Quick answer
Fuel economy converts between distance per fuel (mpg) and fuel per distance (L/100 km). 30 miles per US gallon equals 7.8405 L/100 km, since L/100 km = 235.215 ÷ mpg.
Enter a value, choose the units and decimal places. The calculator returns the converted fuel economy.
Related tools: Fuel Cost Calculator: Trip fuel cost from distance, fuel economy and price. Fuel Savings Calculator: Yearly fuel savings from a more efficient vehicle. Fuel Emissions Calculator: CO₂ from burning gasoline or diesel.
How to use the Fuel Economy Converter
- Enter value.
- Choose from: Miles per US gallon (mpg), Miles per imperial gallon (mpg UK), Kilometres per litre (km/L) and Litres per 100 km (L/100 km).
- Choose to: Miles per US gallon (mpg), Miles per imperial gallon (mpg UK), Kilometres per litre (km/L) and Litres per 100 km (L/100 km).
- Enter decimal places (between 0 and 10). It starts at 4.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Value | A number | — |
| From | Choose one: Miles per US gallon (mpg); Miles per imperial gallon (mpg UK); Kilometres per litre (km/L); Litres per 100 km (L/100 km) | Starts at Miles per US gallon (mpg) |
| To | Choose one: Miles per US gallon (mpg); Miles per imperial gallon (mpg UK); Kilometres per litre (km/L); Litres per 100 km (L/100 km) | Starts at Miles per imperial gallon (mpg UK) |
| Decimal places | A number | ≥ 0, ≤ 10, starts at 4 |
Formula and method
Formula
Convert to the base unit, then to the target unit (see methodology).
IEEE-754 double precision; displayed to the chosen number of decimal places (default 4). Very small or large results are shown in scientific notation.
Every unit is defined by an exact or published factor to the base unit (kilometre per litre). The value is multiplied by the source factor and divided by the target factor. L/100 km is inversely related to km/L, so 0 is not a valid value.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Value = 30; From = Miles per US gallon (mpg); To = Litres per 100 km (L/100 km); Decimal places = 4.
| Result | Value |
|---|---|
| Miles per US gallon (mpg) → Litres per 100 km (L/100 km) | 7.8405 |
| Input | 30 Miles per US gallon (mpg) |
Example 2
Inputs: Value = 8; From = Litres per 100 km (L/100 km); To = Kilometres per litre (km/L); Decimal places = 4.
| Result | Value |
|---|---|
| Litres per 100 km (L/100 km) → Kilometres per litre (km/L) | 12.5 |
| Input | 8 Litres per 100 km (L/100 km) |
Example 3
Inputs: Value = 1; From = Miles per imperial gallon (mpg UK); To = Miles per US gallon (mpg); Decimal places = 4.
| Result | Value |
|---|---|
| Miles per imperial gallon (mpg UK) → Miles per US gallon (mpg) | 0.8327 |
| Input | 1 Miles per imperial gallon (mpg UK) |
The first line is the headline result (miles per US gallon (mpg) → Litres per 100 km (L/100 km)). Below it the fuel Economy Converter lists input, which describe the same case from other angles. IEEE-754 double precision; displayed to the chosen number of decimal places (default 4). Very small or large results are shown in scientific notation.
A reciprocal relationship
mpg and L/100 km are inverses, so doubling mpg halves L/100 km. Zero is invalid for L/100 km because it would mean infinite mpg.
US versus UK gallons
A UK (imperial) gallon is 4.546 L against 3.785 L for a US gallon, so 30 UK mpg is about 25 US mpg. Always check which gallon a figure uses.
How the result changes with value
Holding every other input at the first worked example, this table changes only value. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Value | Miles per US gallon (mpg) → Litres per 100 km (L/100 km) | Input |
|---|---|---|
| 15 | 15.681 | 15 Miles per US gallon (mpg) |
| 30 | 7.8405 | 30 Miles per US gallon (mpg) |
| 60 | 3.9202 | 60 Miles per US gallon (mpg) |
| 120 | 1.9601 | 120 Miles per US gallon (mpg) |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Value: left empty shows "Enter value." beside that field and no result.
Choosing between the Fuel Economy Converter and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Fuel Cost Calculator](/automotive/fuel-cost-calculator/) | distance, fuel economy and fuel price | Trip cost |
| [Fuel Savings Calculator](/automotive/fuel-savings-calculator/) | miles driven per year, current fuel economy and new fuel economy | Annual savings |
| [Fuel Emissions Calculator](/science/fuel-emissions-calculator/) | fuel and fuel burned | CO₂ emitted |
| [Volume Converter](/converters/volume-converter/) | value, from and to | US gallon → Litre (L) |
| [Weight Converter](/converters/weight-converter/) | value, from and to | Pound (lb) → Gram (g) |
Background: unit conversion
Many customary units are defined exactly in terms of metric ones: since the 1959 International Yard and Pound Agreement an inch is exactly 2.54 cm and a pound is exactly 0.45359237 kg (NIST SP 811 lists these). So most length and mass conversions are exact apart from the rounding of the displayed result.
To convert, multiply the value by the source unit's factor to reach the base unit, then divide by the target unit's factor; the unit table on this page lists every factor used. Temperature and fuel economy are the exceptions, because they are not simple multiples.
Assumptions and limitations
- Negative values are allowed
- Zero converts to zero (except L/100 km, where zero is invalid)
- Units from different quantities cannot be converted
- Very large or tiny results switch to exponent notation
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for value to confirm this.
Frequently asked questions
What is 40 mpg in L/100 km?
About 5.88 L/100 km.
How do I convert km/L to mpg?
Multiply km/L by 2.352.
Why are UK mpg figures higher?
The imperial gallon is larger.
What does the Fuel Economy Converter calculate?
Convert mpg (US and UK), km/L and L/100 km. It returns Miles per US gallon (mpg) → Litres per 100 km (L/100 km) and Input.
What formula does the Fuel Economy Converter use?
Convert to the base unit, then to the target unit (see methodology). Every unit is defined by an exact or published factor to the base unit (kilometre per litre).
What does the “From” option change?
It sets how the fuel Economy is worked out. The choices are Miles per US gallon (mpg), Miles per imperial gallon (mpg UK), Kilometres per litre (km/L) and Litres per 100 km (L/100 km).
What is an example of a fuel Economy result?
With value 30, from Miles per US gallon (mpg), to Litres per 100 km (L/100 km) and decimal places 4, the miles per US gallon (mpg) → Litres per 100 km (L/100 km) is 7.8405.
Where does the method come from?
The method follows: Derived from exact mile (1609.344 m), US gallon (3.785411784 L), imperial gallon (4.54609 L).
Related calculators
- Fuel Cost Calculator — Trip fuel cost from distance, fuel economy and price.
- Fuel Savings Calculator — Yearly fuel savings from a more efficient vehicle.
- Fuel Emissions Calculator — CO₂ from burning gasoline or diesel.
- Volume Converter — Convert litres, gallons, cups, spoons and more.
- Weight Converter — Convert kilograms, pounds, ounces, stone and more.
- Length Converter — Convert between metric and imperial lengths.
Sources
- Derived from exact mile (1609.344 m), US gallon (3.785411784 L), imperial gallon (4.54609 L)
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.