MPG Calculator
Enter the odometer reading and fuel added at each fill-up to get your fuel economy in US mpg, UK mpg and L/100 km, with a true average across several tanks.
MPG Calculator
Your fill-ups
Units
US: miles and US gallons. UK: miles and liters, as sold at UK pumps. Metric: km and liters. Switching only changes the labels. Numbers you've typed stay as they are.
Starting reading
The reading when you last filled the tank all the way. Don't enter the fuel from that fill-up: it was burned before this log starts. Using the trip meter instead? Enter 0 here and the trip meter reading in Fill-up 1.
Fill-ups
Fill the tank all the way each time. Type the odometer with its tenths and no commas, like 33001.3, and the gallons from the pump. Leave unused rows blank.
Fill-up 1
Distance 356.2 mi 26.4 mpg
Fill-up 2
Distance 157.6 mi 32.8 mpg
Fill-up 3
Distance 293.1 mi 23.1 mpg
Fill-up 4
Distance – –
3 tanks, 806.9 mi on 31.0 gal: 26.0 mpg overall.
Each tank is measured from the fill-up before it. The overall figure is total distance ÷ total fuel, not an average of the tank figures. Results are rounded to one decimal.
Road44
Driving Hours Tracker & Logger
Log fuel and miles on every drive in Road44
Start a drive with one tap or add it afterward. Road44 totals your hours or miles and exports the logbook as a PDF or CSV.
Free · No sign-up needed
Road44, the driving log app mentioned below, is our app. Methods and conversion factors were checked on fueleconomy.gov, NIST, UK legislation and Natural Resources Canada in September 2026.
How to work out mpg from your fill-ups
The method fueleconomy.gov gives needs two full tanks and four steps:
- Fill the tank all the way and write down the odometer reading.
- At your next fill-up, fill it all the way again. Write down the gallons it took and the new reading.
- Subtract the old reading from the new one. That's the miles you drove on that tank.
- Divide the miles by the gallons.
Their own example goes from 32,645.1 to 33,001.3. That's 356.2 miles, and 356.2 ÷ 13.5 gallons = 26.4 mpg. It's also the calculator's first tank, so you can check the result against a government source.
The gallons that count are the ones you pump at the end of the stretch. The fuel from the fill-up at the start was burned before you began counting. Pair the miles with the wrong fill-up and the number goes badly wrong. In the calculator's example, the second tank's 157.6 miles on the earlier 13.5 gallons would read 11.7 mpg. With the right 4.8 gallons it's 32.8 mpg.
Write the tenths digit down too. The calculator works in tenths of a mile, so the tank distances add up exactly to the total. If you'd rather use the trip meter, reset it at a full tank and read it at the next one. Enter 0 as the starting reading and the trip meter figure in Fill-up 1.
Averaging several tanks the right way
One tank doesn't tell you much on its own, so some drivers track several fill-ups "to get a better average estimate," as fueleconomy.gov puts it. How you average them matters. The calculator's default log has three tanks:
- 356.2 miles on 13.5 gallons: 26.4 mpg
- 157.6 miles on 4.8 gallons: 32.8 mpg
- 293.1 miles on 12.7 gallons: 23.1 mpg
Add up the miles and the gallons and you get 806.9 miles on 31.0 gallons, or 26.0 mpg. Average the three mpg figures instead and you get 27.4 mpg, which is 1.4 mpg too high. A simple average gives the short 157.6-mile tank as much weight as the long ones. Total distance ÷ total fuel counts every mile and every gallon exactly once.
The gap can be much bigger. 100 miles on 2 gallons is 50 mpg, and 300 miles on 15 gallons is 20 mpg. The average of those is 35, but the car went 400 miles on 17 gallons: 23.5 mpg. The same rule holds for L/100 km. The default tanks read 8.9, 7.2 and 10.2 L/100 km, which average to 8.8, but the real figure is 9.0. The EPA's Automotive Trends data uses the same idea for whole fleets, reporting "harmonic production-weighted averages" of each vehicle's fuel economy.
Partial fills. fueleconomy.gov warns: "If you do not fill up your tank completely, your MPG estimate will not be accurate." A partial fill makes its own tank read high and the next one read low. Say you start full at 32,645.1, add 8.0 gallons at 33,001.3, then fill up with 14.2 gallons at 33,300.0. Entered as two fill-ups, the tanks show 44.5 and 21.0 mpg. The overall figure is still 29.5 mpg, the same as 654.9 miles on 22.2 gallons entered as one. That holds as long as the first and last fills in your log are full.
Missed fill-ups. If you forget to write one down, the miles for that stretch get counted but the fuel doesn't. The My MPG help page says that without the missing record, "the number of miles traveled will be too high and your MPG will not be accurate." The calculator flags any tank above 100 mpg (US) as unusually high, so 698.7 miles on 4.8 gallons (145.6 mpg) gets a note. When that happens, start a fresh log from the next full fill-up.
US mpg, UK mpg and L/100 km
All three figures come from exact definitions:
- A US gallon is 231 cubic inches, which is 3.785411784 liters (NIST Handbook 44).
- An imperial gallon is 4.54609 liters (Weights and Measures Act 1985).
- A mile is exactly 1.609344 km (NIST).
From those, an imperial gallon is about 1.20095 US gallons, so UK mpg reads about 20% higher than US mpg for the same drive. To get L/100 km, divide 235.215 by US mpg or 282.481 by UK mpg. Natural Resources Canada rounds these to 235.21 and 282.48. The calculator works from the exact definitions, and the default log's tiles read 26.0 US mpg, 31.3 UK mpg and 9.0 L/100 km.
UK drivers have miles on the dashboard and liters on the pump receipt. The UK button takes both and divides the liters by 4.54609 to get imperial gallons. 383.6 miles on 38.24 liters is 45.6 UK mpg, or 38.0 US mpg.
L/100 km measures consumption, not economy, so it runs the other way. As Natural Resources Canada puts it, "the lower the litres per 100 kilometres (L/100 km) rating, the better the fuel consumption." Consumption also makes savings easier to compare. The EPA's fuel economy label guide notes that going from 10 to 15 mpg saves "about 33 gallons" over 1,000 miles, while going from 30 to 35 mpg saves "only about 5 gallons." Put as consumption, the default log's 806.9 miles on 31.0 gallons works out to 3.84 gallons per 100 miles.
The unit buttons change the labels, not your numbers. 13.5 liters is a very different tank from 13.5 gallons, so the calculator never converts what you've typed. Pick your units first, then enter your readings.
Keep fuel and miles in one log with Road44
Road44 is the driving log we built to keep those numbers with each drive:
- Each drive can store the fuel you added and what it cost, next to its distance: a start and end odometer reading or a typed trip distance. Every field is optional.
- Miles or kilometers and gallons or liters, set in the menu and seeded from your phone's region.
- The Insights screen (Pro) has a Fuel card that shows fuel economy as distance per unit of fuel, such as miles per gallon, for all time or the last 7, 30 or 90 days. Free users see Insights blurred behind an unlock card.
- A vehicle label on each drive, with recent names suggested.
- History, the calendar and the logbook export (Text, CSV, PDF or PNG, with no watermark) are free.
For the app's fuel economy to mean something, log the distance of every drive and the fuel from every fill-up. Road44 is a personal record, not an official or certified logbook. There's no account, and drives stay on your phone. For more, see our driving calculators.
Frequently Asked Questions
Common questions about mpg calculator
How do I calculate my car's mpg?
Fill the tank all the way and write down the odometer reading. At your next fill-up, fill it all the way again and write down the new reading and the gallons it took. Subtract the old reading from the new one to get the miles, then divide by the gallons. fueleconomy.gov's example goes from 32,645.1 to 33,001.3, which is 356.2 miles, on 13.5 gallons: 356.2 ÷ 13.5 = 26.4 mpg. Those are the calculator's first two readings.
Which fill-up's gallons do I divide by?
The gallons you pump at the end of the stretch, when you read the new odometer. The fuel from the fill-up at the start was burned before you started counting, so leave it out. It's an easy mix-up, and it throws the result way off. In the calculator's example, 157.6 miles divided by the earlier fill-up's 13.5 gallons gives 11.7 mpg. With the right 4.8 gallons it's 32.8 mpg.
How do I average mpg over several fill-ups?
Add up all the miles, add up all the gallons, and divide the first by the second. Don't average the mpg figures. The calculator's example has three tanks at 26.4, 32.8 and 23.1 mpg. Their simple average is 27.4 mpg, but the car went 806.9 miles on 31.0 gallons, which is 26.0 mpg. The short 157.6-mile tank counts as much as the long ones in a simple average, and it shouldn't. Tracking several tanks gives what fueleconomy.gov calls "a better average estimate".
What if I didn't fill the tank all the way?
That tank's figure will be off. fueleconomy.gov says: "If you do not fill up your tank completely, your MPG estimate will not be accurate." A partial fill makes its own tank read high and the next one read low. The fix is to skip the partial fill's reading and add its gallons to the next full fill-up. For example, you start full at 32,645.1, put in 8.0 gallons at 33,001.3, then fill up with 14.2 gallons at 33,300.0. That's 654.9 miles on 22.2 gallons, or 29.5 mpg. If you enter both fill-ups instead, the tanks show 44.5 and 21.0 mpg, but the overall figure is still 29.5, because the first and last fills were full.
What if I forgot to write down a fill-up?
Then the miles for that stretch get counted but the fuel doesn't, so your mpg comes out too high. My MPG's help puts it this way: without the missing record, "the number of miles traveled will be too high and your MPG will not be accurate." The calculator marks any tank over 100 mpg (US) as unusually high. Start again with the first full fill-up after the gap as your starting reading.
How do I convert mpg to L/100 km?
Divide 235.215 by US mpg, or 282.481 by UK mpg. The same division works in reverse. 26.4 US mpg is 235.215 ÷ 26.4 = 8.9 L/100 km. Natural Resources Canada rounds these to 235.21 and 282.48. For km per liter, divide 100 by L/100 km, or multiply US mpg by 0.425144: 26.4 mpg is about 11.2 km/L. With L/100 km, lower is better.
Why is UK mpg higher than US mpg for the same car?
The UK (imperial) gallon is bigger. By law it's exactly 4.54609 liters, while the US gallon is 3.785411784 liters, so one imperial gallon is about 1.2 US gallons. The same drive therefore shows about 20% more miles per gallon in UK units: 26.4 US mpg is 31.7 UK mpg. UK pumps sell fuel in liters. To get UK mpg, divide the liters by 4.54609 and then divide the miles by that. The UK button does this for you. 383.6 miles on 38.24 liters is 8.41 imperial gallons, or 45.6 mpg. If you have figures in imperial gallons, multiply by 4.54609 to get liters.
Why is my mpg lower than the EPA sticker or my dashboard says?
The sticker is for comparing cars. fueleconomy.gov says EPA ratings "may not accurately predict the average MPG you will get." Weather is one reason: in city driving, "a conventional gasoline car's gas mileage is roughly 15% lower at 20°F than it would be at 77°F," and up to 24% lower on short trips. Dashboards are close but not exact. AAA's lab tests found an average absolute difference of 2.3% (0.7 mpg) between displayed and lab-measured fuel economy, in either direction, and up to 6.4% for one car. Fill-to-fill numbers over several tanks are the best check.