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Fuel Efficiency (MPG) Calculator

Use this fuel efficiency calculator to estimate your vehicle’s real-world miles per gallon (mpg) by entering the distance you drive and the amount of fuel you add at the pump.

Beyond a simple mpg number, the tool converts your result to liters per 100 kilometers (L/100 km) and kilometers per liter (km/L), estimates your trip fuel cost, and projects how much fuel and money you are likely to spend over a typical driving year.

You can also enter the expected fuel efficiency of an alternative vehicle to see how many gallons, how much money, and how many kilograms of CO2 you could save each year if you switch, assuming your chosen annual distance (for example 15,000 km per year).

Updated Dec 1, 2025

Results

Fuel efficiency

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Fuel consumption

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Fuel efficiency (metric)

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Estimated fuel cost

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Annual fuel use with current vehicle

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Annual fuel cost with current vehicle

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Annual fuel use with alternative vehicle

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Annual fuel cost with alternative vehicle

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Estimated annual fuel cost savings when switching vehicles

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Estimated CO2 emissions for this trip

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Annual CO2 emissions with current vehicle

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Annual CO2 emissions with alternative vehicle

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Methodology

Fuel economy in automobiles is fundamentally a ratio between distance traveled and fuel consumed. When expressed in miles per gallon (mpg), the calculation is distance in miles divided by fuel volume in U.S. gallons. When expressed as fuel consumption, such as liters per 100 kilometers (L/100 km), the relationship is inverted: fuel volume divided by distance, multiplied by 100 to normalize to a 100 km distance.

This calculator assumes distance is measured in statute miles and fuel volume in U.S. gallons. To provide metric outputs, miles are converted to kilometers using the exact definition 1 mile = 1.609344 kilometers, and U.S. gallons are converted to liters using 1 U.S. gallon ≈ 3.785411784 liters, consistent with standard engineering and metrology tables. The tool then computes km/L as distance in kilometers divided by fuel in liters, and L/100 km as fuel in liters divided by distance in kilometers, multiplied by 100.

For the most reliable real-world fuel efficiency estimate, use full-tank measurements: fill the tank, reset your trip odometer, drive normally until the next fill, then record the miles traveled and the gallons needed to refill the tank. Repeating this process over multiple tanks and averaging the results helps smooth out day-to-day variation due to traffic, weather, driving style, and fuel formulation.

Regulatory fuel economy labels, such as those produced under U.S. Environmental Protection Agency (EPA) dynamometer test procedures and published on fueleconomy.gov, are based on standardized laboratory drive cycles. These controlled tests are essential for comparing vehicles, but real-world fuel economy often differs from label values as a result of driving patterns, vehicle loading, road grade, temperature, and accessory use. Using your own odometer and fuel pump data is a practical way to understand how your vehicle performs in your typical conditions.

To translate a single tank or test drive into an annual picture, the calculator scales your measured efficiency to the annual distance you enter. It converts your annual distance from kilometers to miles, divides by your measured mpg to estimate gallons per year for your current vehicle, and multiplies by the fuel price per gallon to estimate annual fuel cost.

If you provide the expected fuel efficiency of an alternative vehicle in mpg, the same steps are applied to that value so you can compare annual fuel use, annual fuel cost, and the estimated annual savings from switching. This "if I change car" view is particularly helpful when you are evaluating whether a more efficient vehicle’s purchase price is justified by fuel savings.

For CO2 emissions, the calculator uses the fuel volume and a user-specified tailpipe emission factor in kilograms of CO2 per liter of fuel. Typical values are about 2.3 kg CO2 per liter of gasoline and about 2.6–2.7 kg CO2 per liter of diesel, as reported by national energy and environment agencies. Multiplying fuel volume in liters by this factor gives an estimate of the CO2 emitted on the trip and, when scaled by annual distance, over a typical year.

Further resources

Expert Q&A

How do I calculate miles per gallon (mpg) with this tool?

To calculate mpg, record the distance you drive in miles and the fuel you add in U.S. gallons. Enter those values in the calculator and it will divide distance by fuel volume to produce your fuel efficiency in miles per gallon (mpg), alongside equivalent values in km/L and L/100 km.

What is the difference between U.S. mpg and U.K. or Imperial mpg?

U.S. mpg uses the U.S. gallon (about 3.785 liters), while U.K. or Imperial mpg uses the larger Imperial gallon (about 4.546 liters). Because the Imperial gallon is larger, the same vehicle will show a higher numeric mpg value when expressed in Imperial mpg than in U.S. mpg. This calculator uses U.S. gallons; if you are using data based on Imperial gallons, convert to liters or U.S. gallons before entering values.

Why does my real-world mpg differ from the value on the window sticker or from fueleconomy.gov?

Official fuel economy ratings are determined from standardized laboratory tests on a chassis dynamometer, using prescribed drive cycles that simulate city and highway driving. These controlled conditions rarely match real-world driving exactly. Short trips, high speeds, aggressive acceleration, heavy loads, stop-and-go traffic, extreme temperatures, and underinflated tires can all reduce real-world fuel economy compared with label values.

How can I measure my fuel efficiency accurately over time?

For better accuracy, base your calculation on multiple full-tank intervals. Each time you fill up, note the odometer or trip reading and the gallons added. Compute mpg as miles driven divided by gallons used for each interval, and then average several intervals. This helps reduce the impact of pump shutoff variation, unusual trips, or atypical driving conditions on any single tank.

What do liters per 100 kilometers (L/100 km) and kilometers per liter (km/L) tell me?

L/100 km is a fuel consumption metric that indicates how many liters of fuel are needed to drive 100 kilometers; lower numbers mean better efficiency. km/L is the inverse ratio, similar to mpg but in metric units; higher numbers mean better efficiency. This calculator converts your inputs from miles and U.S. gallons into both L/100 km and km/L so you can compare your vehicle’s performance to international ratings and technical reports.

Can this calculator estimate my fuel cost for a trip?

Yes. If you also enter a fuel price per U.S. gallon, the calculator multiplies the gallons used by the price to estimate your fuel cost for the measured distance. For trip planning, you can rearrange the relationship by combining your expected mpg, planned distance, and local fuel price to estimate total fuel expenses.

The calculator shows that my consumption is much higher than expected. What can I do?

If your real-world fuel use is significantly above the value you see on official labels or manufacturer brochures, start by checking basic maintenance and driving habits. Ensure tire pressures match the values on the door placard, remove unnecessary roof racks or heavy cargo, avoid extended idling and very aggressive acceleration, and verify that air and fuel filters are serviced according to schedule. If consumption remains high, a trusted mechanic can check for dragging brakes, misfires, or other faults that increase fuel use.

How should I interpret the CO2 emissions numbers from this calculator?

The CO2 values are engineering-style estimates based on fuel burned and a typical tailpipe emission factor per liter of fuel. They are not a regulatory inventory, but they give a useful order-of-magnitude view of the climate impact of a single trip or a full year of driving. Comparing the current vehicle and an alternative vehicle helps you see how much CO2 could be avoided each year if you switch to a more efficient model or reduce your annual driving distance.

Sources & citations