MPG calculator (miles per gallon)

What is an MPG calculator?

An MPG (miles per gallon) calculator measures how far a vehicle travels on each gallon of fuel. It is one of the most common ways to express fuel economy, especially in the United States and the United Kingdom. The higher the MPG value, the more efficient the vehicle.

The calculation only needs two pieces of information: the distance you drove and the amount of fuel you used. Because the calculator handles unit conversion for you, you can enter the distance in miles or kilometers and the fuel in US gallons, UK gallons, or liters, and read the result in MPG (US), MPG (UK), km/L, or L/100km.

Tracking your real-world MPG is useful for budgeting trips, comparing vehicles, spotting maintenance problems early, and improving your driving habits.

Formula

The fundamental formula for fuel economy in miles per gallon is:

$$\text{MPG} = \frac{\text{Distance}}{\text{Fuel Used}}$$

Where:

• Distance — the distance driven, in miles
• Fuel used — the amount of fuel consumed, in gallons

If you measure the distance in kilometers and the fuel in liters, convert first using these relationships:

$$1 \text{ mile} = 1.60934 \text{ kilometers}$$ $$1 \text{ gallon (US)} = 3.78541 \text{ liters}$$

To switch between fuel-economy units, the calculator applies the following conversions, all based on US miles per gallon:

$$\text{MPG (UK)} = \text{MPG (US)} \times 1.20095$$ $$\text{km/L} = \text{MPG (US)} \times 0.425144$$ $$\text{L/100km} = \frac{235.215}{\text{MPG (US)}}$$

How the calculator works

The calculator updates automatically — there is no button to press. As soon as both the distance and the fuel amount are filled in, the fuel economy appears in the unit you selected.

The steps are as follows:

1. Enter the distance you drove (e.g., 300 miles).
2. Enter the fuel used (e.g., 10 US gallons).
3. Choose the result unit (MPG US, MPG UK, km/L, or L/100km).
4. The calculator instantly displays your fuel economy.

Examples

Example 1: Basic MPG (US)

Given:

• Distance = 300 miles
• Fuel used = 10 gallons (US)

$$\text{MPG} = \frac{300}{10} = 30$$

The vehicle achieves 30 MPG (US).

Example 2: Converting to L/100km

Using the same 30 MPG (US) from Example 1:

$$\text{L/100km} = \frac{235.215}{30} = 7.84$$

The vehicle consumes about 7.84 L/100km.

Example 3: A second trip

If you drive 420 miles on 12 gallons:

$$\text{MPG} = \frac{420}{12} = 35$$

The vehicle achieves 35 MPG (US).

Related concepts

Fuel economy vs. fuel consumption

Fuel economy (MPG or km/L) expresses distance per unit of fuel, so a higher number is better. Fuel consumption (L/100km) expresses fuel per fixed distance, so a lower number is better. They describe the same thing from opposite directions, which is why a high MPG corresponds to a low L/100km value.

US gallon vs. UK gallon

A UK (imperial) gallon is larger than a US gallon — about 4.546 liters versus 3.785 liters. Because of this, the same vehicle always shows a higher MPG figure when measured in UK gallons. Always confirm which gallon a quoted MPG figure refers to before comparing vehicles.

Useful conversion formulas

$$1 \text{ gallon (UK)} = 4.54609 \text{ liters}$$ $$\text{MPG (US)} = \frac{235.215}{\text{L/100km}}$$ $$\text{km/L} = \frac{\text{L/100km figure as MPG}}{1}$$

These relationships let you compare a vehicle rated in one system against one rated in another without re-running the whole calculation.

Notes

• Real-world MPG varies with driving style, traffic, terrain, vehicle load, and weather.
• For the most accurate result, measure fuel economy over a full tank rather than a single short trip.
• City driving usually produces lower MPG than steady highway driving.
• Properly inflated tires, regular maintenance, and gentle acceleration all improve fuel economy.

Frequently asked questions

How do I calculate MPG?

Divide the miles you drove by the gallons of fuel you used. For example, 300 miles on 10 gallons gives 300 / 10 = 30 MPG.

What is a good MPG?

For a modern gasoline car, 30–40 MPG (US) is considered good, while many hybrids exceed 50 MPG (US). Large SUVs and trucks often fall in the 15–25 MPG (US) range.

Why is my UK MPG higher than my US MPG?

A UK gallon is larger than a US gallon, so you travel more miles on one UK gallon. The same efficiency therefore reads higher in MPG (UK) — multiply MPG (US) by 1.20095 to convert.

How do I convert MPG to L/100km?

Divide 235.215 by your MPG (US) figure. For example, 30 MPG (US) equals 235.215 / 30 ≈ 7.84 L/100km.

Does air conditioning lower my MPG?

Yes. Running the air conditioner can reduce fuel economy by roughly 5–10%, with the largest effect in slow city traffic and a smaller effect at steady highway speeds.