Fuel Efficiency (Mileage) Calculator: km/L, MPG, L/100km + Fill-up Log

Fuel Efficiency Calculator

km/l · MPG · l/100km · fuel cost · CO₂ · fill-up log

Gallon standard

Fuel type (for CO₂)

Fuel Price (per litre) — your local price Prices vary by region and date — always enter your own current local value. No fixed price is assumed.

Distance & Fuel

Distance (km)

Fuel Consumed (litres)

Planned Trip Distance (km, optional)

Each row: distance since last fill-up (km) + fuel added (L). Per-tank mileage and a running average are computed across all rows.

Result

Enter distance and fuel to get mileage in km/l, km/gallon, MPG and l/100km at once, plus cost per km, optional trip cost and a CO2 estimate — or use the fill-up log for a running average over time

Mileage figures are only as accurate as your distance and fuel readings — for best results, brim the tank to the same cut-off each time and measure over several hundred kilometres. US and UK gallons differ (3.785 L vs 4.546 L), so MPG figures are not comparable across the two standards. The CO2 estimate uses a typical tailpipe emission factor per litre and does not include well-to-tank (production/transport) emissions. Fuel prices change frequently, so the cost figures are only as current as the price you enter.

Fuel Efficiency Calculator: km/L, MPG, L/100km, Fuel Cost and a Fill-up Log

Different parts of the world read fuel economy in completely different units, km/l across most of Asia, MPG in the US and UK (which use different-sized gallons) and L/100km across most of Europe, so comparing a car’s efficiency across regions usually means a manual conversion. This fuel consumption calculator and MPG calculator reports all four at once, km/l, km per gallon, MPG and L/100km calculator results, from a single distance-and-fuel entry and correctly separates US and UK gallons since a US gallon (3.785 L) and a UK gallon (4.546 L) are genuinely different volumes. A multi-fill-up tracking mode logs odometer readings and fuel added across a trip history to compute per-tank mileage and a running average over time, exportable to CSV. A cost-per-distance calculator and trip-cost estimator use your own current local fuel price, never an assumed fixed value, an efficiency comparison table shows where your figure falls among small car, sedan, SUV and truck categories and a bonus CO2 estimate covers petrol, diesel and CNG.

How to Use

This tool has two tabs: Single Calculation and Fill-up Log.

Setting up your shared settings

Select your Gallon standard: US gallon (3.785 L) or UK gallon (4.546 L), since MPG figures are not comparable across the two without knowing which one applies.
Select your Fuel type (for CO2): Petrol (2.31 kg/L), Diesel (2.68 kg/L) or CNG (2.75 kg/kg), used only for the CO2 estimate.
Enter your Fuel Price (per litre), your own current local price. No fixed value is assumed, since prices vary constantly by region and date.

Using Single Calculation mode

Enter your Distance in km and Fuel Consumed in litres for the period you’re checking.
Optionally enter a Planned Trip Distance in km if you want to estimate the fuel cost for an upcoming trip using this same efficiency figure.
Tap Calculate Mileage. The result shows your mileage in km/l, km per gallon, MPG and l/100km simultaneously, your cost per km, the estimated cost for your planned trip if entered, a CO2 estimate based on your selected fuel type and where your figure falls in the small car/sedan/SUV/truck comparison table.

Using Fill-up Log mode

Switch to the Fill-up Log tab. Tap Add Fill-up for each entry and enter the distance since the last fill-up in km and the fuel added in litres for that specific tank.
Repeat for every fill-up in your tracking history, then tap Calculate Log.
The result shows per-tank mileage for every individual fill-up plus a running average across your entire log, since a single tank can be skewed by traffic, weather or a short trip, while several tanks together give a far more reliable long-term figure.

Exporting your result

Use Download CSV (available for the Fill-up Log), Print / PDF for a clean printable copy or Copy to paste the figures elsewhere.

Key Features

Reports mileage in km/l, km per gallon, MPG, and l/100km simultaneously, from one single entry
US and UK gallon standards handled separately, since they’re genuinely different volumes (3.785 L vs 4.546 L)
Multi-fill-up tracking mode, logging odometer distance and fuel added across a trip history, computing both per-tank mileage and a running average
CSV export for your fill-up log, ready for a spreadsheet or fleet record
Cost-per-distance and trip-cost estimator, always using your own entered local fuel price, never a fixed assumption
Efficiency comparison table, showing where your figure sits among small car, sedan, SUV and truck categories
Bonus CO2 estimate covering petrol, diesel and CNG with their own emission factors.

Formula / Logic Used

Core Mileage (from Distance and Fuel)

$km/l = \frac{Distance\ (km)}{Fuel\ (L)}, \qquad l/100km = \frac{100 \times Fuel\ (L)}{Distance\ (km)}$

MPG (US and UK, Different Constants)

 $MPG_{US} = km/l \times 2.35215, \qquad MPG_{UK} = km/l \times 2.82481$

The two constants differ because a US gallon (3.785 L) and a UK/imperial gallon (4.546 L) are genuinely different volumes, so the same underlying km/l figure produces two different MPG numbers.

Running Average (Fill-up Log)

$Running\ Average = \frac{\sum Distance}{\sum Fuel}$

Averaging across multiple tanks this way, rather than averaging the individual per-tank km/l figures, correctly weights each tank by how much distance it actually covered.

Cost per Distance and Trip Cost

 $Cost\ per\ km = \frac{Fuel\ Price}{km/l}, \qquad Trip\ Cost = Cost\ per\ km \times Trip\ Distance$

CO2 Estimate

$CO_2\ (kg) = Fuel\ (L\ or\ kg) \times Emission\ Factor$

Where the emission factor is approximately 2.31 kg/L for petrol, 2.68 kg/L for diesel, or 2.75 kg/kg for CNG, reflecting the carbon content of each fuel type. This is a tailpipe-only estimate and does not include well-to-tank production and transport emissions.

Who Should Use This Tool

Vehicle owners tracking real-world fuel economy across fill-ups rather than relying on the factory-rated figure alone. Also useful for anyone comparing fuel economy figures across regions that report in different units and for fleet managers logging multiple vehicles’ running costs over time.

Frequently Asked Questions (FAQs)

1. How do I calculate my car’s fuel efficiency?

Reset your trip odometer at a full tank, drive normally, then note the distance covered and the exact litres it took to fill back up at the next stop. Divide distance by fuel to get km/l, this tool converts that single figure into MPG and l/100km simultaneously as well.

2. Why are US MPG and UK MPG different for the same car?

A US gallon is 3.785 litres while a UK (imperial) gallon is 4.546 litres, a meaningfully larger volume, so the identical underlying fuel economy produces two different MPG numbers depending on which gallon standard is used. This tool calculates both separately so you’re never comparing figures across an incompatible standard.

3. Why does a single tank of fuel give an unreliable mileage figure?

One tank can be skewed by unusually heavy traffic, a short trip that never reaches an efficient cruising temperature, weather or towing, any of which can distort that single reading. This tool’s Fill-up Log mode calculates a running average across every logged tank, which smooths out this kind of one-off variation.

4. How do I estimate the fuel cost for an upcoming trip?

Calculate your current mileage figure first, then enter your planned trip distance in the optional field alongside it. This tool multiplies your calculated cost per km by that planned distance to give you an estimated total trip cost using your own entered local fuel price.

5. Does the CO2 estimate include the emissions from producing and transporting the fuel?

No, this is a tailpipe-only estimate based on a typical carbon content per litre (or per kg for CNG) at the point of combustion. It does not include well-to-tank emissions from extracting, refining, and transporting the fuel, which would add further to the true total lifecycle emissions.

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