Convert fuel economy between US MPG, UK MPG, litres per 100 km and kilometers per litre. You can also calculate consumption from your own fill-up records and estimate fuel for a trip at the pump price you enter. Choose the gallon definition explicitly: the same number of US MPG and UK MPG represents different fuel consumption.
Choose a conversion or calculate actual fuel economy
- For a published economy figure, choose its unit and enter the value.
- For actual consumption, enter distance traveled and fuel used for each full-tank interval, one pair per line.
- Choose the distance and fuel units used in those records.
- Read all four economy representations from the same calculation.
- Enter a trip distance and optionally a pump price, with its price-per-litre or price-per-gallon unit.
Fuel economy calculator formulas
MPG = miles traveled ÷ gallons used. L/100 km = litres used ÷ kilometers traveled × 100. The first is distance per fuel volume; the second is fuel volume per distance. Higher MPG means less fuel used for the same distance, while lower L/100 km means less fuel used.
The conversions use NIST’s unit definitions: 1 mile = 1.609344 km and 1 US liquid gallon = 3.785411784 L. The UK imperial gallon is 4.54609 L. NIST SP 811, Appendix B.9 also lists the reciprocal US MPG/L-per-100-km relationship. The tool derives its factor from the units rather than rounding that factor before every calculation.
L/100 km = 100 × litres per gallon ÷ (MPG × 1.609344). For kilometers per litre, L/100 km = 100 ÷ km/L. Use the reciprocal formula to convert back.
US and UK MPG conversion chart
| US MPG | L/100 km | UK MPG | km/L |
|---|---|---|---|
| 20 | 11.76 | 24.02 | 8.50 |
| 25 | 9.41 | 30.02 | 10.63 |
| 30 | 7.84 | 36.03 | 12.75 |
| 35 | 6.72 | 42.03 | 14.88 |
| 40 | 5.88 | 48.04 | 17.01 |
| 50 | 4.70 | 60.05 | 21.26 |
These are unit conversions for the stated economy, not predictions for a specific vehicle. A vehicle’s actual consumption changes with the conditions and the measurement interval.
Calculate MPG from fill-up records
Start with a full tank and record the odometer. Drive, refill to a comparable full level, then use the distance since the previous fill and the fuel required at this refill. The first fill establishes the baseline; its purchased fuel should not be included as fuel used for an interval with no matching distance.
For 300 miles traveled and 10 US gallons used, MPG is 300 ÷ 10 = 30 US MPG, equivalent to approximately 7.840 L/100 km. Partial fills can distort a single-interval calculation unless several fills are combined between comparable full-tank endpoints.
Average MPG across multiple tanks
Add distance and fuel first. If one interval is 100 miles on 5 gallons and another is 300 miles on 10 gallons, the combined result is 400 ÷ 15 = 26.667 MPG. Averaging the separate values, 20 and 30, gives 25 MPG and weights unequal intervals incorrectly.
The refill mode uses total distance and total fuel from the entered rows. Keep the measurement units consistent and avoid including overlapping intervals. If one row records an odometer reading rather than distance traveled, subtract the previous reading before entering it.
Trip fuel and cost
Litres required = trip kilometers × L/100 km ÷ 100. Fuel cost = required fuel × price per matching fuel unit. For an illustrative 300-mile trip at 30 US MPG and $4 per US gallon, the estimate is 10 gallons and $40. The price is a hypothetical input, not a current market quote.
The price unit is independent of your fill-up records. A US-gallon price cannot be used as a UK-gallon price without conversion. If the price field is blank, the tool still shows fuel required but does not show a cost estimate. FuelEconomy.gov’s cost-per-100-miles formula follows the same distance/economy relationship.
Compare fuel savings carefully
Fuel cost depends on the reciprocal of MPG. A change in MPG is not a linear change in fuel saved across every starting economy. Compare fuel needed for the same trip distance, then compare the costs. A result based on your measured refill intervals is useful for planning, but it is not a guarantee for a future drive.
For gallon weight rather than consumption, use the weight per gallon calculator. For mileage recorded as steps, the steps-to-miles calculator is a separate walking-distance estimate.
Frequently asked questions
Are US MPG and UK MPG the same?
No. A UK gallon contains more litres than a US gallon. Choose the gallon definition used by the source.
How do I convert MPG to L/100 km?
Multiply 100 by litres per gallon, then divide by MPG multiplied by 1.609344 km per mile. The result is reciprocal, so higher MPG produces lower L/100 km.
Should I average MPG from several fill-ups?
Add the distances and fuel quantities first, then divide total distance by total fuel. A simple average of separate MPG values can be wrong.
Why does a partial fill give an unexpected result?
Fuel purchased at a partial fill may not equal fuel consumed since the previous reading. Use comparable full-tank endpoints or combine intervals appropriately.
Does the calculator use live fuel prices?
No. Enter the pump price and its unit. A blank price leaves the cost estimate hidden.
Can I enter a trip in kilometers and a price per gallon?
Yes. Trip distance and price-unit choices are independent, and the calculation converts them to consistent units.
Sources & method
- NIST SP 811, Appendix B.9 — MPG/L per 100 km and UK gallon
- NIST Handbook 44 (2026), Appendix C — exact US gallon and mile units
- U.S. DOE/EPA FuelEconomy.gov — fuel cost per 100 miles
Results are estimates for general information. Found an error? It helps everyone — see our methodology.