Gas Mileage Calculator
Find out your real fuel economy in MPG and L/100km from the miles you drove and the gallons you burned.
Example
Drive 300 miles on 12 gallons: MPG = 300 / 12 = 25.0 MPG, which is about 9.4 L/100km. At $3.50/gallon that is $0.140 per mile, or $14.00 per 100 miles.
How it works
MPG = miles driven / gallons used. L/100km is converted using 1 US gallon = 3.785411784 L and 1 mile = 1.609344 km, so L/100km = (gallons x 3.785411784) / (miles x 1.609344) x 100. Cost per mile = (gallons x price) / miles.
Good to know
The Gas Mileage Calculator turns three numbers from a single tank or trip — miles driven, US gallons used, and the price you paid per gallon — into the figures that actually tell you how thirsty your vehicle is and what it's costing you. Alongside MPG it returns L/100km, km/L, cost per mile, and cost per 100 miles, so it's useful whether you think in US, metric, or pure dollars-and-cents terms. It suits commuters tracking real-world economy, road-trippers budgeting fuel, and anyone comparing two cars or two driving styles.
The most reliable way to use it is the fill-to-fill method: fill the tank completely, reset your trip odometer, drive normally, then refill and note the miles on the trip meter and the gallons the pump added. That gallon figure is exactly what you enter here, which is why it tends to be more accurate than your dashboard's built-in estimate. Repeat over several tanks and you'll smooth out the effect of one unusually short or long trip.
Reading the results, remember the two cost numbers move in opposite directions to the economy numbers: a higher MPG (or a lower L/100km) means each mile is cheaper. Cost per mile is handy for reimbursement math or comparing a long route against a shorter, slower one, while cost per 100 miles is an easy mental yardstick for a typical week of driving. Because km/L and L/100km describe the same thing differently, use whichever your region quotes.
One caveat: this tool assumes US gallons of 3.78541 liters. If you measured in Imperial (UK) gallons your MPG will look artificially low, and partial fills, a pump that clicks off early, or a steep parking spot can all skew a single reading — so treat one tank as a snapshot and an average of several as the truth.
Frequently asked questions
Does this use US or Imperial gallons?
It uses US gallons (3.78541 liters). One Imperial (UK) gallon is larger at 4.54609 liters, so if you measured in Imperial gallons your MPG figure here will read lower than a UK MPG rating.
How do I convert my MPG to L/100km?
Divide 235.215 by your MPG (US). For example, 25 MPG = 235.215 / 25 = about 9.4 L/100km. This calculator does that conversion automatically from your miles and gallons.
Is my data uploaded anywhere?
No — this calculator runs entirely in your browser. Your inputs never leave your device, and it works offline once loaded.
Is this calculator free?
Yes, completely free with no sign-up and no limits.
People also ask
What is a good MPG for a car?
It depends on vehicle type, but many modern gasoline cars fall in the 25 to 35 MPG range, hybrids often exceed 40 to 50 MPG, and large trucks or SUVs may sit below 20 MPG. Compare your result against your vehicle's EPA rating rather than a universal number.
Why is my real MPG lower than the EPA window-sticker rating?
Lab-based EPA ratings use controlled cycles, while real driving adds cold starts, idling, hills, heavy loads, air conditioning, high speeds, and aggressive acceleration. A real-world figure 10 to 20 percent below the sticker is common.
How many gallons does it take to drive 100 miles?
Divide 100 by your MPG. At 25 MPG you'd use 4 gallons per 100 miles, and at 40 MPG you'd use 2.5 gallons. This calculator shows the dollar version of that as cost per 100 miles.
How do I calculate gas mileage without a trip odometer?
Record the main odometer reading at a full fill-up, then subtract it from the odometer reading at the next full fill-up to get miles driven; the gallons added at the second fill is your fuel used. Enter both into the calculator.
Does driving slower actually improve gas mileage?
Aerodynamic drag rises sharply with speed, so for most vehicles fuel economy peaks somewhere in the roughly 45 to 60 mph range and drops noticeably at higher speeds. Steady speeds and gentle acceleration also help.
How do I convert L/100km back into MPG?
Divide 235.215 by the L/100km value to get US MPG. For example, 9.4 L/100km equals about 25 MPG. The two are inverse measures, so a lower L/100km means a higher MPG.
Why do my fuel economy results change every tank?
Single-tank readings vary with weather, traffic, terrain, cargo weight, tire pressure, and exactly how full each fill was. Averaging several consecutive tanks gives a far more stable and trustworthy number.
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