How to calculate the cost of a UK car journey

Business & Money Desk 2026-10-17 08:20 4 min read

Aerial view of a scenic winding road through the hilly landscape of Wales UK.

A journey’s estimated fuel cost can be expressed as a simple calculation: distance divided by miles per gallon, converted from imperial gallons to litres, then multiplied by the local price per litre. That gives drivers a reusable way to budget, while the UK average provides context rather than a guaranteed price at any particular forecourt.

Calculate fuel cost from distance, mpg and pump price

Start with the total distance you expect to drive, your vehicle’s fuel economy in miles per gallon (mpg), and the price per litre for the fuel grade you use. Include the return leg if you are estimating a round trip. For journeys with planned stops or detours, include those miles too.

UK mpg figures generally use imperial gallons. One imperial gallon is about 4.546 litres, so the calculation converts the estimated gallons used into litres before applying the price shown at the pump:

Fuel cost = (distance in miles ÷ mpg × 4.546) × price per litre

For a quick per-mile estimate, divide 4.546 by your mpg to get approximate litres used per mile, then multiply by the price per litre. At 40 mpg, that is about 0.114 litres per mile. At a hypothetical price of £1.50 per litre, the estimate is roughly 17p per mile. The price is only an illustration, not a current UK pump price.

How to calculate the cost of a UK car journey

Use a realistic mpg figure where possible. Recent fuel economy from your own driving may better reflect your usual journeys than a dashboard or manufacturer figure, which is still an estimate. Traffic, speed, load and other driving conditions can change consumption.

Worked example: estimate a 20-mile return trip

Suppose a driver travels 10 miles each way, for 20 miles in total. Their car averages 40 mpg, and they want to calculate the cost using an assumed price of £1.50 per litre.

Estimated fuel: 20 ÷ 40 × 4.546 = about 2.27 litres.
Estimated cost: 2.27 × £1.50 = about £3.41.

This is an estimate of fuel for the trip, not the full cost of running the car. It excludes parking, tolls, maintenance and other expenses. Actual spending may differ if the route changes, the car uses more fuel in traffic, or the pump price is different from the assumed figure.

The same method works for comparing routes: calculate each route using its own distance, then compare the estimates. A longer route can use more fuel even if it avoids traffic, so distance alone does not show which option will cost less in practice.

Use the UK average as a benchmark, not a station quote

The Department for Energy Security and Net Zero publishes UK road fuel price data and related petroleum product statistics. Its road fuel and other petroleum product prices collection is the official place to check the latest available figures. When using a figure as a benchmark, note the release date and fuel type it covers.

A national average summarises prices across the UK for the period and fuel grade covered by the statistic. It does not tell you what an individual station charges today. It can help put a local price in context, but it should not be substituted for the price you expect to pay on your journey.

For a local comparison, check forecourts you could realistically visit and compare the same fuel type and grade. Note each price per litre and when you checked it. If a cheaper station requires a detour, include the extra distance in your journey calculation: the lower pump price may not reduce the overall fuel cost if getting there uses more fuel.

Recalculate when the journey or price changes

For a commute or another repeated journey, first calculate the one-way or return trip, then multiply that estimate by the number of trips. If comparing options, keep the assumptions consistent: use the same mpg and fuel grade, and update the distance or price when those inputs change.

Treat the result as a planning figure, not a guaranteed receipt total. Stop-start traffic, cold starts, hills, speed, tyre condition, passengers and luggage can affect fuel use. Before setting off, check the latest official UK benchmark for context and verify the price at forecourts you might use; then recalculate with the local price and planned distance.

Source: UK Government, Department for Energy Security and Net Zero

Context & actions About this article

Source check How the estimate works

The trip example uses an assumed pump price; consult the latest government figures and check local forecourts for current prices.

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