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Fuel Efficiency (Mass) Converter

Science

Convert mass-based fuel efficiency between km/kg, miles/kg, and kg/100km — used for hydrogen and CNG/LNG vehicles measured by fuel mass, not volume.

Reviewed by the thecalcu.com team · Last updated July 20, 2026

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What is a Fuel Efficiency (Mass)?

The Fuel Efficiency (Mass) Converter converts fuel efficiency measured by fuel mass rather than fuel volume, kilometres per kilogram, miles per kilogram, and kilograms per 100 kilometres. This distinction matters for hydrogen fuel cell and CNG/LNG vehicles, which are fuelled and metered by weight rather than the litres or gallons used for gasoline and diesel.

Because km/kg (and mi/kg) measure distance per unit fuel while kg/100km measures fuel per unit distance, converting between them is a reciprocal relationship, not a simple multiplication, the same pattern as the existing Fuel Efficiency Converter uses for volume-based L/100km versus km/L.

Why Use a Fuel Efficiency (Mass) Converter?

Hydrogen and CNG vehicle efficiency figures are reported inconsistently across manufacturers and regions, some in km/kg, others in kg/100km, and international comparisons sometimes need miles per kilogram. Manually inverting a km/kg figure to get kg/100km (or vice versa) is easy to get backwards, especially since the relationship isn't a simple multiplier the way most unit conversions are.

Who Should Use This Converter?

  • Hydrogen fuel cell vehicle owners and shoppers comparing efficiency figures reported in different mass-based units across manufacturers.
  • CNG/LNG fleet operators tracking fuel consumption and comparing vehicles using consistent units.
  • Automotive engineers and researchers working with alternative fuel vehicle efficiency data across international unit conventions.

What Insights Does the Fuel Efficiency (Mass) Converter Give You?

The main output is the converted value in your chosen target unit. Remember that km/kg and mi/kg are "bigger is better" metrics (more distance per unit fuel), while kg/100km is a "smaller is better" metric (less fuel needed per fixed distance), the same inverted relationship as km/L versus L/100km for liquid fuels.

How to use this Fuel Efficiency (Mass) calculator

  1. Choose your starting unit from the source dropdown, for example, "Kilometres per Kilogram (km/kg)".
  2. Enter the numeric value you want to convert in the input field.
  3. Choose your target unit from the destination dropdown, for example, "Kilograms per 100 km (kg/100km)".
  4. Read the converted result, which updates instantly as you type or change units.
  5. Use the swap (⇅) button if you need to reverse the conversion direction.
  6. Use the copy button to grab the result for a vehicle comparison or fleet efficiency report.
Show formula & methodology ↓Show less ↑

Formula & Methodology

The converter's canonical unit is km/kg. Conversions follow:

- mi/kg → km/kg: multiply by 1.609344 (exact)
- kg/100km ↔ km/kg: reciprocal relationship, km/kg = 100 ÷ kg/100km, and kg/100km = 100 ÷ km/kg

Worked example, converting 100 km/kg (a typical hydrogen fuel cell vehicle) to kg/100km:

kg/100km = 100 ÷ 100 = 1.0 kg/100km

This means the vehicle consumes about 1 kilogram of hydrogen for every 100 kilometres travelled, a useful reference figure for comparing against other hydrogen vehicles or fuel cost calculations.

For background on the underlying term, see our glossary entry on Fuel Efficiency.

Related Tools

You may also find these useful: MPG to L/100km Converter, Natural Gas Unit Converter.

Frequently Asked Questions

Why do some vehicles measure fuel efficiency by mass instead of volume?
Hydrogen, CNG (compressed natural gas), and LNG (liquefied natural gas) vehicles typically fuel by mass (kilograms) rather than volume (litres or gallons), since gaseous fuels are compressed to varying densities and mass is the more consistent, meaningful measurement for how much fuel was actually delivered.
How do I convert km/kg to kg/100km?
Divide 100 by the km/kg value, since these two units are reciprocals of each other, a higher km/kg value (more distance per kilogram) corresponds to a lower kg/100km value (less fuel needed per 100 km), and vice versa.
Why isn't km/kg to kg/100km a simple multiplication?
km/kg measures distance per unit of fuel mass, while kg/100km measures fuel mass per unit of distance, these are inverse relationships, not proportional ones, so converting between them requires division (100 ÷ value) rather than multiplication by a fixed factor, the same pattern as L/100km versus km/L for liquid fuel.
How does this differ from the standard Fuel Efficiency Converter?
The [Fuel Efficiency Converter](/fuel-efficiency-converter/) covers volume-based fuel efficiency (km/L, L/100km, MPG) for liquid fuels like gasoline and diesel, while this converter covers mass-based fuel efficiency (km/kg, kg/100km) for gaseous fuels like hydrogen and CNG, which are dispensed and measured by weight rather than volume.
What is a typical km/kg figure for a hydrogen fuel cell vehicle?
Hydrogen fuel cell vehicles commonly achieve roughly 90 to 120 km per kilogram of hydrogen, though this varies by vehicle model and driving conditions, use this converter to compare that figure against a kg/100km or mi/kg equivalent from a different source.
How do I convert miles per kg to km per kg?
Multiply the mi/kg value by 1.609344, since one mile equals 1.609344 kilometres. Enter your value with 'Miles per Kilogram (mi/kg)' as the source and 'Kilometres per Kilogram (km/kg)' as the target to apply this automatically.
Can I compare hydrogen fuel efficiency directly against gasoline MPG?
Not directly, since they use different fuel measurement bases (mass for hydrogen, volume for gasoline) with very different energy content per unit, a fair comparison requires converting both to an energy-equivalent basis (like cost or energy per 100 km) rather than comparing km/kg against MPG directly.
Why does kg/100km use 100 kilometres as the reference distance?
Using a fixed 100 km reference distance (rather than expressing efficiency as a per-kilometre rate) keeps the numbers in a convenient, easily comparable range, mirroring the same convention used for L/100km in liquid-fuel efficiency reporting in most of the world outside the US.
Is fuel efficiency by mass used anywhere besides hydrogen and CNG vehicles?
Yes, any application where fuel is measured and dispensed by weight rather than volume, including some industrial and aviation contexts, can use mass-based efficiency metrics, though hydrogen and natural gas vehicles are the most common consumer-facing example.
How accurate is the mile-to-kilometre conversion used here?
The 1.609344 km-per-mile conversion factor is exact by international agreement, so converting mi/kg to km/kg introduces no rounding error beyond the precision of your original figure.
Also known as
km per kg converterkg per 100km converterhydrogen fuel economy convertercng fuel efficiency convertermass based fuel economy converter