Everyday Life · Unit Conversions · Speed & Velocity
Fuel Efficiency Converter (MPG to L/100km)
Converts fuel efficiency between miles per gallon (MPG) and litres per 100 kilometres (L/100km), supporting both US and UK gallon standards.
Calculator
Formula
L/100km is the fuel consumption in litres per 100 kilometres. MPG is the fuel efficiency in miles per gallon. C is the conversion constant: 282.48 for US gallons (1 US gallon = 3.78541 L, 1 mile = 1.60934 km) and 282.48 adjusted to 282.48 × (4.54609 / 3.78541) ≈ 235.21 for UK (Imperial) gallons. Specifically: C = 100 × gallons-in-litres / km-per-mile. For US: C = 100 × 3.78541 / 1.60934 ≈ 235.21. For UK: C = 100 × 4.54609 / 1.60934 ≈ 282.48.
Source: NIST Special Publication 811 (2008 Edition) — Guide for the Use of the International System of Units; UK Weights and Measures Act 1985.
How it works
Fuel efficiency can be expressed in two fundamentally different ways. MPG (miles per gallon) is a higher-is-better measure — the more miles you travel on a gallon of fuel, the more efficient the vehicle. L/100km is a lower-is-better measure — it tells you how many litres of fuel are consumed for every 100 kilometres driven. These two metrics are inversely proportional, which is why the conversion involves dividing a constant by the MPG value rather than simply multiplying.
The core formula is: L/100km = C ÷ MPG. The constant C depends on which gallon standard you're using. For US gallons, 1 gallon equals exactly 3.78541 litres, and 1 mile equals 1.60934 km, giving C = (100 × 3.78541) / 1.60934 ≈ 235.21. For UK Imperial gallons, 1 gallon equals 4.54609 litres, giving C = (100 × 4.54609) / 1.60934 ≈ 282.48. This distinction matters enormously: a car rated at 40 MPG (UK) is not the same as 40 MPG (US) — the UK figure is about 20% more generous because Imperial gallons are larger.
This conversion is practically important when buying imported vehicles, comparing manufacturer specifications across markets, or calculating running costs in a different country. For example, the EU mandates fuel economy be published in L/100km, while the US uses MPG. Fleet operators calculating total fuel budgets across mixed international fleets also depend on consistent units. The secondary output — km per litre (km/L) — is commonly used in Japan, India, and parts of Latin America.
Worked example
Example 1 — US Vehicle: A Honda Civic is rated at 36 MPG (US combined). To convert to L/100km:
L/100km = 235.21 ÷ 36 = 6.53 L/100km
This is a very efficient result by European standards, where the average new car sits around 6–7 L/100km.
Example 2 — UK Vehicle: A Ford Focus is advertised at 50 MPG (UK). Using the Imperial constant:
L/100km = 282.48 ÷ 50 = 5.65 L/100km
Note that 50 MPG (UK) ≠ 50 MPG (US). The same figure in US gallons would be: 282.48 ÷ 235.21 × 50 = 41.6 MPG (US) — a significant difference of over 8 MPG.
Example 3 — Reverse check: If a European car is rated at 7.0 L/100km, its US MPG equivalent is: 235.21 ÷ 7.0 = 33.6 MPG (US). This matches what you'd find on the US EPA window sticker for a comparable vehicle.
Limitations & notes
This converter performs a precise mathematical conversion between units but cannot account for real-world driving variability. Manufacturer-published MPG or L/100km figures are typically measured under standardised test cycles (EPA in the US, WLTP in Europe) that may not reflect real driving conditions — actual fuel consumption can differ by 10–25% depending on speed, load, terrain, driving style, tyre pressure, and climate. Always treat converted figures as equivalent estimates rather than guaranteed real-world performance. Additionally, this calculator does not convert for diesel vs. petrol energy differences — both fuel types use the same volumetric units, but their energy content per litre differs slightly. For precise energy-based comparisons, a fuel energy content correction would be required.
Frequently asked questions
Why are there two different MPG standards — US and UK?
The US gallon (3.78541 L) and the UK Imperial gallon (4.54609 L) are different volumes, a legacy of diverging measurement standards after American independence. A UK gallon is approximately 20% larger, so a car rated in UK MPG will always show a higher (more flattering) number than the same car rated in US MPG, even if it's the same vehicle. Always check which standard a manufacturer is using.
Is a lower L/100km always better?
Yes — L/100km is a consumption figure, meaning it tells you how much fuel you use. A car consuming 5.0 L/100km is more fuel-efficient than one consuming 8.0 L/100km. This is the opposite logic from MPG, where higher numbers are better. The inverse relationship can cause confusion when comparing specs across markets.
What is a good fuel efficiency figure in L/100km?
For a modern petrol car, under 6.0 L/100km is considered excellent, 6–8 L/100km is average, and above 10 L/100km is poor or typical of large SUVs and trucks. Diesel engines typically achieve 4.5–6.5 L/100km for similar-sized vehicles. Hybrid vehicles can reach below 4.0 L/100km, while electric vehicles are rated in kWh/100km instead.
How do I convert L/100km back to MPG?
The conversion is symmetric: MPG = C ÷ (L/100km), using the same constant (235.21 for US, 282.48 for UK). For example, 6.5 L/100km equals 235.21 ÷ 6.5 = 36.2 MPG (US). Because the relationship is an inverse, you simply divide the constant by whichever value you have.
Does this conversion work for electric vehicles or hybrids?
For plug-in hybrids, the liquid fuel portion (MPG or L/100km) can still be converted normally. However, fully electric vehicles are rated in MPGe (miles per gallon equivalent) in the US or kWh/100km in Europe — these involve energy equivalence calculations, not a direct volume conversion. This calculator is designed for conventional petrol and diesel (and standard hybrid) fuel economy figures only.
Last updated: 2025-01-15 · Formula verified against primary sources.