Calculator
What the Miles per Gallon Converter does
Miles per Gallon Converter: converts Miles per gallon (US) to Litres per 100 km through fuel economy's base unit, so the result is exact rather than rounded. Example: 30 mpg (US) = 7.8404861 L/100km.
With Value, From and To entered, the Miles per Gallon Converter works out Result, How it was calculated and Reverse conversion. Everything recomputes live, which makes it easy to test a few scenarios in a row.
Converting once from the original figure beats chaining approximations. The Miles per Gallon Converter exists to take that particular chore off your hands.
Your data stays on your machine. Once the page has loaded you could disconnect entirely and it would still work.
Field definitions, the method, and a worked example from the default values are all set out further down the page.
What do the Miles per Gallon Converter fields mean?
The Miles per Gallon Converter uses 3 inputs. It opens with a complete example loaded, which is usually the quickest way to understand what a field means.
| Field | What to enter | Default |
|---|---|---|
| Value | required | 30 |
| From | choose from Litres per 100 km (L/100km), Kilometres per litre (km/L), Miles per gallon (US) (mpg (US)), Miles per gallon (imperial) (mpg (imp)), Miles per litre (mi/L) or Gallons per 100 miles (US) (gal/100mi) | Miles per gallon (US) (mpg (US)) |
| To | choose from Litres per 100 km (L/100km), Kilometres per litre (km/L), Miles per gallon (US) (mpg (US)), Miles per gallon (imperial) (mpg (imp)), Miles per litre (mi/L) or Gallons per 100 miles (US) (gal/100mi) | Litres per 100 km (L/100km) |
How does the Miles per Gallon Converter work?
Because they are reciprocals, you cannot convert between them by multiplying: halving mpg (US) does not halve L/100km. The conversion goes through litres per 100 km, and small differences matter: the US and imperial gallons are not the same size.
Exact definitions are preferred throughout: the metre, the inch and the pound all have them. Only where a unit depends on a measured constant is an accepted published value substituted.
Worked example
These are the values the Miles per Gallon Converter loads by default, and the result it produces from them. The numbers below come straight from the default inputs, so you can verify them against the live tool.
| Value | 30 |
|---|---|
| From | Miles per gallon (US) (mpg (US)) |
| To | Litres per 100 km (L/100km) |
| Result | 7.8404861 L/100km |
|---|---|
| How it was calculated | 30 mpg (US) → 7.8404861 L/100km (via L/100km) |
| Reverse conversion | 7.8404861 mpg (US) (if you meant 30 L/100km) |
Miles per gallon: US vs. imperial
US miles per gallon (mpg US) and imperial/UK miles per gallon (mpg UK) describe the same physical fuel efficiency using genuinely different-sized gallons. The imperial gallon is about 20% larger than the US gallon, so an identical car’s efficiency produces a noticeably higher "mpg" number when quoted in the UK convention.
Worth knowing
- A car that gets 40 mpg (US) would show roughly 48 mpg if the identical fuel consumption were expressed in mpg (imperial) instead. Not because the car is more efficient, but purely because the imperial gallon holds more fuel, so fewer of them are needed to travel the same distance.
- This is a genuinely common source of confusion when comparing car reviews or spec sheets between US and UK/Commonwealth sources, always check which gallon a quoted mpg figure assumes before comparing it directly to another source.
- Also remember the inverse relationship between mpg and metric L/100km figures (covered in the fuel-economy-converter facts): averaging mpg figures directly across multiple trips or vehicles produces a mathematically incorrect result regardless of which gallon is used.
Definitions: BIPM SI Brochure · NIST reference on units
How to use it
- Type Value.
- Pick a value for From: the options are Litres per 100 km (L/100km), Kilometres per litre (km/L), Miles per gallon (US) (mpg (US)), Miles per gallon (imperial) (mpg (imp)), Miles per litre (mi/L) or Gallons per 100 miles (US) (gal/100mi).
- Pick a value for To: the options are Litres per 100 km (L/100km), Kilometres per litre (km/L), Miles per gallon (US) (mpg (US)), Miles per gallon (imperial) (mpg (imp)), Miles per litre (mi/L) or Gallons per 100 miles (US) (gal/100mi).
- The answer follows your cursor, in effect. Change a field and read the new figure.
- Look at Result, How it was calculated and Reverse conversion in the output panel.
- To keep the result, either copy the link or print the page. Both preserve what is on screen.
What this tool does not do
- Conversion factors use SI definitions; some industries use their own rounded factors.
- Repeated conversions compound rounding, so convert once from the original value where possible.
- Precision beyond what the inputs justify is not meaningful, however many digits appear.
- Garbage in, garbage out: confirm each figure means what you think it does before trusting the result.
Frequently asked questions
3 things: Value, From and To. Defaults are loaded for all of them, so nothing is mandatory before you see a result.
With the values loaded when the page opens, result comes out as 7.8404861 L/100km. Try changing one value. The answer refreshes without a page reload.
Not at all: the whole thing runs offline once the page has loaded, which tells you nothing is being sent.
It is free with no conditions: no sign-up, and no limit on how often you use it.
It is exact within the stated method. Anything the method does not model is simply out of scope.