Calculate your actual MPG from miles driven and fuel added, use odometer readings, convert MPG to L/100 km or km/L, and compare your real fuel economy with a rated MPG.
Gas Mileage Calculator
Choose a mode, enter your values, and press Calculate for an instant result.
Applies to Fill-Up MPG, Odometer MPG, and the Multi-Fill Tracker. US MPG is always shown alongside your chosen unit system using the exact US gallon conversion (1 US gallon = 3.785411784 liters).
Optional: Fuel Price & Rated MPG
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Price for the fuel unit shown. Used only to estimate cost per distance and fuel cost for this tank.
If entered, your measured US MPG is compared against this rated figure.
Optional: Fuel Price & Rated MPG
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A US gallon (3.785411784 L) and an Imperial gallon (4.54609 L) are different volumes, so US MPG and Imperial MPG are never interchangeable without converting. This mode clearly labels which gallon each result uses.
Enter both figures in the same MPG unit (for example, both US MPG). Use Convert Units above first if your rated figure is in a different unit.
Add each fill-up's distance and fuel added. Overall MPG is calculated as total distance divided by total fuel, not by averaging each row's individual MPG. Up to 10 rows.
Optional: Fuel Price
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Applied to total fuel across all rows for an estimated total cost and cost per distance. Cost is secondary to the measured MPG.
Planning a future trip instead of measuring a past one? Use the Fuel Cost Calculator with your measured MPG to estimate cost.
Gas mileage in the United States is commonly expressed as miles per gallon, or MPG. The basic formula is MPG equals miles driven divided by gallons of fuel used.
Suppose you drive 320 miles and use 12.5 US gallons. Calculate 320 divided by 12.5, which equals 25.6. Your measured fuel economy is 25.6 MPG.
Visual: The MPG Formula
320 miles divided by 12.5 gallons equals 25.6 MPG. The formula is simple; getting reliable input data is the harder part.
The MPG formula applied to a 320-mile, 12.5-gallon example
Quantity
Value
Miles driven
320
Gallons used
12.5
MPG
25.6
The formula is simple. Getting reliable input data is the more important part.
How to Calculate MPG at the Gas Pump
The most practical method is often called the full-tank method. Start by filling the tank. Reset your trip odometer or record the vehicle's odometer reading. Drive normally. The next time you need fuel, refill the tank to approximately the same full level. Record the miles driven and the gallons added. Then divide miles by gallons added to get MPG.
Visual: The Full-Tank Method
Fill the tank, drive, refill to the same level, then divide miles driven by the gallons added at the refill.
Steps in the full-tank method for measuring MPG
Step
What Happens
1
Fill tank to a consistent full level
2
Drive normally
3
Refill to the same full level
4
Divide miles driven by gallons added at the refill
5
Result is MPG
The gallons from the second fill approximate the amount of fuel consumed during the distance you just drove.
Why Do I Need Two Fill-Ups?
The first fill creates the starting point. You do not know how much fuel was actually in the tank before that initial fill. After driving and refilling, the amount needed to return to full gives you an estimate of fuel consumed during the measured distance. That is why the full-tank method should not calculate MPG from tank capacity, or from the amount purchased before measuring a trip. Tank capacity is not the same as fuel consumed.
Calculate MPG From Odometer Readings
If you do not use a trip odometer, record the starting odometer and the ending odometer. Distance equals ending minus starting. Suppose the starting reading is 45,200 and the ending reading is 45,520. Distance is 320 miles. If the refill uses 12.5 gallons, then 320 divided by 12.5 equals 25.6 MPG. The Odometer MPG mode above performs this subtraction automatically, and it will reject an ending reading that is lower than the starting reading rather than silently using the absolute difference.
Why Does Fill Level Matter?
A fuel pump does not necessarily stop at exactly the same tank level every time. Vehicle angle, pump sensitivity, temperature, and topping-off behavior can change the amount dispensed. For one tank, this can create noticeable noise. Over several complete fill-ups, total distance divided by total fuel can provide a more stable picture. Do not continue pumping after the nozzle automatically stops simply to make the math look consistent. Follow safe fueling practices.
How Do I Calculate Average MPG Over Several Fill-Ups?
Do not simply average the MPG numbers. Instead, add all distance, add all fuel, then divide. Overall MPG equals total miles divided by total gallons.
Suppose Trip A is 100 miles using 2 gallons, which equals 50 MPG. Trip B is 100 miles using 10 gallons, which equals 10 MPG. A simple average says 30 MPG. But the vehicle actually traveled 200 miles using 12 gallons. 200 divided by 12 equals 16.67 MPG.
Visual: Do Not Average MPG Directly
Averaging 50 MPG and 10 MPG gives an incorrect 30 MPG. Total distance (200 mi) divided by total fuel (12 gal) gives the correct 16.67 MPG.
Why simple MPG averaging is incorrect
Method
Result
Trip A (100 mi / 2 gal)
50 MPG
Trip B (100 mi / 10 gal)
10 MPG
Simple average of 50 and 10 (incorrect)
30 MPG
Total distance (200 mi) / total fuel (12 gal) (correct)
16.67 MPG
This is why a multi-fill MPG tracker should total the underlying miles and fuel rather than average MPG readings directly. The Multi-Fill Tracker mode above does exactly this.
MPG vs Gallons Per 100 Miles
MPG measures distance per unit of fuel. Higher MPG means less fuel is generally needed for a fixed distance. Fuel consumption can instead be expressed as gallons per 100 miles, calculated as 100 divided by MPG. At 10 MPG, the vehicle uses 10 gallons per 100 miles. At 20 MPG, it uses 5 gallons. At 25 MPG, it uses 4 gallons. At 50 MPG, it uses 2 gallons.
Visual: MPG vs. Gallons Per 100 Miles
As MPG rises, gallons used per 100 miles falls, but not in a straight line. This makes the amount of fuel saved easier to compare.
MPG compared with gallons used per 100 miles
MPG
Gallons per 100 Miles
10 MPG
10 gallons
20 MPG
5 gallons
25 MPG
4 gallons
50 MPG
2 gallons
Why MPG Is Nonlinear
Imagine improving from 10 MPG to 20 MPG. For 100 miles, 10 MPG uses 10 gallons, and 20 MPG uses 5 gallons. That saves 5 gallons. Now improve from 40 MPG to 50 MPG. 40 MPG uses 2.5 gallons per 100 miles, and 50 MPG uses 2. Savings: 0.5 gallon. Both examples involve a 10 MPG increase, but the amount of fuel saved is very different. EPA discusses this relationship in its MPG educational materials.
What Is L/100 km?
Liters per 100 kilometers measures fuel consumption. Unlike MPG, lower L/100 km means better fuel economy. The formula is L/100 km equals liters used divided by kilometers driven, multiplied by 100. For example, 40 liters over 500 km is 40 divided by 500, times 100, which equals 8 L/100 km.
Visual: US MPG vs. L/100 km (Inverse Relationship)
Higher MPG is more efficient. Lower L/100 km is more efficient. As MPG rises, L/100 km falls, an inverse relationship, so the two numbers move in opposite directions.
US MPG compared with the equivalent L/100 km, showing the inverse relationship
US MPG
L/100 km
10 MPG (less efficient)
23.52 L/100 km (more fuel used)
25 MPG
9.41 L/100 km
50 MPG (more efficient)
4.70 L/100 km (less fuel used)
What Is km/L?
Kilometers per liter is distance in km divided by liters used. For example, 500 km divided by 40 L equals 12.5 km/L. Higher km/L means greater distance per liter.
How Do I Convert MPG to L/100 km?
For US MPG, the commonly used conversion is approximately L/100 km equals 235.214583 divided by US MPG. At 25 MPG, 235.214583 divided by 25 is approximately 9.41 L/100 km. Because the relationship is inverse, doubling MPG does not double L/100 km. Instead, higher MPG produces a lower consumption figure. The Convert Units mode above performs the exact version of this calculation, along with km/L and Imperial MPG.
US MPG vs Imperial MPG
This distinction matters. A US gallon (3.785411784 liters) and an Imperial gallon (4.54609 liters) are not the same volume. Therefore, 30 US MPG is not the same fuel economy as 30 Imperial MPG. A converter must know which gallon definition is being used. CalculateThisWay explicitly labels US MPG and Imperial MPG separately in the Convert Units mode above, and does not display a generic MPG conversion when the gallon type matters.
How Accurate Is EPA MPG?
EPA vehicle labels are designed as a standardized way to compare fuel economy among vehicles. EPA testing accounts for several driving conditions and provides city, highway, and combined fuel-economy estimates. But EPA also explains that individual real-world mileage can vary. That is expected. Your measured MPG reflects your specific combination of vehicle, route, speed, traffic, weather, terrain, load, maintenance, and driving behavior. A single tank should not automatically be treated as proof that the EPA label is wrong.
Why Is My MPG Lower Than the Rated MPG?
Possible reasons include short trips, cold weather, heavy traffic, high speeds, rapid acceleration, towing or cargo, roof racks, air conditioning, hills, low tire pressure, and vehicle condition. There can also be measurement noise from one fill-up. Track several full tanks before deciding that a small difference represents a long-term pattern. Actual fuel economy can differ from a label or reference estimate because of driving conditions, speed, weather, vehicle load, maintenance, terrain, and other factors.
Can My MPG Be Higher Than the EPA Rating?
Yes. A driver may exceed a label estimate under some conditions. For example, a particular route may have steady speeds, mild weather, low traffic, and favorable terrain. Again, EPA MPG is a standardized estimate, not a promise that every driver will always receive the exact number.
How Do I Compare My Actual MPG With Rated MPG?
Suppose rated is 30 MPG and actual is 27 MPG. Difference: 27 minus 30 equals -3 MPG. Percentage difference relative to rated: -3 divided by 30, times 100, equals -10%. But the fuel-consumption comparison provides additional perspective. At 30 MPG, 100 divided by 30 equals 3.33 gallons per 100 miles. At 27 MPG, 100 divided by 27 is approximately 3.70 gallons per 100 miles. Difference: about 0.37 gallon for every 100 miles. That makes the additional fuel use easier to understand. The Compare to Rated mode above performs this calculation automatically.
How Do I Calculate Gas Cost Per Mile?
If the fill-up cost is known, cost per mile equals fuel cost divided by miles driven. Example: 12.5 gallons at $3.50 costs $43.75. Over 320 miles, $43.75 divided by 320 is approximately $0.1367 per mile, or about 13.7 cents per mile in fuel. This does not include maintenance, insurance, depreciation, or other vehicle expenses.
Gas Mileage Calculator vs Fuel Cost Calculator
Use the Gas Mileage Calculator after driving when you want to measure actual MPG, L/100 km, km/L, and fuel consumption. Use the Fuel Cost Calculator when planning ahead and you already know or estimate your fuel economy. A useful workflow: calculate your real MPG from several fill-ups here, then use that real MPG in the Fuel Cost Calculator to estimate future trip or commute fuel cost using your own observed efficiency.
What MPG Is Good?
There is no single MPG number that defines a "good" vehicle in every situation. Fuel economy depends strongly on vehicle class, size, powertrain, model year, and intended use. A pickup truck and a small hybrid should not be judged against the same simple MPG threshold. For vehicle comparisons, use EPA ratings for similar vehicles and consider fuel consumption rather than relying only on a generic label such as good or bad.
The Bottom Line
To calculate US gas mileage, miles driven divided by gallons used equals MPG. For the most useful real-world estimate, start with a full tank, measure the distance, refill, record the gallons added, and divide. For multiple fill-ups, total miles divided by total gallons is better than directly averaging separate MPG values. CalculateThisWay shows the measured MPG alongside L/100 km, km/L, fuel consumption per 100 miles, and optional cost information so you understand more than one number.
Worked Examples
The examples below use the exact same calculation engine as the calculator above. Values are computed live in JavaScript rather than typed in as fixed text, so they will always match what the calculator itself would return for these inputs.
320 Miles, 12.5 Gallons
500 Miles, 20 Gallons
500 km, 40 Liters
25 US MPG to L/100 km
Compare: Rated 30, Actual 27
Multi-Fill: Three Fill-Ups
Try It Yourself
Starting odometer 45,200. Ending odometer 45,520. Refill used 12.5 gallons.
What is the distance driven, and what is the MPG?
Distance: 45,520 minus 45,200 equals 320 miles. MPG: 320 divided by 12.5 equals 25.6 MPG.
Which Calculator Do I Need?
Gas Mileage Calculator (this page)
I know my actual miles driven and gallons used and want to calculate my vehicle's real MPG, L/100 km, or km/L.
Divide miles driven by gallons of fuel used. MPG equals miles driven divided by gallons used.
What is the MPG formula?
MPG equals miles driven divided by gallons of fuel used, using US gallons for US MPG.
How do I calculate gas mileage from a fill-up?
Record the miles driven since your last fill, then divide by the gallons added at the next fill-up using the full-tank method.
Why do I need to fill my tank twice?
The first fill establishes a consistent starting point. The gallons used in the calculation come from the second, refill, after you have driven and are returning the tank to the same full level.
How do I calculate MPG from odometer readings?
Subtract the starting odometer reading from the ending odometer reading to get distance, then divide that distance by the gallons added at the refill.
Do I divide miles by gallons?
Yes. MPG equals miles driven divided by gallons of fuel used during that distance.
Do I use tank capacity to calculate MPG?
No. Tank capacity is not the same as fuel consumed. Use the gallons actually added at the refill after driving, not how many gallons your tank holds.
How do I calculate average MPG?
Add up all the distance driven and all the fuel used across your fill-ups, then divide total distance by total fuel. Do not average the individual MPG numbers.
Should I average individual MPG numbers?
No. Averaging individual MPG values from separate fill-ups overstates your real fuel economy when fill-ups and distances differ. Use total distance divided by total fuel instead.
How do I track MPG over several fill-ups?
Use the Multi-Fill Tracker mode above to add each fill-up's distance and fuel. It totals the figures and divides total distance by total fuel for an overall MPG.
What is L/100 km?
Liters used per 100 kilometers driven, calculated as liters used divided by kilometers driven, multiplied by 100. Lower L/100 km means better fuel economy.
What is km/L?
Kilometers traveled per liter of fuel, calculated as distance in kilometers divided by liters used. Higher km/L means better fuel economy.
How do I convert MPG to L/100 km?
For US MPG, L/100 km is approximately 235.214583 divided by MPG. At 25 MPG that is about 9.41 L/100 km.
How do I convert L/100 km to MPG?
Divide approximately 235.214583 by the L/100 km figure to get US MPG.
How do I convert MPG to km/L?
Multiply US MPG by approximately 0.425143707 to get km/L.
Is US MPG the same as Imperial MPG?
No. A US gallon and an Imperial gallon are different volumes, so the same vehicle has a different numeric MPG depending on which gallon is used. Always check which gallon definition a figure uses.
Why is my actual MPG lower than EPA MPG?
Real-world conditions such as short trips, cold weather, heavy traffic, high speeds, rapid acceleration, towing, cargo, roof racks, air conditioning, hills, low tire pressure, and vehicle condition can all reduce measured MPG below the EPA estimate.
Can my real MPG be higher than EPA MPG?
Yes. Steady speeds, mild weather, light traffic, and favorable terrain can all produce a measured MPG higher than the EPA label for some drivers and trips.
What affects fuel economy?
Driving conditions, speed, weather, vehicle load, maintenance, terrain, tire pressure, and driving behavior can all affect fuel economy.
Does cold weather affect MPG?
Yes. Cold weather commonly reduces fuel economy because of factors such as engine warm-up time, increased idling, and denser cold air.
Does speed affect MPG?
Yes. Fuel economy typically declines at higher speeds because of increased aerodynamic drag.
Does tire pressure affect MPG?
Yes. Underinflated tires increase rolling resistance, which can reduce fuel economy.
Does towing affect MPG?
Yes. Towing or carrying additional cargo adds weight and aerodynamic drag, which typically reduces fuel economy.
How do I compare actual MPG with rated MPG?
Enter your actual measured MPG and the rated or reference MPG into the Compare to Rated mode above. It calculates the MPG difference, percentage difference, and fuel consumption in gallons per 100 miles for both figures.
What is gallons per 100 miles?
A fuel consumption figure calculated as 100 divided by MPG. It expresses how much fuel is used over a fixed distance rather than how far a gallon travels.
Why is MPG nonlinear?
Because MPG is a rate, equal increases in MPG do not save equal amounts of fuel. Improving from 10 to 20 MPG saves far more fuel over a fixed distance than improving from 40 to 50 MPG.
What MPG is good?
There is no single good MPG number for every vehicle. Fuel economy depends on vehicle class, size, powertrain, model year, and intended use, so compare your MPG against EPA ratings for similar vehicles rather than a universal threshold.
How do I calculate fuel cost per mile?
Divide the fuel cost for a fill-up by the miles driven on that fill-up. The calculator can do this automatically when you enter a fuel price.
Can one fill-up give an inaccurate MPG?
Yes. Fill level, pump sensitivity, vehicle angle, and temperature can all introduce some noise into a single tank's reading. Tracking several fill-ups and using total distance divided by total fuel gives a more stable picture.
What is the best way to measure real MPG?
Use the full-tank method consistently: fill to the same level, record the distance driven, refill to the same level, and divide the miles driven by the gallons added at the refill. Tracking several fill-ups this way is more reliable than relying on a single tank.
What is the difference between Gas Mileage Calculator and Fuel Cost Calculator?
The Gas Mileage Calculator measures your actual MPG from miles driven and fuel added. The Fuel Cost Calculator instead uses a known or estimated MPG to calculate trip or commute fuel cost. Use Gas Mileage Calculator after driving to measure, and Fuel Cost Calculator before driving to plan.
U.S. EPA / U.S. Department of Energy: FuelEconomy.gov, the official federal fuel economy resource - fueleconomy.gov
Methodology & Limitations
Fill-Up MPG and Odometer MPG calculate MPG as distance divided by fuel used at the refill, never from tank capacity or from fuel purchased before a trip began.
Metric inputs are converted to US MPG using the exact US gallon constant (1 US gallon = 3.785411784 liters) and the exact mile constant (1 mile = 1.609344 kilometers). No approximate rounding is applied internally.
Convert Units distinguishes US MPG from Imperial MPG using their different gallon volumes (US gallon 3.785411784 L, Imperial gallon 4.54609 L) rather than reusing one constant for both.
Odometer MPG rejects an ending reading lower than the starting reading rather than silently using an absolute difference.
The Multi-Fill Tracker calculates overall MPG as total distance divided by total fuel across all rows, never as an average of each row's individual MPG.
Compare to Rated presents MPG and percentage differences neutrally and never suggests the vehicle is faulty; it attributes differences to driving conditions, speed, weather, vehicle load, maintenance, terrain, and similar factors.
Fuel price is always user-entered. This calculator never hardcodes a current or national average fuel price.
This calculator measures your actual fuel economy from your own data. It is not the Fuel Cost Calculator, which instead estimates cost from a known or estimated MPG.
This calculator estimates fuel economy from the distance and fuel figures you enter. Actual fuel economy can differ from a label or reference estimate because of driving conditions, speed, weather, vehicle load, maintenance, terrain, and other factors, as EPA explains in its fuel economy materials. A single fill-up can also include measurement noise from fill level, pump sensitivity, and temperature. For official rated fuel economy by vehicle year, make, and model, consult FuelEconomy.gov.