What the Mileage Calculator (MPG and km/L) does
This mileage calculator works out your vehicle's real fuel economy from what you actually put in the tank: enter your fill-ups - odometer reading and fuel added - and it gives the average in kilometres per litre, litres per 100 km, US miles per gallon and UK miles per gallon, the figure for every tank, the trend across tanks, and the fuel cost per kilometre and per mile at the prices you paid.
It uses the full-tank method recommended by fuel-economy guides, which is far more reliable than the dashboard estimate: fill to full, note the odometer, and at the next full fill-up the fuel you add is the fuel you used. Top-ups in between are handled automatically. For a single trip, or to convert a figure such as 50 UK mpg into km/L, switch mode at the top. Your log stays in your browser.
How to use it
- Choose your units: kilometres or miles, and litres, US gallons or UK gallons. Type the currency symbol you pay in if you want costs.
- Fill the tank to full and enter the odometer reading and the fuel added; this first row is only the starting point. Leave Full tank ticked.
- Each time you fill up, add a row with the new odometer reading, the fuel added and the price per litre or gallon. For a top-up that does not fill the tank, untick Full tank.
- Read the average at the top in your main unit, the other three units in the tiles, and the cost per km and per mile. The table and chart show each full tank separately.
- Check the trend note: after four or more tanks it tells you whether economy is steady, improving or getting worse, and how the latest tank compares with the average.
Reading the results
Kilometres per litre and miles per gallon are 'more is better' figures; litres per 100 km is 'less is better', which is why Europe uses it on official labels - it tells you directly how much fuel a journey needs. At 10 km/L the two numbers happen to be equal: 10 km/L is 10 L/100 km.
US and UK mpg are not the same unit. A UK gallon is 4.546 litres and a US gallon 3.785 litres, so the same car shows about 20% more miles per gallon on a British figure. Always check which gallon a quoted mpg uses.
The average is total distance divided by total fuel, not the average of the per-tank figures, so a long motorway tank counts for more than a short town tank - just as it did in the fuel you actually bought.
Tank-to-tank figures vary by several per cent for ordinary reasons: pumps shut off at slightly different levels, and traffic, weather, air conditioning and loads change. Look at the trend over several tanks rather than reacting to one.
Worked example: five fill-ups in a small petrol car
The first fill-up at 24,310 km is the starting point. At 24,822 km the tank took 30.4 litres: 512 km / 30.4 L = 16.84 km/L, or 5.94 L/100 km.
At 25,060 km the driver added only 12.0 litres (a top-up), then filled to full at 25,371 km with 15.9 litres. Those two fills cover 549 km on 27.9 L: 19.68 km/L. Counting the top-up as its own tank would have produced two nonsense figures instead of one correct one.
The last tank, 507 km on 31.2 L, is 16.25 km/L. Overall the car covered 1,568 km on 89.5 L, an average of 17.52 km/L - 5.71 L/100 km, 41.2 US mpg or 49.5 UK mpg. At the prices paid the fuel cost 9,202.97, or about 5.87 per kilometre.
Formulas and scoring rules
- Kilometres per litre
km/L = distance (km) / fuel (L)- Litres per 100 km
L/100 km = 100 / (km/L) = fuel (L) / distance (km) x 100- US miles per gallon
mpg(US) = km/L x 3.785411784 / 1.609344 = km/L x 2.35211 US gallon = 3.785411784 L and 1 mile = 1.609344 km, both exact.- UK miles per gallon
mpg(UK) = km/L x 4.54609 / 1.609344 = km/L x 2.82481 imperial gallon = 4.54609 L exactly.- Full-tank interval
distance = odometer now - odometer at the last full tank; fuel = all fuel added since thenPartial fills are added to the next full fill. The first full tank only sets the starting point.- Average over the log
total distance / total fuel- Cost per distance
fuel spend (excluding the first tank) / distanceShown to 2 decimal places, rounded half away from zero.- Trend
least-squares slope of economy against tank number, as % of the average per tankNeeds at least four full tanks; under 2% per tank is reported as steady.
How this differs from the fuel cost and mileage reimbursement calculators
The Fuel Cost Calculator prices a trip from an economy figure you already know, and the Mileage Reimbursement Calculator applies an allowance rate per mile or kilometre. This page is the step before both: it measures the economy itself from your own fill-up records and tracks how it changes from tank to tank, so the number you plug into a trip budget is your car's real figure rather than the brochure's.
Why your result differs from the official figure
Official economy figures - WLTP in Europe, the UK and India's newer type approvals, EPA ratings in the US - come from standard laboratory test cycles so that cars can be compared fairly. Real driving includes cold starts, short trips, hills, idling, roof boxes and air conditioning, so most drivers record somewhat worse figures than the label. A logged figure that is consistently far worse than usual for your own car is the useful warning sign: under-inflated tyres, a dragging brake or a sensor fault can all show up there first.
Limitations: what the result does not prove
- Accuracy depends on filling to the same level each time. Pump auto shut-off varies, so treat a single tank as a sample and the multi-tank average as the result.
- It measures liquid fuel. Electric vehicles use kWh per 100 km or miles per kWh, and gas vehicles (CNG, LPG) are often sold by mass; enter LPG in litres only if that is how you bought it.
- Odometer errors of a few per cent are normal and pass straight into the economy figure. A GPS-measured distance for a long trip is a good check.
- Costs are only the fuel you entered. Maintenance, tyres, depreciation and insurance are part of the true running cost per kilometre but are not included.
Privacy: where your data goes
Everything you paste, type or drop is processed in this browser tab. It is not uploaded, logged, stored or sent to analytics. Session recording and tag-manager scripts are switched off on this page.
Standards and sources
- US DOE / EPA - fueleconomy.gov vehicle efficiency figures - checked 19 Sep 2026
- US Department of Energy and EPA - fueleconomy.gov
- NIST Special Publication 811 - Guide for the Use of the SI (US gallon and mile definitions)
- UK Weights and Measures Act 1985, Schedule 1 (gallon = 4.54609 litres)
Frequently asked questions
How do I calculate my car's mileage in km per litre?
Fill the tank to full and note the odometer. At the next full fill-up, divide the kilometres driven since then by the litres you put in. 512 km on 30.4 litres is 16.8 km/L. Doing this over several tanks gives a dependable average.
How do I convert L/100 km to mpg?
For US mpg divide 235.215 by the L/100 km figure; for UK mpg divide 282.481 by it. So 6 L/100 km is about 39.2 US mpg or 47.1 UK mpg. The Convert a figure mode does this for any of the four units.
Why is UK mpg higher than US mpg for the same car?
Because the UK (imperial) gallon is bigger: 4.546 litres against 3.785 litres for the US gallon. The car goes further on the larger gallon, so its UK mpg is about 1.2 times its US mpg even though the fuel used per kilometre is identical.
What should I do with a partial fill-up?
Enter it and untick Full tank. Its fuel is added to the next full fill-up, so the interval from one full tank to the next stays correct. A partial fill on its own cannot be measured because you do not know how much fuel was left in the tank.
Is my dashboard fuel economy reading accurate?
It is an estimate from the engine's fuel-injection data and is often a few per cent optimistic. Comparing it with the full-tank figure from your fill-up log over several tanks shows how far off your particular car's display is.
How much does my car cost per kilometre in fuel?
Add the price per litre (or gallon) to each fill-up and the calculator divides what you spent by the distance covered, excluding the first tank, which only sets the starting point. It shows the cost per kilometre and per mile at the prices you actually paid.
Does the mileage calculator save my fill-up log?
No. The log exists only in this open page, so download the CSV if you want to keep it, and paste it back next time with the Paste a log option. Nothing is sent to a server or stored in the browser.
Last reviewed by the A2Z.Tools team against the sources listed above.
