MPG to L/100 km Converter

Convert fuel economy between US mpg, UK mpg, L/100 km and km/L.

Supports US & imperial gallons with inverse-proportional conversion.

Updated August 20, 2026

CreatorFrank Zhao

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Introduction / overview

This calculator translates fuel economy between the four units you actually meet in listings, manuals, and road trips: $\mathrm{US\ mpg}$ , $\mathrm{UK\ mpg}$ , $\mathrm{L/100\,km}$ , and $\mathrm{km/L}$ . Enter any one field and the intelligent bidirectional engine fills in the other three instantly — no mode switching, no "convert" button.

Who this is for

Comparing a US-spec car advertised in mpg with a European model listed in L/100 km.
Converting a UK brochure (imperial gallons) before you compare it to US EPA figures.
Turning an efficiency claim into the cost or range number you actually plan with.

Inverse relationship: mpg and km/L go up when efficiency improves, but L/100 km goes down. A "better" car means a smaller L/100 km number — a frequent source of mistakes when scanning tables.

How to use (quick start)

Enter any one fuel-economy figure

Type your value into the field you already know — US mpg, UK mpg, L/100 km, or km/L. The other three update automatically.

Read all four equivalents

No extra clicks. Change any field at any time — the engine re-derives the rest in the same instant.

Clear to start a new comparison

Erase a field to reset that value. Empty fields stay blank — they are not treated as zero.

Worked example — US 32 mpg to the other three units

A US listing claims $32\ \mathrm{US\ mpg}$ . What is that in the units you will see elsewhere?

L_{100}

=

\frac{235.21458}{32}

=

7.35\ \mathrm{L/100\,km}

\mathrm{UK_{mpg}}

=

32 \times 1.20095

=

38.43\ \mathrm{UK\ mpg}

k_{L}

=

\frac{100}{7.35}

=

13.6\ \mathrm{km/L}

So the same car is 32 US mpg ≈ 38.4 UK mpg ≈ 7.35 L/100 km ≈ 13.6 km/L. If you only looked at the mpg numbers you might think the UK figure is a different car — it is not, the gallon changed.

How to read the result

L/100 km: lower is better. $5\!-\!6$ is efficient, $10+$ is thirsty for a passenger car.
US mpg / UK mpg / km/L: higher is better. They all scale together, just with different gallon or distance factors.

Real-world examples

Example 1 — Importing a UK spec

A UK Autotrader listing says $42.8\ \mathrm{UK\ mpg}$ . You want the US and metric numbers before you compare with local cars.

L_{100}=282.48/42.8

=

6.60

\mathrm{US}=235.21/6.60

=

35.6\ \mathrm{mpg}

k_{L}=100/6.60

=

15.15\ \mathrm{km/L}

Result: about 35.6 US mpg / 6.60 L/100 km / 15.2 km/L — not directly comparable to the raw 42.8 number.

Example 2 — From a fuel log to cost

Your log shows $8.2\ \mathrm{L/100\,km}$ . On a $420\ \mathrm{km}$ trip that is:

\mathrm{fuel}=8.2 \times 420/100

=

34.4\ \mathrm{L}

\mathrm{US\ mpg}=235.21/8.2

=

28.7

The same consumption is 28.7 US mpg ≈ 34.5 UK mpg ≈ 12.2 km/L. Use the L/100 km value directly for trip fuel and cost estimates.

Calculation method

All four fields describe the same physical efficiency. The calculator keeps everything exact by carrying the full decimal precision internally and only rounding for the preview you see.

Exact constants (by definition)

1\ \mathrm{mile}=1.609344\ \mathrm{km}\quad;\quad 1\ \mathrm{US\ gal}=3.785411784\ \mathrm{L}\quad;\quad 1\ \mathrm{UK\ gal}=4.54609\ \mathrm{L}

One mile, the US gallon, and the imperial gallon are defined exactly. The factors below follow directly — there is no approximation in the stored constants.

US mpg ↔ L/100 km

L_{100}=\frac{235.21458333\ldots}{\mathrm{US_{mpg}}}\qquad\Longleftrightarrow\qquad \mathrm{US_{mpg}}=\frac{235.21458333\ldots}{L_{100}}

where $C_{\mathrm{US}}=100\cdot 3.785411784 / 1.609344$

UK mpg ↔ L/100 km

L_{100}=\frac{282.48093633\ldots}{\mathrm{UK_{mpg}}}\qquad\Longleftrightarrow\qquad \mathrm{UK_{mpg}}=\frac{282.48093633\ldots}{L_{100}}

where $C_{\mathrm{UK}}=100\cdot 4.54609 / 1.609344$

km/L ↔ L/100 km & cross-conversions

k_{L}=\frac{100}{L_{100}}\quad;\quad \mathrm{UK_{mpg}}=\mathrm{US_{mpg}}\cdot\frac{4.54609}{3.785411784}\quad;\quad k_{L}=\mathrm{US_{mpg}}\cdot\frac{1.609344}{3.785411784}

$\mathrm{UK/US}\approx 1.20095$ and $\mathrm{US\to km/L}\approx 0.42514$ are not rounded inputs — they are consequences of the two exact gallon definitions above.

Supported directions

Every field can be input or output. The four fields form a single connected group through $L_{100}$ as the hub, so entering any one value derives the other three. Values must be $>0$ ; zero or negative inputs show a validation message and leave dependent fields blank.

Tips & best practices

Check which gallon the source uses

US listings, EPA, and most American reviews use the 3.785 L gallon. UK brochures and many Commonwealth sources use the 4.546 L gallon. A 42 UK mpg car is only about 35 US mpg — the gap is about 20 %.

Don't compare mpg and L/100 km by eye

Because they are inversely proportional, a drop from 8 to 6 L/100 km saves more fuel than a drop from 6 to 4 L/100 km, even though both are "2 L". Convert first, then compare.

Measure consistently if you validate

If you calculate mpg from a fill-up, fill to the same pump cut-off, use the same trip odometer, and prefer a longer distance — short trips exaggerate small volume errors when converted to L/100 km.

Use the right unit for the question

Use L/100 km or km/L for fuel-needed and cost questions ( $\mathrm{L}=L_{100}\times \mathrm{km}/100$ ). Use mpg when comparing historic fleet averages quoted in miles per gallon.

Related: For distance steps in the same workflow, try our Length Converter or Volume Conversion calculators.

Frequently asked questions

Why is UK mpg always higher than US mpg for the same car?

The UK (imperial) gallon is 4.54609 L, about 20% larger than the US gallon at 3.78541 L, while the mile is the same. Each gallon therefore takes you farther: $\mathrm{UK_{mpg}} = \mathrm{US_{mpg}} \times 1.20095$ . Both numbers describe the same efficiency — only the gallon changes.

Is fuel economy the same as fuel consumption?

Not quite. Fuel economy ( $\mathrm{mpg}$ , $\mathrm{km/L}$ ) is distance per volume — higher is better. Fuel consumption ( $\mathrm{L/100\,km}$ ) is volume per distance — lower is better. They are reciprocals ( $k_{L}=100/L_{100}$ ), so swapping one for the other without converting flips the meaning of "better".

What is a good fuel economy in L/100 km?

For a modern petrol passenger car, about $6\!-\!8\ \mathrm{L/100\,km}$ (roughly $29\!-\!39\ \mathrm{US\ mpg}$ ) is typical, and anything under $6\ \mathrm{L/100\,km}$ (over $39\ \mathrm{US\ mpg}$ ) is efficient. Hybrids and small diesels often sit around $4\!-\!5\ \mathrm{L/100\,km}$ . The "good" range shifts a lot for SUVs, vans, or performance cars.

Limitations

•Unit conversion only: This calculator converts equivalent economy figures. It does not estimate real-world fuel use, which also depends on driving style, load, temperature, and road conditions.
•Positive values only: Inputs must be greater than zero. Zero or negative numbers are flagged and leave related fields blank rather than producing a misleading result.
•Standards are exact, driving is not: The mile and gallon definitions are exact, but a converted brochure figure does not guarantee you will achieve that economy on your own routes.

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