To calculate MPG, divide the miles driven by the gallons of fuel used. To calculate L/100 km, divide litres used by kilometres driven, then multiply by 100. The most useful fuel-purchase measurement compares the distance between two full refills with the fuel added at the second refill.
For example, 360 miles using 12 U.S. gallons gives 30 U.S. MPG. A separate metric example, 540 km using 42 litres, gives 7.78 L/100 km. These are hypothetical records, rather than ratings for a named vehicle. Below, we explain how to collect the inputs, convert the result and avoid averaging errors.
Have your distance and refill amount? The Fuel Economy Calculator gives you U.S. and imperial MPG, L/100 km and km/L from those two inputs.
Once you know your fuel economy, enter it with a distance and local fuel price in our Trip Fuel Cost Calculator. The calculator estimates a fuel budget from an efficiency figure; this guide shows how to obtain that figure from your records.
In this guide
- Understand MPG and L/100 km
- Measure consumption between full refills
- Follow the Canadian metric example
- Follow the U.S. MPG example
- Combine several fill-ups correctly
- Convert U.S. MPG, imperial MPG and L/100 km
- Compare your result with dashboard figures and ratings
MPG and L/100 km measure different sides of fuel economy
MPG measures distance per unit of fuel. A higher value means you travel farther on each gallon. L/100 km measures fuel per fixed distance. A lower value means you use fewer litres to travel 100 km. A change in the opposite numerical direction can therefore mean the same improvement.
Always label the gallon. A U.S. gallon and an imperial gallon have different volumes, so two figures both labelled “30 MPG” may not describe the same consumption. Litres remove that particular ambiguity, but you must still distinguish L/100 km from km/L.
Kilometres per litre is another distance-per-fuel measure: divide kilometres by litres. To turn km/L into L/100 km, divide 100 by km/L. Do not enter a km/L reading into a calculator field asking for L/100 km.
Measure fuel economy between two full refills
The U.S. Department of Energy and EPA explain in their My MPG guidance that fuel-purchase calculations need at least two consecutive records where the tank is filled completely. That provides a starting and ending point for the fuel interval.
- At the starting refill, follow your vehicle’s normal filling procedure, record the odometer and note the units. Reset a trip odometer if you want an additional distance record.
- Drive normally. Keep receipts for every fuel purchase during the interval. You do not need to empty the tank before measuring.
- At the next full refill, record the new odometer reading and the quantity of fuel dispensed. Keep the starting and ending fill levels as comparable as practical.
- Calculate distance travelled by subtracting the starting odometer from the ending odometer. Divide by fuel volume for MPG, or divide fuel volume by distance and multiply by 100 for L/100 km.
The fuel bought at the starting refill belongs before the measured interval. For a simple interval without intermediate purchases, use the quantity bought at the ending refill. Do not add the initial full tank’s volume to it, and do not substitute the tank’s advertised capacity for measured fuel used.
If you make partial purchases before the next full refill, keep all those receipts. Add the intermediate quantities and the ending full-refill quantity, then use the complete distance from the initial full refill to the final one. Missing a purchase makes the calculated economy look better than it really was.
Canadian example: calculate L/100 km from a refill
| Record | Hypothetical value |
|---|---|
| Starting odometer at full refill | 80,000 km |
| Ending odometer at next full refill | 80,540 km |
| Fuel added at ending refill | 42.0 L |
The distance is 80,540 − 80,000 = 540 km. Consumption is 42 ÷ 540 × 100 = 7.777… L/100 km, which rounds to 7.78 L/100 km. The equivalent distance-per-litre result is 540 ÷ 42 = about 12.86 km/L.
Keep the unrounded result for later calculations when possible. Displaying 7.8 L/100 km is reasonable for a simple log, but repeatedly rounding intermediate figures can create small differences when estimating fuel for a long journey.
U.S. example: calculate gas mileage in MPG
Suppose your starting odometer is 24,100 miles. At the next full refill it reads 24,460 miles, and the pump dispenses 12.0 U.S. gallons. There were no purchases between the two refills.
Distance travelled is 24,460 − 24,100 = 360 miles. Fuel economy is 360 ÷ 12 = 30 U.S. MPG. The fuel quantity comes from the pump receipt, not the amount of money paid. Price changes affect your bill, but they do not change the definition of MPG.
If your receipts show litres while your odometer shows miles, convert the fuel volume to the intended gallon first. Alternatively, convert the distance to kilometres and calculate L/100 km. Writing the units beside every input makes mistakes easier to spot.
Combine several fill-ups using total distance and total fuel
For a combined MPG figure, add the miles and gallons separately, then divide total miles by total gallons. A simple arithmetic average of the displayed MPG values can give the wrong answer.
| Measured interval | Distance | U.S. gallons |
|---|---|---|
| First interval: 30 MPG | 300 miles | 10 |
| Second interval: 20 MPG | 100 miles | 5 |
| Combined records | 400 miles | 15 |
The combined result is 400 ÷ 15 = 26.67 U.S. MPG. Averaging 30 and 20 would give 25 MPG, which does not represent these records. The first interval accounts for more of the driving.
Use the same totals-first principle for metric records: combined L/100 km = total litres ÷ total kilometres × 100. For example, 42 litres over 540 km plus 35 litres over 500 km gives 77 ÷ 1,040 × 100 = 7.40 L/100 km. Do not give every individual tank an equal weight unless that weighting actually fits the calculation.
Convert between U.S. MPG, imperial MPG and L/100 km
MPG and L/100 km are reciprocal measures, so the conversion uses division rather than a fixed multiplier. Using rounded conversion constants:
- L/100 km = 235.215 ÷ U.S. MPG.
- U.S. MPG = 235.215 ÷ L/100 km.
- L/100 km = 282.481 ÷ imperial MPG.
- Imperial MPG = 282.481 ÷ L/100 km.
NIST’s conversion tables provide the U.S. MPG relationship and the relevant distance and gallon definitions. Natural Resources Canada also publishes U.S. and imperial conversion formulas, using constants rounded to two decimal places.
| L/100 km | U.S. MPG | Imperial MPG |
|---|---|---|
| 5.0 | 47.04 | 56.50 |
| 8.0 | 29.40 | 35.31 |
| 10.0 | 23.52 | 28.25 |
For the earlier U.S. example, 30 U.S. MPG is about 7.84 L/100 km. In contrast, 30 imperial MPG is about 9.42 L/100 km. They are not interchangeable. The same 30 U.S. MPG can also be expressed as about 36.03 imperial MPG, without any change to the vehicle’s actual fuel use.
Dashboard averages, published ratings and your own log
Check the distance period before comparing figures. A dashboard average since your last reset, a single refill interval and several months of fuel records may cover different journeys. An instantaneous display is a momentary reading, rather than an average for the whole tank.
If the dashboard and refill calculation disagree, first check the units, reset dates, distance covered and whether every fuel purchase was recorded. Differences in the starting and ending fill levels can also distort a single interval. A repeated discrepancy deserves a careful review of the records; one result alone does not prove a fault in the vehicle or dashboard.
Natural Resources Canada explains that standardized ratings help compare vehicles, while actual consumption varies with driving, speed, weather, traffic, vehicle condition and accessories. Compare your records with the correct model and relevant city or highway conditions rather than expecting every trip to match a combined rating.
Track enough comparable intervals to see a pattern. Note unusual traffic, a roof load, towing or very different seasonal conditions. That makes the log useful for planning similar journeys, instead of treating the best recorded tank as your guaranteed everyday result.
To put your result in context, compare gas mileage benchmarks by vehicle type. The guide shows official ratings for a sedan, SUV, pickup and hybrid, plus what different MPG figures can mean for your annual fuel bill.
Use the measured result in a cost calculator
Choose the units carefully in the Trip Fuel Cost Calculator: use L/100 km with kilometres and a price per litre, or U.S. MPG with miles and a price per U.S. gallon. Add your one-way distance, choose a return trip if appropriate and enter the people sharing fuel costs.
For a fuller itinerary and shared-expense example, read How to Calculate Fuel Cost for a Road Trip. For annual gas-versus-hybrid fuel savings, the Hybrid vs Gas Cost Calculator uses your driving distance, consumption and price assumptions. Neither tool measures MPG directly from refill records.
For a plug-in hybrid, a high liquid-fuel MPG figure can exclude electricity used between refills. Keep charging energy and its cost in the comparison. Battery-electric vehicles use electricity-consumption measures such as kWh/100 km or kWh/100 miles, rather than ordinary liquid-fuel MPG. Our home EV charging cost guide explains how to budget for that electricity.
Methodology and sources
All example odometers, distances and fuel volumes in this guide are hypothetical. The refill calculations, combined averages and conversion table are original calculations by Drive Cost Calculator. Results are rounded for display, with unrounded values retained for the underlying arithmetic. This guide does not assign a universal “good MPG” threshold or claim a particular vehicle will achieve an example figure.
- U.S. DOE and EPA: My MPG Help, for fuel-purchase records and consecutive full refills.
- Natural Resources Canada: Understanding the tables, for rating units and conversion formulas.
- Natural Resources Canada: Understanding fuel consumption ratings, for the relationship between published ratings and actual driving.
- NIST: Guide to the SI, Appendix B.9, for unit conversion factors.
Read our Methodology & Data Sources for the site’s calculation approach, or contact us if you find an error.
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