Gas Mileage Calculator
Calculate your real-world gas mileage (MPG, L/100 km or km/L) from a single fill-up. Enter the distance driven and the fuel it took to refill the tank.
Calculate Your Real Fuel Economy from a Fill-Up
Gas Mileage Formula
Where:
- \(D\) — distance driven since the last fill-up (miles or kilometers)
- \(F\) — fuel needed to refill the tank (US gallons, imperial gallons or liters)
The calculator converts your inputs to kilometers and liters, computes fuel economy in three common units, and shows the one matching your input choice first.
Gas Mileage – Calculation Example
Example: you filled the tank, drove 380 miles, and it took 15.5 US gallons to fill it back up.
\[ \text{Distance} = 380 \times 1.609344 = 611.55~\text{km}, \qquad \text{Fuel} = 15.5 \times 3.785411784 = 58.67~\text{L} \]
\[ \text{MPG}_{US} = \frac{380}{15.5} = 24.5, \qquad \text{L/100 km} = \frac{58.67}{611.55} \times 100 = 9.6, \qquad \text{km/L} = \frac{611.55}{58.67} = 10.42 \]
Result: 24.5 MPG (US) — equivalent to 9.6 L/100 km or 10.42 km/L. That is an average figure, typical for a mid-size petrol car in mixed driving.
How to Use the Calculator
- Fill your tank completely and reset the trip odometer (or write down the mileage).
- Drive as you normally would, then refill the tank to full again.
- Enter the distance covered in the Distance driven field and select miles or kilometers as the unit.
- Enter the amount of fuel the second fill-up required in Fuel used, choosing US gallons, imp. gallons (UK) or liters.
- Read your Your fuel economy result in the unit matching your input, plus the Equivalent unit, Economy in km/L and a short comparison against typical passenger cars.
What Is Gas Mileage?
Gas mileage (also called fuel economy or fuel consumption) measures how far a vehicle travels on a given amount of fuel. In the United States and the UK it is expressed as distance per volume — miles per gallon (MPG) — where higher is better. In most other countries it is expressed as fuel used per distance, typically liters per 100 km (L/100 km), where lower is better. The two scales are inversely related, so a car that does 24.5 MPG (US) consumes 9.6 L/100 km.
Be careful with gallons: a US gallon is 3.785 liters while an imperial gallon is 4.546 liters. The same car therefore shows a larger MPG figure with UK gallons — 24.5 MPG (US) equals about 29.4 MPG (UK). This calculator handles both, so you can compare figures from different sources without conversion errors.
The manufacturer's official economy rating is measured in a laboratory, so real-world results are usually 10–25% worse. Calculating your own figure from actual fill-ups is the only reliable way to know what your car really costs to run — and a sudden drop in mileage is often the first visible symptom of a mechanical problem, underinflated tires, or a change in driving conditions.
Tips for an Accurate Measurement
- Use the full-to-full method. Fill the tank to the click, drive, then fill to the click again. The fuel you add the second time equals what you burned, regardless of tank shape.
- Don't trust partial fills. Filling from half to half, or trusting the fuel gauge, introduces large errors because tank sensors and pump shutoff points are inconsistent.
- Measure over a longer distance. 300 miles (500 km) or more smooths out rounding at the pump; short trips give noisy results.
- Compare like with like. Winter fuel blends, cold starts, roof racks, towing, heavy loads and city driving all lower mileage. For a fair comparison, measure over similar routes and seasons.
- Track the trend, not one number. A single fill-up is a snapshot; a slow decline over several fill-ups is what points to maintenance issues such as dirty air filters, worn spark plugs, or low tire pressure (see the Tire Pressure Temperature Calculator).
Putting the Result to Work
Once you know your real fuel economy, you can estimate fuel costs for any trip: divide the trip distance by your MPG (or multiply the distance in km by L/100 km ÷ 100) and multiply by the fuel price. To plan where you will need to refuel on a longer journey, use the Road Trip Fuel Stops Calculator, and to budget the whole trip including fuel, lodging and food, try the Travel Budget Calculator. If you also drive for work, the Mileage Reimbursement Calculator helps you turn those same miles into a reimbursement claim.
If you are weighing a car against an electric alternative, remember that an EV's efficiency is measured very differently. The EV Charging Cost Calculator gives you the electricity cost per charge, which you can compare directly with your fill-up cost here.
Frequently Asked Questions
How do I calculate gas mileage by hand?
Divide the distance driven since your last full fill-up by the gallons needed to refill the tank. For example, if you drove 380 miles and added 15.5 US gallons: \( 380 \div 15.5 = 24.5 \) MPG. If you work in metric, divide the liters added by the kilometers driven and multiply by 100 to get L/100 km: \( 58.67 \div 611.55 \times 100 = 9.6 \) L/100 km.
What is a good gas mileage for a car?
As a rough guide for a modern passenger car: below 6 L/100 km (about 39 MPG US or more) is very good, 6–8 L/100 km (29–39 MPG) is good, 8–10 L/100 km (24–29 MPG) is average, and above 12.5 L/100 km (under 19 MPG) is high. Larger SUVs and trucks naturally sit higher; a sudden change in your car's usual figure matters more than the absolute number.
How do I convert MPG to L/100 km (or the other way)?
The units are inversely related, so you cannot just multiply by a fixed factor. Use \( \text{L/100 km} = \frac{235.215}{\text{MPG}_{US}} \) or \( \text{MPG}_{US} = \frac{235.215}{\text{L/100 km}} \). For imperial gallons, the constant is 282.481 instead. For example, 24.5 MPG (US) is \( 235.215 \div 24.5 = 9.6 \) L/100 km.
Why is my calculated MPG lower than the manufacturer's rating?
Official ratings come from standardized laboratory tests that don't reflect real conditions. Hard acceleration, stop-and-go traffic, cold engines, hills, heavy loads, roof racks, winter fuel blends and air conditioning all reduce mileage, typically by 10–25%. Your own fill-up-based figure is the realistic one to use for budgeting fuel costs.
Do I have to fill the tank completely to get an accurate result?
Yes, the full-to-full method is strongly recommended. Fill until the pump clicks off, drive, then fill to the click again — the fuel added the second time equals what you burned. Filling from half to half or trusting the fuel gauge is unreliable, because gauge sensors are imprecise and pump shutoff points vary, which can skew your result significantly.
Are US gallons and UK (imperial) gallons the same?
No. A US gallon is 3.785 liters while an imperial gallon is 4.546 liters — about 20% larger. The same car therefore shows a bigger MPG figure in UK gallons: 24.5 MPG (US) equals roughly 29.4 MPG (UK). When comparing figures from American and British sources, always check which gallon is used or the comparison will be misleading.
Why did my gas mileage suddenly drop?
A noticeable decline often has a mechanical or behavioral cause: underinflated tires, a dirty air filter, worn spark plugs, faulty oxygen sensors, or changes such as colder weather, more city driving, a roof rack, or heavier loads. Track mileage over several fill-ups — a one-off dip may just be measurement noise, but a steady downward trend is worth investigating at a workshop.