About this calculator
The MPG Calculator helps UK drivers calculate miles per gallon from miles driven and fuel used. It is useful after filling the tank, checking whether your car is performing normally, comparing official MPG with real-world MPG, or estimating fuel use before a long trip. The calculator uses UK imperial gallons because UK car economy is normally quoted in miles per gallon while fuel is bought in litres.
MPG Calculator method
This calculator uses straightforward fuel economy, vehicle measurement, or unit conversion formulas. Results are estimates for planning and comparison, so real-world driving, vehicle condition, load, weather, route type, and manufacturer data can change the outcome.
- UK gallons = litres / 4.54609
- MPG = miles driven / UK gallons used
- litres per 100km = 282.481 / UK MPG
How to use the MPG Calculator
- Enter the main vehicle or journey figures requested by the calculator.
- Use UK units where shown, especially miles, litres, UK MPG, and prices per litre.
- Check whether the calculator is asking for measured real-world figures or official brochure figures.
- Review the headline result and the supporting breakdown below it.
- Compare at least two scenarios if you are choosing between vehicles, journeys, or running-cost assumptions.
- Adjust one assumption at a time to see which input changes the result most.
- Use the result as an estimate, then confirm important decisions with real quotes, vehicle records, or specialist advice.
Worked examples
Tank-to-tank MPG
Input: 420 miles driven and 45 litres used
Calculation: 45 / 4.54609 = 9.90 UK gallons; 420 / 9.90
Result: About 42.4 MPG
Motorway trip
Input: 180 miles and 17 litres used
Calculation: 17 / 4.54609 = 3.74 UK gallons; 180 / 3.74
Result: About 48.1 MPG
UK MPG versus US MPG
A UK gallon is larger than a US gallon, so UK MPG and US MPG are not the same number. This calculator uses UK imperial gallons for UK-style car economy. If you compare data from a US source, convert the units before drawing conclusions.
| Unit | Litres |
|---|---|
| 1 UK gallon | 4.54609 litres |
| 1 US gallon | 3.78541 litres |
Best way to measure real MPG
The most reliable everyday method is to fill the tank, reset the trip meter, drive normally, then fill the tank again and record the litres added. The litres added are a better estimate of fuel used than the dashboard fuel gauge.
Why real-world results vary
Fuel economy and running costs can shift noticeably from the calculated result. Short trips, cold starts, roof boxes, tyre pressure, motorway speed, stop-start traffic, heavy loads, air conditioning, and poor servicing can all increase fuel use.
For buying decisions, compare a conservative scenario as well as an optimistic one. A car that looks cheaper on official MPG may cost more if your real-world driving pattern is mainly urban or short-distance.
Common mistakes and edge cases
- Do not mix UK MPG and US MPG when comparing results.
- Dashboard MPG can be optimistic or rounded.
- A small fuel top-up over a short distance can produce noisy results.
- Hybrid and plug-in hybrid results depend heavily on battery use and charging habits.
Limitations
This calculator is for general information only. It is not financial, engineering, insurance, valuation, or mechanical advice.
- The calculator assumes the litres entered were actually used over the distance entered.
- It does not account for fuel quality, driving style, weather, vehicle load, or mechanical condition.
- Official WLTP figures may not match your driving pattern.
Frequently asked questions
Does this use UK MPG?
Yes. UK MPG uses imperial gallons, where one UK gallon equals 4.54609 litres.
Why is my real MPG lower than the official figure?
Official tests follow standard conditions. Cold starts, traffic, speed, tyres, and load can lower real-world MPG.
Can I use diesel and petrol?
Yes. MPG is based on distance and fuel volume, not fuel type.
Is higher MPG better?
Yes, for fuel economy. Higher MPG means more miles from each gallon of fuel.
How often should I check MPG?
Checking over several full tanks gives a more reliable average than one short journey.
Related calculators
- Fuel Cost Calculator
- L/100km Calculator
- Fuel Savings Calculator