UK MPG Calculator

The MPG figure on a car's spec sheet is a useful starting point, but it rarely matches what you actually get once you are driving in the real world. This calculator works out your actual fuel economy from a real journey, using the distance you drove and the fuel you genuinely used.

It is a handy way to check whether your car is performing as expected, compare two vehicles fairly, or simply understand what a typical journey is really costing you. It is especially useful before a longer trip, when knowing your real MPG rather than an optimistic official figure gives a much more accurate idea of what the fuel is actually going to cost.

How to use the MPG calculator

Enter the distance you drove in miles, and how much fuel you used, in litres or gallons. Add your fuel type and the price you paid per litre, and the calculator works out your MPG in both imperial and US terms, your litres per 100km, and an estimated cost per mile for that journey.

How the calculation works

If you enter fuel used in litres, the calculator converts your distance and fuel figures directly into miles per UK gallon by applying the standard conversion of 4.54609 litres to one UK gallon. It also works out miles per US gallon, using the smaller US gallon of 3.78541 litres, which is why the US figure always comes out higher for the same underlying fuel economy. Litres per 100km is calculated by converting your mileage to kilometres and scaling your fuel used accordingly.

The cost figures work the other way round from a typical fuel cost calculator. Instead of estimating fuel use from an assumed MPG figure, this tool starts from what you genuinely put in the tank, so the cost per mile it shows reflects your actual driving rather than a manufacturer estimate. That makes it a useful reality check against the official figures quoted when you were shopping for the car.

Petrol, diesel and hybrid MPG in practice

Diesel engines generally return higher MPG than equivalent petrol engines, particularly on motorway journeys, though diesel is often priced a few pence higher per litre at the pump, so the real cost saving depends on both figures together, not MPG alone. Hybrids tend to perform best in stop start town driving, where the electric motor can take some of the load, and closer to a standard petrol car's figures on sustained motorway driving where the engine does most of the work. Running your own numbers through this calculator for each car you are considering is the fairest way to compare them for your typical journeys.

Worked example

Say you drive 300 miles and use 35 litres of petrol, at £1.45 a litre. That works out at 39.0 MPG in imperial terms, or 32.4 MPG in US terms, and 7.2 litres per 100km. The total fuel cost for the journey comes to around £50.75, or roughly 17p a mile.

If your car's official combined figure was advertised at 45 MPG, this real world result of 39.0 MPG is a meaningful gap, worth knowing about when budgeting for fuel, especially over a year of regular driving.

Common mistakes to avoid

A common mistake is calculating MPG from a partial tank rather than a full fill to full fill measurement, which can give a misleading result if the pump does not stop at exactly the same point each time. It is also easy to judge fuel economy from a single short journey, which tends to be less efficient than a longer, steadier drive, since the engine spends more of a short trip warming up.

Another mistake is comparing your MPG directly against a car's official WLTP figure and assuming something is wrong if it is lower. Some gap between official and real world figures is completely normal, so it is more useful to track your own MPG over time and notice genuine changes, rather than comparing against the spec sheet alone.

Related calculators

To plan the cost of a specific trip rather than check your real world economy, try our fuel cost calculator. Once you know your MPG, our road tax calculator and car tax guide can help you understand the rest of your annual motoring costs. If you drive into London regularly, our Congestion Charge and ULEZ calculator covers those additional costs too, and our take-home pay calculator can help you see how motoring costs fit into your wider monthly budget.