How UK MPG is calculated
UK miles per gallon = miles driven × 4.54609 ÷ litres used, because one imperial gallon is 4.54609 litres. In odometer mode, miles driven is the ending reading minus the starting reading. The calculator requires a positive distance and positive fuel volume; an ending odometer at or below the starting reading is rejected.
A US liquid gallon is 3.785411784 litres, so the same journey has a smaller numerical US MPG value. These gallon definitions are shown separately. Litres per 100 km = litres used ÷ kilometres driven × 100, with 1 mile = 1.609344 km. Higher MPG and km/L mean greater distance per fuel volume; lower L/100 km means less fuel used for a fixed distance.
Worked UK fill-up example
A 300-mile journey using 30 litres gives 45.46 UK MPG, 37.85 US MPG and 6.21 L/100 km after rounding. If the pump price was a hypothetical £1.50 per litre, the fuel cost would be £45.00, or £0.15 per mile. The example price is not a current market price; enter what you actually paid.
Recording a useful measurement
Use fuel added between two comparable full-tank fill-ups, with the tank returned to the same level. Fuel bought is only a proxy for fuel consumed when the starting and ending tank levels match. A short distance or an inconsistent fill level can distort the result. The calculator measures the entered journey; it does not predict future economy or correct for traffic, weather, load or tyre changes.
For several tanks, enter the total miles and total fuel used over the whole interval. Do not take a simple average of individual MPG figures: combine the underlying distance and fuel quantities first. The optional cost estimate covers fuel only, excluding parking, tolls, depreciation and other motoring costs.
Use the distance unit buttons for kilometre records and the fuel unit button for imperial or US liquid gallons. The distance converter rounds to 0.1 mile, while fuel conversions retain six decimal places. Keep the original measurements when greater precision is needed. US dry gallons are not supported.
To plan a journey from an economy estimate and pump price, use the UK fuel cost calculator.
For a headline result and pump-price entry in US conventions, open the US MPG calculator.
Inputs for the UK MPG calculator
Distance entry
Choose a trip distance or the starting and ending odometer readings for the same fill-up interval.
Distance and odometer readings
The base unit is miles. The inline controls also accept kilometres. The ending odometer must be greater than the starting reading.
Fuel used
Enter litres added to restore the original tank level. The converter also accepts clearly labelled imperial gallons or US liquid gallons.
Estimate fuel cost
Choose whether to include the pump price you paid. Enter pounds per litre: £1.50 means 150p per litre. The page does not retrieve live prices.
CALCULATOR QUESTIONS
UK MPG calculator questions
Is UK MPG the same as US MPG?
They use the same international mile but different gallons. The imperial gallon is 4.54609 litres; the US liquid gallon is 3.785411784 litres. Always identify the gallon definition before comparing figures.
Can I use kilometres instead of miles?
Yes. Open the unit button beside the distance or odometer field and enter kilometres. The converted field remains in miles and is rounded to one decimal place.
How do I average several fill-ups?
Add the distances and fuel volumes for the full interval, then calculate MPG from those totals. A simple average of the individual MPG figures can give the wrong combined economy.
Does the fuel cost include other running costs?
It includes only the fuel volume multiplied by your entered pump price. It excludes parking, tolls, maintenance, insurance and depreciation.