racks when you don't need them.
• Compare prices before you fill up. Gas near highways and in city centers often costs more than a few minutes away.
• Avoid long idling, and plan routes that skip heavy traffic.

Frequently asked questions

How do I calculate how much gas a trip will take?

Divide the total distance by your car's miles per gallon. A 600-mile trip at 28 mpg takes about 21.4 gallons. Multiply by the price per gallon for the cost.

How much gas money should I give the driver?

Work out the total fuel cost and divide it by the number of people in the car, including the driver. For the 600-mile trip at $3.50 a gallon, each of three people would chip in $25.

How do I convert mpg to L/100 km?

Divide 235.2 by the US mpg figure: 28 mpg is about 8.4 L/100 km. For UK mpg, divide 282.5 by the mpg instead.