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Fuel Efficiency Calculator

Use this free fuel efficiency calculator to find out how far your car really goes on a gallon or a litre. Enter the distance you drove and the fuel you used, in miles or kilometres and gallons or litres, and it shows all three standard measures at once: miles per gallon (MPG), litres per 100 km (L/100 km), and kilometres per litre (km/L). It also estimates the fuel cost for the trip and the cost per mile, so you can compare cars and driving habits in plain numbers.

How it works

Fuel efficiency is a simple division: distance divided by fuel. If you drive 300 miles and use 10 gallons, your efficiency is 300 divided by 10, or 30 miles per gallon. That is the figure used in the United States, and it is the one most drivers know. The same trip in metric units is 482.8 km on 37.85 litres, which is 12.75 km per litre, and the other metric figure, litres per 100 km, is 100 divided by 12.75, or 7.84. All three describe the same trip; they just frame it differently.

The conversions between the three figures are fixed. To go from MPG to litres per 100 km, divide 235.214 by the MPG: 235.214 divided by 30 is 7.84. To go the other way, divide 235.214 by the L/100 km figure. To get km per litre from MPG, multiply by 0.425144: 30 MPG is 12.75 km/L. The 235.214 constant comes from converting both sides at once, 100 km in miles and a litre in gallons, and it is the same for every car. This calculator converts your inputs to miles and US gallons first, then works out all three figures, so you can mix units freely, for example kilometres with gallons, or miles with litres.

Note the two kinds of gallon. A US gallon is 3.785 litres, and a UK (imperial) gallon is 4.546 litres, so the same car shows a higher MPG number in the UK than in the US: 30 US MPG is about 36 UK MPG. If you are comparing a car reviewed in a British magazine with one sold in the US, check which gallon the figure uses, or the comparison will be off by 20 percent. This calculator lets you choose the gallon, and it labels the result as US MPG unless you pick UK gallons.

The most accurate way to measure your own car is the fill-up method. Fill the tank until the pump clicks off, reset the trip meter, and drive normally until the tank is low. Fill up again until it clicks off, and note the fuel added and the trip distance. Divide the distance by the fuel added. Do not trust a single tank: a pump that clicks off early, a different pump angle, or a week of short cold trips can move the number by several MPG. Track three or four tanks and average them, and compare city tanks with city tanks and highway tanks with highway tanks.

What changes the number in real life is mostly how you drive and where. Short trips from a cold start use far more fuel per mile than a warm engine on the highway, because the engine runs rich while it warms up. Idling, air conditioning, a roof box, under-inflated tires, and a heavy load all cost fuel, and driving at 75 mph uses noticeably more than driving at 55 mph because air resistance rises with the square of speed. Hybrids shine in city driving because they recover braking energy and shut the engine off at lights, which is why a hybrid that gets 50 MPG in town may only get 40 MPG on the highway, the opposite pattern from a gasoline car.

Worked example

A driver fills up, drives 300 miles, and fills up again, adding 10 gallons at $3.50 per gallon.

Step 1: MPG — 300 ÷ 10 = 30.0 MPG.
Step 2: L/100 km — 235.214 ÷ 30 = 7.84 L/100 km.
Step 3: km/L — 30 × 0.425144 = 12.75 km/L.
Step 4: Trip cost — 10 × $3.50 = $35.00, or $35.00 ÷ 300 = $0.117 per mile.

Enter 300 miles and 10 gallons in the calculator and it gives exactly those three figures, plus the $35.00 trip cost. If the same car had used 12 gallons for the same 300 miles, the result would be 25.0 MPG, 9.41 L/100 km, and 10.63 km/L, costing $42.00 for the trip.

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