How it works
mpg to l/100km converter — the short version
Use this MPG ↔ L/100 km Converter when you need a mpg to l/100km converter you can trust — clean inputs, transparent steps, zero fluff.
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.
On this page you will see Vehicles & Motoring, GOV.UK and RAC Foundation treated as first-class terms — each one is linked to the calculators and references that use it, so you can follow the thread without retyping queries into a search bar.
The formula we run is L/100 km = 282.481 / mpg (UK). You'll see each term laid out in the worked example below.
If it helps, jump straight to the Vehicles hub or compare with the Fuel Cost Calculator and the Speed Converter — those two calcs are the ones readers usually open right after this page.
From inputs to answer, in full
Consider a realistic scenario and follow it through:
A Ford Focus 1.0 EcoBoost rated at 52 mpg UK converts to 282.481 / 52 = 5.43 L/100 km. On a 12,000-mile year that's roughly 1,050 litres at UK pump prices.
A diesel van advertised at 35 mpg UK is 282.481 / 35 = 8.07 L/100 km. The same van quoted at 35 mpg US (the number American brochures print) is actually 6.72 L/100 km — the UK gallon is 21% larger.
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.
Watch-outs before you trust the number
Every time you work it out for a new scenario, one of these creeps in — it's worth knowing them ahead of time.
- Confusing UK and US mpg — they are 20% apart. Always verify which gallon the source is using before converting.
- Forgetting that fuel economy degrades at motorway speeds above 70 mph and in cold weather — manufacturer figures are optimistic.
- Comparing petrol and diesel mpg directly. Diesel carries more energy per litre, so the same mpg on paper costs different money at the pump.
- Using old WLTP/NEDC numbers on pre-2018 cars without adjusting — real-world returns on older cycles were typically 15–25% below label.
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 Calculator — 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.
