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MPG Calculator

Find your real gas mileage from the distance you drove and the fuel you used. See it in mpg (US and UK), L/100 km and km/L.

Level: Grade 6

Printable worksheet

MPG Calculator

Know the answer you need? Pick an input to work backward.

Inputs
What do you want to do?

The amount needed to refill the tank after driving that distance.

The price for one unit of the fuel you chose above. Adds the fill-up cost and the cost per mile or kilometer.

Result

Miles per US gallon

30 mpg (US)

That is 30.00 mpg (US), 36.03 mpg (imperial), 7.84 L/100 km or 12.75 km/L.

Miles per imperial gallon
36.03 mpg (imp)
Liters per 100 km
7.84 L/100 km
Kilometers per liter
12.75 km/L

See the step-by-step solution ↓

Step-by-step solution

  1. Change the distance to kilometers, so we can work out the metric figures too (1 mi = 1.609344 km)

    342 mi×1.609344=550.3956 km

  2. Change the fuel to liters for the metric figures (1 US gal = 3.785411784 L)

    11.4 US gal×3.785412=43.1537 L

  3. Change the fuel to imperial gallons for the UK mpg figure (÷ 4.54609)

    43.1537 L4.54609=9.4925 imp gal

  4. Miles per US gallon: divide the miles by the US gallons to see how far each gallon took you

    34211.4=30 mpg (US)

  5. Miles per imperial gallon: divide the miles by the imperial gallons

    3429.4925=36.0285 mpg (imp)

  6. Liters per 100 km: divide the liters by the km, then multiply by 100 to see the fuel for every 100 km

    43.1537550.3956×100=7.8405 L/100 km

  7. Kilometers per liter: divide the km by the liters to see how far each liter took you

    550.395643.1537=12.7543 km/L

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

Please note: Results are estimates for planning and comparison. Lenders, banks, and tax authorities may round differently or include fees not modeled here, so confirm figures with your provider before deciding.

What this calculator does

Find out how many miles your car really gets per gallon, and what each mile costs you in fuel. Fuel economy tells you how far a vehicle goes on a set amount of fuel. Or it tells you how much fuel it needs for a set distance. Measuring it yourself gives a more honest number than the official rating. You use the distance on your trip meter and the fuel it takes to refill the tank. That way it reflects your own driving, roads and weather.

The same fuel economy is written four common ways: miles per US gallon, miles per imperial (UK) gallon, liters per 100 kilometers and kilometers per liter. This calculator gives all four at once. So you can compare a number from a US window sticker with one from a UK or European review. A second mode converts a number you already have.

Where it's used

Why this math matters outside the classroom:

  • Measuring fuel economy. Driving 300 miles on 10 gallons works out to 30 miles per gallon.
  • Metric units. A car rated at 30 miles per US gallon uses about 7.84 liters per 100 kilometers.
  • Tracking a vehicle. Drivers fill the tank, reset the trip meter and divide the miles driven by the gallons used at the next fill-up.
  • Fleet records. Fleet managers compare fuel economy across vehicles to spot one that is using more fuel than the others.

Grade level and standards: Grade 6; Common Core State Standards for Mathematics 6.RP.A.3.b, 6.RP.A.3.d. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.

How to use it

  1. Choose Calculate to work from a distance and an amount of fuel. Choose Convert to change a fuel economy number into the other units.
  2. To calculate, enter the distance you drove since the last full tank and choose miles or kilometers. Then enter the fuel it took to refill, and choose its unit.
  3. If you like, enter the fuel price per unit. You'll see what the fill-up cost and what each mile or kilometer cost you.
  4. Read the result in the unit that matches your fuel unit. The other measures are listed below it.

What the inputs mean

Distance driven
How far you drove between two fill-ups. Reset the trip meter when you fill the tank, and read it at the next fill-up.
Fuel used
The amount pumped at the second fill-up, which is the fuel the car burned over that distance. Fill to the same point both times, such as the first click of the pump, so the two fills match.
Fuel price (optional)
The price per gallon or liter, in the same unit as the fuel you entered. Leave it blank if you only need the fuel economy.
Fuel economy
In Convert mode, the number you want to change, with the unit it's given in.

Formula and method

mpg (US) = miles ÷ US gallons
mpg (imperial) = miles ÷ imperial gallons
L/100 km = liters ÷ kilometers × 100
km/L = kilometers ÷ liters
L/100 km = 235.215 ÷ mpg (US) = 282.481 ÷ mpg (imperial)
cost per mile = fuel used × price ÷ miles

In the formulas, mpg is miles per gallon, L/100 km is liters per 100 kilometers, and km/L is kilometers per liter. The distance and fuel are changed into every unit needed, using exact definitions: 1 mile = 1.609344 km, 1 US gallon = 3.785411784 liters and 1 imperial gallon = 4.54609 liters. Each measure is then one division.

Miles per gallon and kilometers per liter measure distance for each unit of fuel, so higher is better. Liters per 100 km measures fuel for a set distance, so lower is better. One is the flip of the other (its reciprocal). So the same gain in mpg saves less fuel when mpg is already high than when it's low.

The numbers 235.215 and 282.481 come from the same exact factors: 100 × 3.785411784 ÷ 1.609344 and 100 × 4.54609 ÷ 1.609344.

Worked example

You drive 342 miles between fill-ups, and the second fill-up takes 11.4 US gallons at $3.50 per gallon.

  1. Miles per US gallon: 342 ÷ 11.4 = 30 mpg (US).
  2. Distance in km: 342 × 1.609344 = 550.3956 km. Fuel in liters: 11.4 × 3.785411784 = 43.1537 L.
  3. L/100 km: 43.1537 ÷ 550.3956 × 100 = 7.84 L/100 km. km/L: 550.3956 ÷ 43.1537 = 12.75 km/L.
  4. Fuel in imperial gallons: 43.1537 ÷ 4.54609 = 9.4925. So 342 ÷ 9.4925 = 36.03 mpg (imperial).
  5. Cost: 11.4 × $3.50 = $39.90. That's $39.90 ÷ 342 = about $0.12 per mile, or $11.67 per 100 miles.

Result: The car got 30 mpg (US). That's the same as 36.03 mpg (imperial), 7.84 L/100 km or 12.75 km/L, at a fuel cost of about 12 cents a mile.

Understanding the result

A single tank can be thrown off by how full the pump filled it, or by an unusual trip. Averaging three or more tanks gives a number you can trust. It's good for budgeting or for noticing a change in the vehicle.

If your fuel economy suddenly drops with the same driving, it's worth looking into. It can point to low tire pressure, a brake that's dragging, or a maintenance problem.

When comparing vehicles, L/100 km shows fuel savings more directly. Going from 10 to 8 L/100 km saves 2 liters every 100 km. The matching mpg change looks different depending on where it starts.

Assumptions and limitations

  • The fuel you entered is exactly the fuel used over the distance you entered. This is called a full-to-full tank measurement.
  • Your trip meter or odometer (the car's mileage counter) is accurate.
  • US and imperial gallons are as each system defines them. No other gallon sizes are used.
  • Electric and plug-in hybrid vehicles aren't covered, because their energy use is measured in kilowatt-hours.
  • Odometer errors of a few percent, common after changing tire size, carry straight into the result.
  • The cost per mile counts fuel only, not maintenance, insurance or the car losing value over time (depreciation).

Frequently asked questions

Why is my mpg lower than the sticker rating?

Official ratings come from standard lab tests. Higher speeds, short trips with a cold engine, heavy loads, roof racks, air conditioning and hilly routes all use more fuel. So most drivers see somewhat lower numbers in real life.

Why does the same car show a higher mpg in the UK?

An imperial gallon is about 20% bigger than a US gallon (4.546 liters against 3.785). So the car travels further on each one. 30 mpg (US) is about 36 mpg (imperial) for the same fuel economy.

Can I measure fuel economy without filling the tank?

Only roughly. The full-to-full method works because the pump measures exactly what the engine used since the last fill. A fuel gauge or a partial fill gives a rough guess at best. Many cars also show a trip computer number. It's handy, but it can be a few percent too hopeful.

How do I convert L/100 km back to mpg?

Divide 235.215 by the L/100 km number for US mpg, or 282.481 for imperial mpg. For example, 6 L/100 km is 235.215 ÷ 6 = 39.2 mpg (US). Convert mode does this for you.

How this calculator is checked

Method: Definitions of fuel economy (distance per volume and volume per distance) with exact definitions of the international mile, US gallon and imperial gallon. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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