How it works
mpg to l/100km converter — the short version
We built MPG ↔ L/100 km Converter because the other tools for this job either cost a subscription or came with a consent banner the size of a small novel.
Dealerships bury totals in fine print — bringing your own numbers changes the conversation. Have the actual fuel price from last fill-up, not a guess — then work it out and the rest of this page explains what the answer means.
The UK uses imperial mpg (gallon = 4.546 L). 40 mpg ≈ 7.06 L/100 km. The US gallon (3.785 L) gives different numbers — 40 US mpg ≈ 5.88 L/100 km.
The formula we run is L/100 km = 282.481 / mpg (UK). You'll see each term laid out in the worked example below.
A worked example, step by step
Consider a realistic scenario and follow it through:
The UK uses imperial mpg (gallon = 4.546 L). 40 mpg ≈ 7.06 L/100 km. The US gallon (3.785 L) gives different numbers — 40 US mpg ≈ 5.88 L/100 km.
Every run comes back to L/100 km = 282.481 / mpg (UK) — change the inputs, the structure of the answer stays.
Moments this tool earns its keep
MPG ↔ L/100 km Converter is aimed at people arriving with questions like these:
- "Mpg to l/100km"
- "L/100km to mpg"
- "Uk vs us mpg"
- "Fuel efficiency conversion"
- "What is mpg to l/100km converter"
- "How to calculate mpg to l/100km converter"
Where the number stops being useful
Every tool has an edge where it stops being the right answer. MPG ↔ L/100 km Converter is no exception:
- For legally binding tax or medical decisions — cross-check with HMRC, NHS or a qualified professional.
- For very large or very small extremes the rounding error outgrows the useful precision.
- When the underlying rate or threshold has changed since the page was last reviewed — always verify with the primary source.
- When the input you have is already a derived figure (net of something) — feeding it in as "gross" will double-subtract.
Five things that trip everyone up
Every time you work it out for a new scenario, one of these creeps in — it's worth knowing them ahead of time.
- Assuming the UK and US versions of the same unit are interchangeable — they're not.
- Typing a comma where the tool expects a dot (or vice versa).
- Rounding early — particularly painful in percentages and compound growth.
- Ignoring the time window: a 'per year' answer makes no sense with a monthly input.
- Treating the answer as private: screenshots are fine, but the URL always reruns cleanly.
The sources behind the numbers
Where the maths needs an external authority, we cross-check against:
- GOV.UK
- RAC Foundation
Works well alongside
If this question keeps coming up for you, the same cluster of tools usually comes next:
- Fuel Cost calculadora — Work out fuel cost for any journey from distance, pence-per-litre and MPG — with a UK-average benchmark for comparison.
- Speed Converter — Convert mph, km/h, m/s and knots with context: UK road signs use mph, most of Europe uses km/h, sailors use knots and SI uses m/s.
How we keep this accurate
Our calculadoras run on pure, unit-tested functions — the same logic lives in the browser and in the CI test suite. When tax rates, thresholds or official figures move, the update lands within 24 hours of the announcement. You can read the editorial policy and corrections policy.
Found an out-of-date number on MPG ↔ L/100 km Converter or anywhere else in the Vehicles toolkit? Send it to the editorial desk and we'll patch it. Or browse the full calculadora directory for the next tool you need.