MotorMath
Fuel & Efficiency

MPG Converter (UK/US/L per 100km)

Convert fuel economy between imperial MPG, US MPG, and liters per 100 km with precise gallon conversions.

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What this tool does

This calculator converts fuel-economy figures between miles per imperial gallon, US miles per US gallon, and liters per 100 kilometers. It applies the standard volume definitions—4.54609 liters per imperial gallon and 3.78541 liters per US gallon—and the reciprocal relationship between distance-per-volume and volume-per-distance metrics. The output includes all three common units plus kilometers per liter.

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How MPG Converter (UK/US/L per 100km) works

Fuel-economy figures appear in three main units worldwide: miles per imperial gallon (used in several countries), US miles per US gallon, and liters per 100 kilometers (L/100km). This tool converts any of these three inputs into the other two, enabling direct comparison of manufacturer figures or trip-computer readings across regions. The calculation hinges on two official volume definitions and the mathematical relationship between distance-per-volume and volume-per-distance formats.

The formula

The converter uses fixed constants derived from the imperial and US gallon definitions:
Imperial MPG ↔ L/100km: L/100km = 282.481 ÷ Imperial MPG
US MPG ↔ L/100km: L/100km = 235.215 ÷ US MPG
Imperial MPG ↔ US MPG: US MPG = Imperial MPG × (3.78541 ÷ 4.54609)
The constants 282.481 and 235.215 are derived by converting one mile (1.60934 km) and the respective gallon volumes into the ratio form (100 × gallon_liters ÷ miles_per_km). The tool applies whichever pair of conversions matches the input unit, then displays all three results plus km/L (the reciprocal of L/100km × 100).

Where this method is most accurate

The conversions are mathematically exact when the input value itself is accurate. Real-world fuel-economy figures depend on driving style, terrain, tire pressure, vehicle load, and ambient temperature; this tool assumes the MPG or L/100km figure entered reflects true consumption under known conditions. The output precision is limited to one decimal place for MPG values and two for L/100km, which is sufficient for trip-computer and manufacturer-quoted data.

What this tool does not do

This calculator performs unit conversion only; it does not measure actual fuel consumption, estimate future economy, or account for differences in test cycles (WLTP, EPA, NEDC). It does not adjust for altitude, ethanol content, or differences between real-world and laboratory conditions. The tool assumes standard temperature and pressure for the gallon definitions and does not incorporate jurisdiction-specific correction factors.

Disclaimer

This tool is provided for educational and informational purposes. It performs mathematical unit conversion and does not constitute vehicle advice, emissions certification, or a guarantee of any specific fuel economy. Users remain responsible for verifying manufacturer specifications and complying with applicable regulations in their jurisdiction.

Questions

Why are imperial MPG and US MPG different?
The imperial gallon contains 4.54609 liters, while the US gallon contains 3.78541 liters—a 20% difference. A car returning 40 imperial MPG delivers approximately 33.3 US MPG because the larger imperial gallon means fewer refills per mile.
How is L/100km calculated from MPG?
L/100km is the reciprocal form of distance per volume. The formula divides a constant (282.481 for imperial, 235.215 for US) by the MPG value. Higher MPG yields lower L/100km, reflecting better fuel economy in both systems.
Which unit is more accurate for comparing cars?
All three units are equally valid; the choice depends on regional convention. L/100km has a linear relationship with fuel cost (doubling L/100km doubles cost over the same distance), whereas MPG is hyperbolic. Neither format affects the underlying consumption.
Does this tool adjust for real-world driving conditions?
No. The calculator converts the number entered without modifying it for temperature, traffic, load, or driving style. Real-world economy typically differs from laboratory test figures; this tool only translates units.
Can I use this for electric-vehicle efficiency (kWh/100km or MPGe)?
No. This converter is calibrated for liquid-fuel volume units (gallons and liters). Electric-vehicle efficiency uses energy units (kilowatt-hours) and requires separate conversion factors that account for energy density rather than volume.

Sources & Methodology

The calculator applies the official imperial gallon (4.54609 L) and US gallon (3.78541 L) definitions, combined with the standard mile-to-kilometer conversion (1.60934 km/mi). The reciprocal relationship L/100km = (100 × gallon_liters) ÷ (MPG × 1.60934) yields the constants 282.481 (imperial) and 235.215 (US). These constants are published in SAE and ISO unit-conversion tables.