Imperial miles per gallon to liters per 100 kilometers

What the units mean

Convert Imperial miles per gallon (Imp mpg) to liters per 100 kilometers (L/100 km) instantly in your browser. See the formula, a worked example and limits of the result.

Fuel-consumption figures use opposing directions. Liters per 100 kilometers measures fuel used to travel a fixed distance, so lower is better. Miles per gallon measures distance traveled on a fixed amount of fuel, so higher is better. The two measures are reciprocals rather than simple multiples.

This page converts one named unit into another within the same physical or digital quantity. The input stays in the browser. The result is a change of representation: it does not estimate an unmeasured property, correct a label, or supply missing information about the object being measured. Identify the original unit before typing a number, especially when a symbol could have more than one informal meaning.

How to calculate

L/100 km = 282.480936332 ÷ Imp mpg. Fuel economy uses a reciprocal formula and requires a value above zero; US and Imperial gallons differ.

Use the displayed relationship with the entered value in the source unit. For an ordinary linear conversion, multiply by the ratio of the source unit to the destination unit. The reverse direction divides by that same ratio. Fuel economy is the exception here: it uses a reciprocal, so the number appears in the denominator. The calculator carries the defined constants through the calculation before shortening the displayed decimal.

As a worked example, enter 6 Imp mpg. The output is 47.0801560553 L/100 km. Put the unit beside both numbers when copying them into a document or spreadsheet. A number without its unit does not reveal the measurement scale, and switching the order of the units changes the operation. The example is illustrative; use the value from your own measurement for a real comparison.

When this conversion is useful

Imperial mpg uses the 4.54609-liter gallon historically used in the United Kingdom. For the same fuel consumption, Imperial mpg is numerically higher than US mpg. When reading a review or an older vehicle manual, identify which gallon the source intended rather than assuming every “mpg” figure shares one definition.

Start with a figure explicitly reported in Imperial miles per gallon (Imp mpg) and read the answer in liters per 100 kilometers (L/100 km). A smaller or larger output number alone says nothing about whether the measured quantity has changed. If the original source gives a rounded value, the calculation cannot recover digits that were never measured. For a critical limit, compare using the unrounded numerical result and round only the final presentation.

Precision and common mistakes

A US gallon is 3.785411784 liters, while an Imperial gallon is 4.54609 liters. That difference makes US mpg and Imperial mpg numerically different for the same vehicle. Both formulas require a value above zero; zero consumption would correspond to unbounded mpg. Real results depend on driving cycle, terrain, weather and vehicle condition, not only the unit.

Conversion factors can be defined exactly even when the decimal displayed on the page has been shortened. Values involving π are approximated by computer arithmetic; a stated measurement may have much greater uncertainty than that arithmetic. Carry the source unit and enough significant figures through intermediate steps. If you need a certified measurement, calibration or commercial settlement, use the applicable standard and the original instrument record.

Questions about the result

Why can a reverse conversion return a slightly different final digit? The visible result is rounded to make it readable; reversing that shortened number starts from a different input. Why does a plausible figure sometimes disagree with a product label? The label may use another convention, a different reference condition or fewer significant figures. Confirm the exact unit symbol and the measurement definition before deciding that either number is wrong.