MPG / Gas Mileage Calculator
Calculate your car's real-world fuel economy from odometer and fuel fill-up data.
Reviewed by the ToolNestr Editorial Team — July 2026
How MPG calculation works
Your car's real-world fuel economy is calculated by dividing the distance travelled between fill-ups by the amount of fuel consumed. This is far more accurate than relying on the manufacturer's quoted figures, which are based on laboratory tests under ideal conditions. Real-world factors like traffic, weather, tyre pressure, and driving style can significantly affect your actual MPG.
The calculator converts all inputs to a common base: kilometres for distance and litres for fuel. If you enter miles, they are converted using 1 mile = 1.60934 km. Gallons are converted using 1 US gallon = 3.785 L or 1 UK gallon = 4.546 L. From the distance in km and fuel in litres, L/100km is calculated, then MPG US, MPG UK, and km/L are derived.
The formula explained
Worked example
300 km driven, 35 L of fuel used.
Personal Mileage Tracking
Track your car's real fuel economy over time. Identify if efficiency changes after servicing or tyre changes.
Vehicle Shopping
Compare a used car's advertised MPG with its real-world figure from a test drive by tracking distance and fuel used.
Fuel Budgeting
Use your calculated MPG to estimate monthly and annual fuel costs for better household budgeting.
Efficiency Diagnostics
A sudden drop in MPG can indicate mechanical issues like underinflated tyres, dirty air filters, or oxygen sensor problems.
How to use the MPG calculator
Fill up completely
Fill your tank to the top and reset your trip odometer. Drive normally until your next fill-up.
Record the data
At your next fill-up, note the trip distance and the litres (or gallons) you put in to fill the tank again.
Get your results
Enter the numbers and see your MPG, L/100km, and km/L instantly. Track over multiple fills for accuracy.
Tips for accurate MPG measurement
Always fill to the same level
Fill until the pump clicks off automatically. Using the same pump and technique each time reduces measurement error.
Average multiple fill-ups
A single tank may include a mix of city and highway driving. Track 5+ tanks for a reliable average of your car's typical economy.
Account for seasonal changes
Winter fuel blends and cold temperatures can reduce MPG by 10–15%. Compare summer and winter figures separately.
Check tyre pressure regularly
Under-inflated tyres increase rolling resistance and reduce fuel economy. Keeping tyres at the recommended pressure can improve MPG by 3–5%.
Related tools
Frequently asked questions
How do I get accurate MPG data?
Fill your tank completely, reset your trip odometer, drive normally, then fill the tank completely again. Use the trip distance and the litres (or gallons) from the second fill.
Should I include the first fill-up?
No. Track from a full tank to the next full tank. The first fill is just a starting reference point.
Can I use this for L/100km?
Yes. The calculator shows both MPG and L/100km simultaneously, plus km/L.
Does it work with UK gallons?
Yes — toggle between US and UK (imperial) gallons for accurate MPG comparisons.
How many fill-ups should I track?
At least 3–5 fill-ups for a reliable average. Single-trip calculations can vary due to driving conditions.
Sources & references
This tool uses standard formulas and reference values from:
- • SAE International standards (e.g. J1349 engine power, J1634 EV range). sae.org
- • U.S. DOE / EPA — fueleconomy.gov, official efficiency, MPGe and charging figures. fueleconomy.gov
- • Vehicle manufacturer service specifications — always defer to the OEM figures for your specific vehicle.
General estimates. Follow your manufacturer’s published specifications and a qualified mechanic for safety-critical work.