FuelTripCost guide
MPG vs L/100km: What's the Difference?
By FuelTripCost · Published 27 September 2026 · Updated 30 September 2026
Quick answer
MPG measures distance traveled per gallon; L/100 km measures litres consumed over 100 kilometres. Higher MPG is more efficient, while lower L/100 km is more efficient.
L/100 km = 235.214583 ÷ US MPG, or 282.480936 ÷ Imperial MPG.
What you need
- The original economy value.
- Whether an MPG figure uses a US gallon or an Imperial gallon.
- The output unit needed for the comparison, specification, or calculator.
Step-by-step method
- Identify whether the source is US MPG, Imperial MPG, L/100 km, or km/L.
- For MPG, confirm the gallon system before choosing a conversion constant.
- Divide the correct constant by the input value; the same operation converts in either direction between MPG and L/100 km.
- Keep several decimal places during the calculation and round only the displayed answer.
- Interpret direction correctly: higher MPG and km/L are more efficient, while lower L/100 km is more efficient.
Worked examples
Core example
A car rated at 30 US MPG uses about 7.84 L/100 km. A vehicle at 6 L/100 km uses about 12 litres on a 200 km trip.
30 US MPG to L/100 km
235.214583 ÷ 30 = 7.8404861, which is about 7.84 L/100 km.
30 Imperial MPG to L/100 km
282.480936 ÷ 30 = 9.4160312, or about 9.42 L/100 km. The result differs because an Imperial gallon is larger.
6 L/100 km to both MPG systems
235.214583 ÷ 6 = about 39.20 US MPG. Using the Imperial constant gives 282.480936 ÷ 6 = about 47.08 Imperial MPG.
Reverse conversion check
Convert 25 US MPG to 9.40858332 L/100 km. Dividing 235.214583 by the unrounded metric result returns 25 US MPG.
Trip fuel from L/100 km
At 6 L/100 km, a 200 km trip uses 200 × 6 ÷ 100 = 12 litres. Conversion changes how economy is written; it does not change the fuel physically used.
| US MPG | L/100 km | Imperial MPG | Interpretation |
|---|---|---|---|
| 20 | 11.76 | 24.02 | Higher consumption |
| 25 | 9.41 | 30.02 | — |
| 30 | 7.84 | 36.03 | — |
| 35 | 6.72 | 42.03 | Lower consumption |
What MPG measures
Miles per gallon is a distance-per-volume measure. It answers how many miles a vehicle travels for one gallon of fuel. A larger number means the vehicle covers more distance with that gallon.
The label is incomplete unless the gallon is identified. US MPG uses a 3.785411784-litre US gallon. Imperial MPG uses a 4.54609-litre Imperial gallon, so the same physical consumption produces a larger Imperial MPG number.
What L/100 km measures
Litres per 100 kilometres is a volume-per-distance measure. It answers how many litres the vehicle consumes to travel 100 kilometres. A smaller number means less fuel is used over that fixed distance.
For example, 8 L/100 km uses 16 litres over 200 km, while 6 L/100 km uses 12 litres. The second vehicle uses four fewer litres over that example distance.
Why the relationship is inverse
MPG places distance in the numerator, while L/100 km places fuel volume in the numerator. Because the ratio is reversed, the conversion is division by a constant rather than multiplication by a fixed factor.
The practical effect is nonlinear. Moving from 20 to 25 US MPG reduces consumption by about 2.35 L/100 km, while moving from 30 to 35 MPG reduces it by about 1.12 L/100 km. A five-MPG improvement does not save the same fuel at every starting point.
Conversion formulas
Use 235.214583 for conversions between US MPG and L/100 km. Use 282.480936 for conversions between Imperial MPG and L/100 km. The formulas work in both directions because each pairs reciprocal measures.
For km/L, divide 100 by L/100 km. To convert km/L back, divide 100 by km/L. Keep the original unit label with every intermediate value.
- L/100 km = 235.214583 ÷ US MPG
- US MPG = 235.214583 ÷ L/100 km
- L/100 km = 282.480936 ÷ Imperial MPG
- Imperial MPG = 282.480936 ÷ L/100 km
- km/L = 100 ÷ L/100 km
How to interpret and use a conversion
Use converted figures to compare the same underlying consumption across regional specifications. In common usage, US vehicle information often uses US MPG, UK references often use Imperial MPG, and many other markets use L/100 km or km/L. Always follow the unit printed by the source rather than assuming from the word MPG alone.
When estimating money, conversion of economy is only one step. Distance and fuel price must also match the chosen system. Pair US MPG with miles and price per US gallon, Imperial MPG with miles and price per Imperial gallon, or L/100 km with kilometres and price per litre. The synchronized calculator presets handle those combinations.
Rounding without changing the comparison
Carry the full conversion result into later calculations and round only what is displayed. Repeatedly converting a value rounded to one decimal place can create a visible difference on the return conversion.
Two displayed values can appear equal after rounding even when their source values differ slightly. For budgeting, sensible display precision is useful; for a multi-step calculation, retain the unrounded value until the final result.
Real-world considerations
- Check whether an MPG value uses US or Imperial gallons.
- The conversion changes units, not the underlying vehicle performance.
- Convert price volume units separately when estimating money.
Common mistakes
- Using the US constant for Imperial MPG or leaving the gallon type unlabeled.
- Thinking higher L/100 km is better because higher MPG is better.
- Multiplying by the constant instead of dividing.
- Converting economy but forgetting to convert distance and price units for a cost calculation.
- Rounding an intermediate conversion too aggressively and then converting it back.
How to make the estimate more useful
- Label US or Imperial MPG every time.
- Compare vehicles only after putting both values in the same unit.
- Use the Fuel Economy Calculator when the original distance and fuel-volume measurements use different systems.
- For trip planning, use a measured or representative economy value and test a conservative scenario.
Limits of this calculation
A conversion changes notation, not vehicle performance. It cannot reconcile different test procedures, driving conditions, loads, seasons, or measurement errors. Rounded displayed values can differ slightly when converted back and forth.
Questions
Why are the numbers inverse?
MPG is distance per fuel volume, while L/100 km is fuel volume per fixed distance. Improving economy raises one and lowers the other.
Why isn't MPG converted with one fixed multiplier?
MPG and L/100 km are reciprocal measures: distance per volume versus volume per distance. Their relationship requires division by a constant.
Is km/L the same as L/100 km?
No. km/L is distance per litre and rises with efficiency; L/100 km is fuel per fixed distance and falls with efficiency.
Which MPG does a UK figure normally use?
UK MPG normally refers to an Imperial gallon, but confirm the source label rather than relying only on location.
Does converting a rating make it a real-world result?
No. Conversion preserves the source figure. It does not account for traffic, weather, load, test method, or driver behaviour.
Use the calculation
Try the Fuel Economy Calculator or review the calculation methodology and unit constants.