What this calculator does
This fuel consumption calculator works out your vehicle's real consumption rate from two numbers you can read directly off a fill-up: the distance you drove since the last full tank, and how much fuel it took to fill it back up. That is the standard way to calculate fuel consumption in practice, since the figure quoted on a car's specification sheet is a laboratory estimate, not what any particular vehicle, driver or trip actually achieves.
The result is shown in litres per 100 kilometres, kilometres per litre and miles per gallon at once, since different countries and different car specifications default to different units. A rough cost per 100 km is included too, using whatever fuel price you enter, so the consumption figure translates into something closer to a running cost.
The formula
Litres per 100 km is fuel used divided by distance driven, multiplied by 100. Kilometres per litre is the reciprocal, scaled to a single litre. Miles per gallon is found by converting through the fixed relationship between litres-per-100km and gallons-per-mile: 235.215 for US gallons, 282.481 for imperial gallons, both exact conversion constants rather than estimates.
| Term | Meaning |
|---|---|
| L/100km | Litres of fuel used per 100 kilometres travelled, the standard consumption figure in most of the world outside North America. |
| km/L | Kilometres travelled per litre of fuel, the inverse of L/100km, sometimes preferred for smaller or more efficient vehicles. |
| mpg | Miles travelled per gallon of fuel. US and imperial (UK) gallons are different sizes, so US mpg and imperial mpg are not directly comparable figures. |
The inputs explained
| Field | What to enter |
|---|---|
| Distance travelled (km) | The distance covered since the tank was last filled. |
| Fuel used (L) | The amount of fuel it took to fill the tank back up, in litres. |
| Fuel price (for cost estimate) ($/L) | The price you paid per litre, used only to estimate a cost per 100 km. |
When to use it
Checking a car against its advertised figure
Manufacturer fuel economy figures are measured under standardised lab conditions. Calculating your own consumption from a real fill-up shows how far your actual driving, terrain and load sit from that laboratory number.
Comparing two vehicles or two driving styles
Running the same trip, or the same commute, through this calculator for two different cars or two different drivers puts the comparison in one consistent unit rather than relying on the dashboard's own estimate.
Budgeting fuel cost for regular driving
Once the actual consumption rate is known from a fill-up, it can be used to estimate ongoing running costs far more accurately than a specification-sheet figure.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does fuel used change the consumption rate over a fixed 500 km?
The same 500 km trip, refuelled with a range of different amounts.
| Fuel used to refill | Fuel consumption | Cost per 100 km at this rate |
|---|---|---|
| 30 L | 6.00 L/100km | $11.70 |
| 35 L | 7.00 L/100km | $13.65 |
| 40 L | 8.00 L/100km | $15.60 |
| 45 L | 9.00 L/100km | $17.55 |
| 50 L | 10.00 L/100km | $19.50 |
| 55 L | 11.00 L/100km | $21.45 |
How does distance change the consumption rate for a fixed 45 L fill-up?
The same 45-litre fill-up, attributed to a range of different distances.
| Distance driven | Fuel consumption | Miles per gallon (US) |
|---|---|---|
| 400 km | 11.25 L/100km | 20.91 mpg |
| 500 km | 9.00 L/100km | 26.14 mpg |
| 600 km | 7.50 L/100km | 31.36 mpg |
| 700 km | 6.43 L/100km | 36.59 mpg |
| 800 km | 5.63 L/100km | 41.82 mpg |
Questions
Why does my calculated consumption differ from the manufacturer's figure?
Manufacturer figures come from a standardised laboratory test cycle, not real roads. Traffic, hills, cold starts, air conditioning, tyre pressure and driving style all push real-world consumption away from that lab number, usually higher.
Is a lower L/100km always better?
For a given vehicle, yes, a lower L/100km means less fuel for the same distance. It is not meaningful to compare L/100km across very different vehicle types (a small hatchback against a loaded truck) as a measure of anything beyond fuel use itself.
What is the difference between US mpg and imperial mpg?
A US gallon is about 3.785 litres and an imperial (UK) gallon is about 4.546 litres, roughly 20 per cent larger. The same car therefore reports a higher mpg figure in imperial gallons than in US gallons, despite identical real consumption.
How accurate is a single fill-up calculation?
One fill-up captures whatever mix of driving happened over that tank, so it can be skewed by an unusual trip. Averaging over several consecutive fill-ups gives a more representative long-run consumption figure than any single one.
To convert an already-known consumption figure between units instead of calculating it from a trip, see the fuel economy converter. To estimate the cost and time for a specific upcoming journey, use trip fuel cost.