Mpg Calculator
MPG tells you how far each gallon carries you, but the number that decides your budget is cost per mile: fuel price divided by MPG. This calculator measures real fuel economy from a fill-up, converts it to L/100 km and km/L, and with an optional fuel price shows your cost per mile plus the annual fuel bill. A 24-MPG car at $3.50 a gallon, for example, costs about $0.15 per mile, or roughly $1,823 a year at 12,500 miles.
Tip: enter both odometer readings and the distance is figured for you, so you can leave the distance field blank.
Quick Answer
MPG is miles driven divided by gallons used: 320 miles on 11.5 gallons is 27.8 MPG, which converts to 11.8 km/L and 8.5 L/100 km. At $3.50 a gallon that works out to $0.126 per mile, or about $1,572 a year for 12,500 miles of driving. Enter your own fill-up above and the calculator reproduces every one of these figures.
Key takeaways
- Measure brim-to-brim over three or four tanks: dashboards read a few percent optimistic, and one tank of city errands is noisy.
- MPG, km/L and L/100 km are the same measurement in different clothes: L/100 km = 235.214583 / MPG (US gallons).
- Cost per mile is the budget number: $3.50 / 24 MPG = $0.146 per mile, about $1,823 a year at 12,500 miles.
- Watch the gallon. A UK-quoted MPG figure in imperial gallons is roughly 20% higher than the same car measured in US gallons.
- Quick Answer
- How to measure your real MPG from a fill-up
- The MPG formula and the metric conversions
- Worked example: 320 miles on 11.5 gallons
- What counts as good MPG in 2026
- Fuel cost per mile and the annual fuel bill
- Five things that move your real MPG
- What competing calculators miss
- Frequently asked questions
- Add This Calculator to Your Website
How to measure your real MPG from a fill-up
The honest method is brim-to-brim, and it takes no equipment at all. Fill the tank completely, reset the trip odometer (or note the odometer reading), drive as you normally would, then fill completely again. Divide the miles covered by the gallons on the second receipt. That division is the whole method: the fuel you burned equals the fuel it took to refill the tank.
1. Fill and reset
Fill to the first automatic shutoff, not topped off, and reset the trip odometer.
2. Drive normally
Run through most of the tank. One long interval beats three short ones.
3. Refill and divide
Fill to the same first shutoff. MPG = trip miles / gallons on this receipt.
Average three or four consecutive tanks before judging a car. A single tank dominated by cold starts and short trips can read 15% below what the same car returns on a relaxed highway week, and that is measurement noise, not a repair bill.
The MPG formula and the metric conversions
The core formula is a division: MPG = miles driven / gallons used. The metric forms describe the same trip in different units: km/L = kilometers / liters and L/100 km = (liters / kilometers) x 100. Note the inversion: more MPG or more km/L is better, but a lower L/100 km is better.
The exact conversion constants, which this calculator uses, are 1 mile = 1.609344 km and 1 US gallon = 3.785411784 liters. That makes L/100 km = 235.214583 / MPG and km/L = MPG x 0.425144. The table below was generated with those constants:
| MPG (US) | L/100 km | km/L | What it feels like |
|---|---|---|---|
| 20 | 11.76 | 8.50 | Full-size SUV, older pickup |
| 25 | 9.41 | 10.63 | Around the new-car average |
| 30 | 7.84 | 12.75 | Compact sedan, efficient SUV |
| 35 | 6.72 | 14.88 | Economy hatchback |
| 50 | 4.70 | 21.26 | Hybrid territory |
One trap catches cross-border shoppers: the imperial gallon is 4.54609 liters while the US gallon is 3.78541, so a UK-quoted 60 MPG is only about 50 US MPG. If a figure looks too good to be true, check which gallon it uses before celebrating.
Worked example: 320 miles on 11.5 gallons
The calculator is pre-filled with this example, so you can press Calculate MPG and watch every figure below appear. The trip: 320 miles driven, 11.5 gallons to refill, fuel at $3.50 a gallon, 12,500 miles a year.
- MPG: 320 / 11.5 = 27.8 MPG
- km/L: (320 x 1.609344) / (11.5 x 3.785411784) = 11.8 km/L
- L/100 km: 235.214583 / 27.826 = 8.5 L/100 km
- Cost per mile: $3.50 / 27.826 = $0.126 per mile
- Annual bill: 12,500 / 27.826 = 449.2 gallons x $3.50 = about $1,572/year
The calculator also rates the result: 27.8 MPG lands in the Good band, around the new-car average. Swap the distance and fuel fields for your own last fill-up and the same five numbers recompute instantly.
What counts as good MPG in 2026
Context matters more than the raw number. The average new gasoline car sits in the mid-20s MPG, and real-world driving typically runs 10-20% below the EPA sticker because the laboratory tests do not include your commute, your right foot, or winter. The calculator rates every result against these bands:
| MPG band | Verdict | Typical vehicles |
|---|---|---|
| 50+ | Excellent | Hybrids, efficient diesels |
| 35-49 | Very good | Economy cars, well-driven compacts |
| 25-34 | Good | Average new gasoline cars |
| 20-24 | Average | Mid-size SUVs, older sedans |
| 15-19 | Below average | Trucks, large SUVs |
| Under 15 | Poor | Time to check tires and maintenance |
A low reading on one tank is rarely a verdict on the car. Check the five factors below before spending money on a diagnosis.
Fuel cost per mile and the annual fuel bill
Cost per mile is where fuel economy becomes a budget: cost per mile = fuel price / MPG. A 24-MPG car at $3.50 a gallon costs $0.146 per mile, and over 12,500 miles that is 520.8 gallons, about $1,823 a year. Two cars five MPG apart can differ by several hundred dollars a year at the same pump price, which is why shoppers should compare cost per mile, not just the MPG badge.
| Your MPG | Cost per mile ($3.50/gal) | Gallons per year (12,500 mi) | Fuel bill per year |
|---|---|---|---|
| 15 | $0.233 | 833.3 | $2,917 |
| 20 | $0.175 | 625.0 | $2,188 |
| 25 | $0.140 | 500.0 | $1,750 |
| 30 | $0.117 | 416.7 | $1,458 |
| 35 | $0.100 | 357.1 | $1,250 |
| 45 | $0.078 | 277.8 | $972 |
If you budget month to month rather than by the year, our gas cost per month calculator turns the same arithmetic into a monthly figure. Drivers who see a fuel surcharge line on freight or delivery bills can check how carriers build it with the fuel adjustment factor calculator. And if driving to work is the expense you really budget for, our commute expense calculator folds fuel into the full cost of the daily drive.
Five things that move your real MPG
These are the levers the U.S. Department of Energy and the EPA point to, and the first two are free to fix today:
Speed
Fuel use climbs fast past about 50 mph. The DOE finds each 5 mph over 50 is like paying an extra $0.30 per gallon for gas.
Tire pressure
A few PSI low costs measurable fuel on every trip. Check monthly against the door-jamb sticker, not the tire sidewall.
Short cold trips
A cold engine burns extra fuel warming up, so a string of two-mile errands is the worst possible duty cycle.
Drag and weight
Roof racks, cargo boxes and extra weight all cost fuel. Take the box off when it is not earning its keep.
Driving style
Hard acceleration and braking waste the fuel that smooth anticipation saves. Cruise control on the highway pays.
What competing calculators miss
Most MPG tools are a bare division box with no context around the number. This one was built against the gaps the top-ranking tools leave open. CalculatorLib's tool silently returns 0 on a blank or zero fuel entry instead of flagging the bad input, and it stops at the metric conversions. Softonic's wizard spreads the same math over a three-step flow that hides your result behind a Next button. CalPro's mileage calculator does add cost per mile, but it has no odometer mode, no efficiency rating and no dedicated unit converter. The UK-oriented tools quote imperial gallons without saying so, which inflates the figure by about 20% for a US reader. Omni's page explains what affects MPG well, yet its tool never prices the mile.
The winning moves on this page: an odometer mode that derives the distance from two readings, a live efficiency verdict badge wired into the calculator, a dedicated three-way converter mode (MPG, L/100 km, km/L), cost per mile plus the annual fuel bill in one click, and an explicit imperial-gallon warning so a UK figure never gets compared against a US one by mistake.
Frequently asked questions
Divide miles driven by gallons used: 320 miles / 11.5 gallons = 27.8 MPG. The calculator above does the division for you, plus the km/L and L/100 km equivalents.
Use L/100 km = 235.214583 / MPG for US gallons. So 27.8 MPG = 8.5 L/100 km. The calculator's converter mode translates between MPG, L/100 km and km/L in one step.
Yes. Enter the start and end odometer readings and leave the distance field blank; the calculator subtracts them to get the distance and divides by your fuel. The end reading must be greater than the start reading.
The average new gasoline car sits in the mid-20s MPG. Above 35 is very good, 50-plus is hybrid territory, and anything under 15 deserves a look at tire pressure and maintenance. Real-world driving usually runs 10 to 20 percent below EPA sticker figures.
EPA tests run controlled laboratory cycles, not your commute. Short cold trips, high speeds, winter fuel blends and aggressive driving all pull the real number below the sticker. A gap of 10 to 20 percent is normal and not a fault.
No, unless your car specifically requires it. Octane measures knock resistance, not energy content, so premium in an engine built for regular buys nothing except a higher receipt. Follow the octane rating in your owner's manual.
More than most drivers expect. The U.S. Department of Energy finds each 5 mph you drive over 50 mph is like paying an extra $0.30 per gallon for gas. Slowing from 75 to 65 on a long trip is one of the cheapest fuel savings available.
The UK quotes miles per imperial gallon, and an imperial gallon is 4.54609 liters against 3.78541 for a US gallon. The same car is rated about 20 percent higher in imperial MPG, so always check which gallon a figure uses before comparing.
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References
- U.S. Department of Energy. How Vehicles Are Tested. Accessed October 23, 2026.
- U.S. Department of Energy. Drive More Efficiently. Accessed October 23, 2026.
- U.S. Department of Energy. Keep Your Car in Shape. Accessed October 23, 2026.
- U.S. Environmental Protection Agency. Automotive Trends Report. Accessed October 23, 2026.
- U.S. Energy Information Administration. U.S. All Grades All Formulations Retail Gasoline Prices. Accessed October 23, 2026.