How to Calculate MPG Conversions in the UK: Step-by-Step Guide

Introduction

Running a car in the UK costs far more than fuel alone. Putting figures on each part of motoring makes it easier to budget, claim what you're owed and compare petrol, diesel and electric.

This guide explains how to calculate mPG Conversions in the UK, step by step. Convert UK MPG to litres per 100km, km per litre and US MPG. You'll see the formula, a fully worked example and practical tips for UK readers.

What this calculation covers

Convert UK MPG to litres per 100km, km per litre and US MPG.

Accurate motoring numbers help you budget journeys, claim mileage relief from HMRC and decide whether an electric car makes financial sense for you.

The method and formula

UK gallons are bigger than US gallons, so the same car shows a higher UK MPG. L/100km = 282.48 ÷ UK MPG.

Every figure on our mPG Converter follows this method, so you can check the working yourself.

Worked example

Here's a typical example using these figures:

Running them through the method gives:

The headline figure is litres per 100km: 6.28. Change any input and the result will move with it.

How to use the calculator

1. Enter or choose uK miles per gallon (mpg).
2. Read your results. The main figure is shown at the top, with a breakdown underneath.
3. Try different values to compare scenarios side by side.

Tips for UK readers

• UK MPG uses imperial gallons (4.546 litres), which differ from US gallons.
• HMRC's approved mileage rate is 45p per mile for the first 10,000 business miles.
• Home EV charging on an off-peak tariff is usually far cheaper than public rapid chargers.
• Depreciation is often the largest hidden cost of owning a newer car.

Frequently asked questions

What's the quickest way to work out mPG Conversions?

Use our free mPG Converter. It applies this method automatically: UK gallons are bigger than US gallons, so the same car shows a higher UK MPG. L/100km = 282.48 ÷ UK MPG.

What MPG should I use?

Use your real-world figure from your car's trip computer if you can; official figures are often optimistic.

Can I claim for commuting miles?

No. Normal commuting to a permanent workplace doesn't count as business mileage.

How often do HMRC mileage rates change?

Rarely. The 45p rate has been unchanged for many years, but always check GOV.UK.

Summary

To work out mPG Conversions, follow the method above: UK gallons are bigger than US gallons, so the same car shows a higher UK MPG. For a quick, accurate answer with your own figures, use the calculator.

Next, read MPG Conversions: Common Mistakes, Tips and FAQs.