How to Calculate Fuel Economy and Mileage (MPG, km/L, Miles per Litre)
NextCodeHub Automotive Team
Quick Formula Reference:
US Miles Per Gallon (MPG) = Miles Driven ÷ US Gallons Used
Kilometers per Liter (km/L) = Kilometers Driven ÷ Liters Used
Miles Per Litre = Miles Driven ÷ Liters Used
Fuel Economy vs. Mileage vs. Fuel Consumption
Before computing your vehicle's performance, let's clarify terminology:
- Fuel Economy: The broad engineering term describing how efficiently a vehicle extracts kinetic motion from fuel.
- Fuel Mileage: The common colloquial term used in North America (MPG) and South Asia (km/L) to describe how far you travel per unit of fuel.
- Fuel Consumption: The inverse metric (L/100km) measuring how much fuel is burned over 100 kilometers.
How to Calculate Miles Per Gallon (MPG)
Miles Per Gallon (MPG) is the primary fuel economy standard in the United States and Great Britain. Here is how you calculate your exact MPG:
- Fill up your tank: Fill up your gas tank completely at a pump station and note the odometer reading (e.g. 54,300 miles).
- Drive your vehicle: Drive for regular commutes until your tank drops to roughly a quarter or half.
- Refill and record: Return to the station, refill until full, and record the gallons pumped (e.g. 11.4 gallons) and your new odometer (e.g. 54,665 miles).
- Subtract miles: 54,665 - 54,300 = 365 miles driven.
- Divide miles by gallons: 365 miles ÷ 11.4 gallons = 32.0 MPG.
You can run this calculation instantly using our dedicated MPG calculator.
Important: US Gallon vs. UK Imperial Gallon
If you live in the United Kingdom or read British automotive publications (such as Top Gear or Autocar), MPG ratings will appear higher than American vehicle listings. This is because the UK Imperial gallon is larger than the US liquid gallon:
- 1 US Gallon = 3.78541 Liters
- 1 UK Imperial Gallon = 4.54609 Liters (approx 20% larger)
A vehicle that scores 40.0 MPG in the UK is equivalent to 33.3 MPG in the US. When checking window specifications, always ensure you are comparing identical gallon standards!
How to Calculate Kilometers Per Liter (km/L)
In metric nations across Asia, South America, and parts of Europe, vehicle mileage is measured in km/L. The formula is identical in concept:
Formula: km/L = Total Kilometers Driven ÷ Liters of Fuel Added
Example: 450 km driven on 30.0 Liters = 450 ÷ 30 = 15.0 km/L
To convert km/L directly to fuel consumption: 100 ÷ 15.0 = 6.67 L/100km. Check your numbers on our fuel mileage calculator.
How to Calculate Miles Per Litre (UK Commuter Method)
In the UK, petrol is purchased in liters at the pump, but speedometers and road signs are measured in miles. Drivers frequently want to know their miles per litre:
Formula: Miles per Litre = Miles Driven ÷ Litres Added
Example: Driving 220 miles on 32 litres = 220 ÷ 32 = 6.88 miles per litre.
To convert to UK MPG: 6.88 × 4.546 = 31.3 UK MPG.
Complete Fuel Economy Conversion Table
Use this reference table to quickly cross-convert between all four major automotive efficiency units:
| km / Liter | Liters / 100km | US MPG | UK Imperial MPG | Miles / Litre |
|---|---|---|---|---|
| 25.0 km/L | 4.0 L/100km | 58.8 MPG | 70.6 MPG | 15.5 mi/L |
| 20.0 km/L | 5.0 L/100km | 47.0 MPG | 56.5 MPG | 12.4 mi/L |
| 16.0 km/L | 6.25 L/100km | 37.6 MPG | 45.2 MPG | 9.9 mi/L |
| 14.0 km/L | 7.14 L/100km | 32.9 MPG | 39.5 MPG | 8.7 mi/L |
| 12.0 km/L | 8.33 L/100km | 28.2 MPG | 33.9 MPG | 7.5 mi/L |
| 10.0 km/L | 10.0 L/100km | 23.5 MPG | 28.2 MPG | 6.2 mi/L |
| 8.0 km/L | 12.5 L/100km | 18.8 MPG | 22.6 MPG | 5.0 mi/L |
Try Our Free Mileage Calculators
Take the hassle out of manual calculations with our free automotive tools:
- Gas Mileage Calculator — Determine MPG and road trip fuel costs.
- Fuel Mileage Calculator — Calculate km/L and metric efficiency.
- Miles Per Gallon Calculator — Compare US and UK MPG side-by-side.
Frequently Asked Questions
How is fuel economy calculated?
Fuel economy is calculated by dividing the total distance traveled by the total amount of fuel consumed: Distance ÷ Fuel. In imperial units, it is Miles ÷ Gallons (MPG). In metric units, it is Kilometers ÷ Liters (km/L).
How do you calculate miles per gallon (MPG)?
To calculate MPG: Fill your gas tank, reset your trip meter, drive until your next fill-up, and note miles driven. Divide the miles driven by the gallons of fuel required to refill the tank: MPG = Miles Driven ÷ Gallons Pumped.
What is the difference between US MPG and UK Imperial MPG?
A UK Imperial gallon is 20% larger than a US liquid gallon (4.546 liters vs 3.785 liters). Therefore, a car achieving 30 US MPG achieves 36.03 UK MPG on the exact same drive.
How do I calculate mileage per litre (Miles per Litre)?
To calculate miles per litre, divide total miles driven by the liters of fuel pumped: Miles per Litre = Miles ÷ Liters. For example, 150 miles on 20 liters = 7.5 miles per litre.
How do you convert km/L to MPG?
To convert km/L to US MPG, multiply your km/L figure by 2.35215 (e.g., 15 km/L × 2.352 = 35.28 US MPG). To convert US MPG to km/L, divide MPG by 2.35215.