How to Calculate Fuel Consumption

This calculator works out your vehicle’s fuel consumption in liters per 100 kilometers or miles per gallon. You enter the distance traveled and the fuel used, and it returns the consumption rate, plus the cost per kilometer or mile if you provide the fuel price. This is the standard way to track efficiency and compare vehicles.

Formula

The core formula for fuel consumption in liters per 100 kilometers is:

Fuel Consumption (L/100km) = (Fuel Used in Liters ÷ Distance in Kilometers) × 100

Where:

• Fuel Used – the amount of fuel consumed, in liters (or gallons, if you convert).
• Distance – the distance traveled, in kilometers (or miles).
• L/100km – liters of fuel per 100 kilometers; lower is better.

To convert to miles per gallon (US), use:

MPG = (Distance in miles) ÷ (Fuel in gallons)

1 mile = 1.609344 km, 1 US gallon = 3.785411784 liters. The calculator handles the conversion automatically.

Worked Example

Using the default values: distance = 500 km, fuel used = 40 L, fuel price = 1.50 per liter.

1. Divide fuel used by distance: 40 ÷ 500 = 0.08 L/km.
2. Multiply by 100: 0.08 × 100 = 8.00 L/100km.
3. For MPG: convert distance to miles: 500 ÷ 1.609344 = 310.686 miles.
4. Convert fuel to gallons: 40 ÷ 3.785411784 = 10.5669 gallons.
5. Divide miles by gallons: 310.686 ÷ 10.5669 = 29.40 mpg.
6. Cost per km: 1.50 × 40 ÷ 500 = 0.12 currency/km.

So the car uses 8.00 L/100km, which is about 29.4 mpg, and costs 0.12 per km at that fuel price.

How to Use the Calculator

Enter the distance you traveled and the exact amount of fuel you put in to refill the tank. The most accurate method is to fill the tank completely, note the odometer reading, drive until you need fuel again, then fill it completely again. The fuel you added on the second fill is the fuel used for the distance you drove between fills.

If you only have a partial fill, the result will be less accurate. For best results, use the full-tank method over a mix of driving conditions (city, highway, hills) to get a representative average.

The calculator gives you three outputs:

• L/100km – the metric standard, common in Europe, Asia, and Canada.
• MPG (US) – miles per US gallon, used in the US. Note that the UK uses a different gallon (imperial), which would give a different number.
• Cost per km – how much you spend on fuel for each kilometer driven, based on the fuel price you enter.

Common Mistakes

• Mixing up gallons: US gallon (3.785 L) vs Imperial gallon (4.546 L). The calculator uses US gallons. If you use imperial, your MPG will be about 20% higher (wrong).
• Using distance from the trip meter incorrectly: Reset the trip meter at the first fill, or note the odometer reading exactly. If you forget, you’ll get a wrong distance.
• Not filling the tank completely: If you don’t fill to the same level each time, the fuel amount will be off. Always fill until the pump clicks off automatically.
• Including fuel used before the first fill: The fuel in the tank at the start is not counted. Only the fuel added between the two fills matters.
• Rounding too early: Keep at least two decimals in intermediate steps to avoid errors.
• Using the wrong units: If your distance is in miles and fuel in liters, you need to convert. The calculator assumes kilometers and liters.

Tips

• Calculate over multiple fill-ups (e.g., 3-5) and take the average for a more reliable figure.
• For a long trip, use the trip meter and fill at the start and end to get highway-specific consumption.
• Check your tire pressure and remove roof racks when not in use – they can affect fuel consumption by several percent.
• Record the date, odometer, and fuel amount in a small notebook or app to track trends over time.
• Fuel consumption varies with speed, traffic, load, and weather. A single figure is a snapshot, not a constant.
• If you use the calculator for a vehicle that runs on diesel or ethanol, the same formula applies – just use the actual fuel amount.

Troubleshooting

The result seems too high or too low. Check your inputs: did you enter the distance in kilometers and fuel in liters? If you used miles or gallons, convert them first. Also, a very short trip (like 10 km) can give a skewed result because of cold starts and idling – use a longer distance.

MPG shows a weird number. If you entered distance in miles and fuel in liters, the calculator will still treat them as kilometers and liters, giving a nonsense MPG. Convert miles to kilometers (multiply by 1.609) or use a different tool.

Cost per km is negative or zero. Ensure the fuel price is positive. If you leave it at 0, the cost will be 0, which is correct but not useful.

The calculator gives “Cannot compute” – this happens if you enter distance = 0. You must drive some distance; otherwise, division by zero is impossible.

You get different results between fills. That’s normal. Variations in traffic, terrain, and even the same pump’s automatic shutoff can cause small differences. Take an average over several fills.

Frequently Asked Questions

Q: What is a good fuel consumption figure?
A: It depends on the vehicle. A small car might achieve 5-6 L/100km (40-47 mpg), a mid-size sedan 7-9 L/100km (26-34 mpg), and an SUV 10-14 L/100km (17-24 mpg). Compare with the official ratings for your car.

Q: How do I convert L/100km to MPG?
A: Use the formula: MPG = 235.215 ÷ L/100km (for US gallons). For example, 8 L/100km = 235.215 / 8 = 29.4 mpg.

Q: Does the calculator work for electric vehicles?
A: No, it’s for liquid fuel. For EVs, you’d use kWh per 100 km or miles per kWh. The principle is similar but with electricity.

Q: Why does my car’s computer show different consumption than this?
A: On-board computers often underestimate consumption by 5-10% because they don’t account for idling, fuel used during start, or variations in fuel density. The manual fill-up method is more accurate.

Q: Should I use the trip meter or the odometer?
A: Use the trip meter if you can reset it at the first fill. If not, note the odometer reading at both fills and subtract. The difference is the distance.

Q: Can I calculate fuel consumption for a boat or airplane?
A: Yes, the same formula applies. Enter the distance in nautical miles or kilometers and the fuel used in liters or gallons. For boats, distance is often in nautical miles (1 nm = 1.852 km), so adjust accordingly.

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in d | Distance | km | 500 | 0 | 100000
in f | Fuel used | L | 40 | 0 | 10000
in p | Fuel price | per L | 1.50 | 0 | 100
out l100 | Fuel consumption | L/100km | f / d * 100 | 2
out mpg | Fuel economy | mpg (US) | (d / 1.609344) / (f / 3.785411784) | 2
out ck | Cost per km | currency/km | p * f / d | 2