Calculator
What the Fuel Economy Converter does
Fuel Economy Converter: converts Litres per 100 km to Miles per gallon (US) through fuel economy's base unit, so the result is exact rather than rounded. Example: 8 L/100km = 29.40182288 mpg (US).
Supply Value, From and To and you get Result, How it was calculated and Reverse conversion back. Edit a field and the answer updates in place.
Typical users are anyone working across two measurement systems. Having the exact factor matters more than a rough mental estimate.
There is no back end here. The calculation runs where you are sitting, not on a server somewhere.
The rest of this page documents each field, the method, and a worked example built from the values the fuel economy converter loads by default.
What do the Fuel Economy Converter fields mean?
The Fuel Economy Converter uses 3 inputs. You can work from the defaults, or clear them and start fresh, both work.
| Field | What to enter | Default |
|---|---|---|
| Value | required | 8 |
| 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) | Litres per 100 km (L/100km) |
| 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) | Miles per gallon (US) (mpg (US)) |
How does the Fuel Economy Converter work?
Because they are reciprocals, you cannot convert between them by multiplying: halving L/100km does not halve mpg (US). The conversion goes through litres per 100 km, and small differences matter: the US and imperial gallons are not the same size.
The constants come from the SI brochure and the 1959 international yard-and-pound agreement. For quantities defined empirically rather than by treaty, the current CODATA or IAU value is used instead.
Worked example
These are the values the Fuel Economy Converter loads by default, and the result it produces from them. Here is the calculation as it stands the moment the page loads.
| Value | 8 |
|---|---|
| From | Litres per 100 km (L/100km) |
| To | Miles per gallon (US) (mpg (US)) |
| Result | 29.40182288 mpg (US) |
|---|---|
| How it was calculated | 8 L/100km → 29.40182288 mpg (US) (via L/100km) |
| Reverse conversion | 29.40182288 L/100km (if you meant 8 mpg (US)) |
Fuel economy: an inverse relationship
Miles per gallon (mpg) and litres per 100 km (L/100km) measure fuel economy in opposite directions (mpg gets bigger as efficiency improves, while L/100km gets smaller), which means the two figures are inversely, not linearly, related, and cannot simply be scaled by a fixed multiplier.
Worth knowing
- This inverse relationship creates a genuine, well-documented trap: you cannot average mpg values directly across a trip. Driving 20 mpg outbound and 40 mpg on the return leg does not average to 30 mpg overall: converting each leg to L/100km (or gallons used), averaging correctly, and converting back gives the true combined figure, which in this example comes out closer to 26.7 mpg.
- The mismatch matters most at the efficient end of the scale: the fuel-consumption difference between 10 and 20 mpg (going from a gas-guzzler to a merely inefficient car) is much larger in absolute fuel saved than the difference between 40 and 50 mpg (already-efficient cars getting a bit better), a genuinely counter-intuitive result of the inverse relationship that L/100km actually represents more transparently.
- Also confirm which gallon a "mpg" figure uses: US and UK/imperial gallons differ by about 20%, and mixing them up compounds directly on top of the inverse-relationship trap above.
Definitions: BIPM SI Brochure · NIST reference on units
How to use it
- Put 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 tool reacts immediately rather than waiting for you to finish the form.
- The results panel then shows Result, How it was calculated and Reverse conversion.
- Copy Link is the quickest way to send the exact scenario to someone else.
What this tool does not do
- Repeated conversions compound rounding, so convert once from the original value where possible.
- Factors use the internationally agreed definitions; some industries round them differently.
- There is no audit trail: if you need one, record the inputs alongside the output.
- Read the field labels once before trusting the answer; they specify the expected unit.
Frequently asked questions
You need Value, From and To. Nothing starts empty, meaning you can explore the calculation before committing to real figures.
With the values loaded when the page opens, result comes out as 29.40182288 mpg (US). Any edit re-runs the calculation, so there is no stale number on screen.
No. Your browser is doing the computing, so we could not see the values even if we wanted to.
Nothing at all. There is no registration, no paywall and no per-use quota.
The conversion is exact for the method described above. Where a value is rounded for display, the underlying figure keeps full precision.