What Does the MPG Calculator Do?
The MPG calculator measures fuel economy from distance and fuel used, then expresses the same consumption in familiar alternative units. The full-to-full mode is intended for a matched pair of fills, not for guessing consumption from the position of a dashboard fuel needle. A second mode estimates journey fuel cost when you already know a representative MPG figure. Keeping measurement and prediction separate helps avoid treating a manufacturer's test figure as your own driving result.
How to Use This Calculator
- For measured MPG, record the distance driven between comparable refuelling points. A full-to-full method uses a starting fill, the intervening miles and the litres needed to return to a comparable fill level.
- Enter those miles and litres in the measurement inputs. Do not enter the fuel tank's capacity unless that is actually the measured quantity consumed. Avoid using a fuel-gauge estimate as if it were a precise volume.
- For the separate journey-cost calculation, enter a planned distance, a known imperial MPG and a price in pence per litre. These inputs answer a different question from the measured-tank calculation.
- Calculate the relevant section and check the units on the result. For a longer record, add miles and litres across fills before calculating an overall MPG, rather than averaging each tank's displayed MPG.
The Maths & Assumptions Behind the Tool
Imperial gallons used = litres ÷ 4.54609. UK MPG = miles ÷ imperial gallons. Equivalently, MPG = miles × 4.54609 ÷ litres. The journey calculation reverses that relationship: litres = journey miles ÷ known MPG × 4.54609, then cost = litres × pence per litre ÷ 100. Fuel remaining at the beginning and end matters to a measured test; a fill quantity is only a consumption measurement when those levels are comparable.
Worked example
Driving 400 miles between matched full-tank fills and adding 40 litres gives 400 ÷ (40 ÷ 4.54609) = 45.46 UK MPG. That is approximately 6.21 L/100km or 37.85 US MPG.
For several fills, add the miles and litres separately before dividing. Do not average the individual MPG figures without accounting for fill size. Fuel added must match the distance recorded; a partial fill without a matched starting level cannot give a reliable tank-to-tank figure.
Why This Matters for UK Drivers
For a tank measurement, record mileage after a consistent fill and the litres needed to return to a comparable fill level. A receipt supplies the litres; an odometer or trip counter supplies distance. You do not need to know total tank capacity. The quantity added on the second fill is useful because it approximates the fuel consumed since the first matching level. A partly filled starting tank would break that relationship.
One tank can be affected by the pump stopping at a different point, a route change or unusually heavy traffic. Combining several comparable fills gives a more useful planning input. Add their miles together and their litres together before calculating. Averaging the individual MPG readings equally can misrepresent fills of different sizes. When copying the result into another tool, preserve the UK imperial label so it is not confused with the lower numerical US MPG value for the same efficiency.
Interpreting the result in practice
Use a consistent filling method rather than trying to force extra fuel into the tank. Pump cut-off behaviour and the angle of the vehicle can change one reading slightly. A longer sequence of comparable fills reduces the influence of a single unusual stop. Keep the date and mileage with each receipt so missing fills can be identified; an omitted fill makes the calculated economy look better than it was.
If the measured result differs from the dashboard display, check whether both cover the same period and use the same gallon convention. A trip computer may have been reset at a different time. The calculator can expose that mismatch, but it cannot diagnose a mechanical fault. A sustained change is a reason to review routes, conditions and records before drawing conclusions about the vehicle.
A numerical cross-check can catch a misplaced unit. If 450 miles uses 45 litres, the fuel volume is approximately 9.90 imperial gallons and economy is about 45.46 UK MPG. Dividing miles directly by litres would give ten miles per litre, which is valid in its own unit but is not ten MPG. Multiplying that miles-per-litre result by 4.54609 returns imperial MPG.
For records covering different drivers, keep the vehicle and operating pattern identified. A combined household figure may be useful for fuel budgeting but may conceal differences between cars. The calculator only knows the totals entered, so it cannot attribute a change to a driver, route, mechanical condition or missing receipt without additional evidence.
One page for measured MPG and journey planning
The fill-up mode turns miles and litres into UK MPG, L/100km and miles per litre. Journey mode starts with your own MPG and estimates fuel volume, fuel cost and pence per mile for a planned distance.
Why real MPG matters for cost calculations
Published test figures can be useful for comparison, but traffic, temperature, speed, load, tyres and driving style all affect actual consumption. For budgeting, a recent real-world average from your own vehicle is normally the more useful input.