How do you calculate mpg?
Short answer. Divide the miles you drove by the gallons you used. Fill the tank, reset the trip meter, drive, then fill to full again and divide the miles by the gallons that second fill took: 312 miles on 11.6 gallons is 26.9 mpg. Over several tanks, divide total miles by total gallons. Never average mpg figures.
Miles per gallon is a division with the answer written in its name:
mpg = miles driven ÷ gallons used
The miles are easy, the car counts them. The hard part is the gallons. A fuel gauge is a vague needle, so the only honest measurement of fuel burned is the pump, and the pump only tells you something useful when it fills the tank to the same level twice.
The full tank method
Fill up until the pump clicks off. Ignore how much went in; that fill just sets the starting line. Zero the trip meter, or write down the odometer. Drive normally. When you next need fuel, fill until the pump clicks off again. The gallons on that receipt are exactly what the car burned since the last click.
Odometer at first fill 41,236 Odometer at second fill 41,548 Miles driven 41,548 − 41,236 = 312 Gallons (second receipt) 11.6 mpg 312 ÷ 11.6 = 26.8966… → 26.9 mpg
Same pump, same click, same level both times. If you top off past the click on one fill and not the other, the measurement inherits the difference, so be boring about it.
Averaging several tanks: the trap
One tank is a sample. Highway one week, school runs the next. To get a real figure, collect a few tanks, and here is where people go wrong:
Tank 1 300 miles ÷ 10 gallons = 30 mpg Tank 2 100 miles ÷ 5 gallons = 20 mpg Wrong (30 + 20) ÷ 2 = 25 mpg Right 400 miles ÷ 15 gallons = 26.666… → 26.67 mpg
Averaging the two mpg figures treats a 100 mile tank as if it counted as much as a 300 mile tank. It does not. Add the miles, add the gallons, divide once. This is a weighted average wearing a car costume, and the same mistake as averaging percentages without their totals.
The trick also rescues partial fills. If you buy fuel three times between two full tanks, add all three receipts together. Only the first and last fills need to reach the click.
mpg to L/100km, and why it flips
Most of the world measures the other way round: liters burned per 100 kilometers. Lower is better, which is the first thing that confuses everyone. Both units are exact, a mile is 1.609344 km and a US gallon is 3.785411784 liters, so the conversion constant is exact too:
100 × 3.785411784 ÷ 1.609344 = 235.2145833… L/100km = 235.215 ÷ mpg mpg = 235.215 ÷ L/100km
The same division goes both ways because the two units are reciprocals of each other, scaled. 30 mpg is 235.215 ÷ 30 = 7.84 L/100km, and 8 L/100km is 235.215 ÷ 8 = 29.40 mpg.
| US mpg | L/100km | UK mpg |
|---|---|---|
| 15 | 15.68 | 18.0 |
| 20 | 11.76 | 24.0 |
| 25 | 9.41 | 30.0 |
| 30 | 7.84 | 36.0 |
| 35 | 6.72 | 42.0 |
| 40 | 5.88 | 48.0 |
| 50 | 4.70 | 60.0 |
Why going from 10 to 20 mpg beats going from 25 to 50
Because mpg is miles over gallons, equal steps in mpg are not equal steps in fuel. Count the gallons for the same 1,000 miles instead:
10 mpg → 1,000 ÷ 10 = 100 gallons 20 mpg → 1,000 ÷ 20 = 50 gallons saves 50 gallons 25 mpg → 1,000 ÷ 25 = 40 gallons 50 mpg → 1,000 ÷ 50 = 20 gallons saves 20 gallons
Doubling a thirsty car's mpg saves more than doubling a frugal car's, even though 25 to 50 looks like the bigger jump. Liters per 100 km does not have this problem, since it already counts fuel. In US terms, gallons per 100 miles is just 100 ÷ mpg: 10 mpg is 10 gallons, 25 mpg is 4, 50 mpg is 2.
US gallons versus UK gallons
A UK gallon is exactly 4.54609 liters, about 20% larger than the US one. More fuel per gallon means more miles per gallon from the same car on the same drive:
UK mpg = US mpg × 4.54609 ÷ 3.785411784 = US mpg × 1.20095 30 US mpg → 36.0 UK mpg 50 UK mpg → 50 ÷ 1.20095 = 41.6 US mpg
So a British review quoting 50 mpg and an American one quoting 42 mpg can be describing the same engine. For L/100km from UK mpg, the constant becomes 282.481 instead of 235.215.
From mpg to cost per mile
Once you have mpg, cost per mile is your price per gallon divided by it. With an example price of $4.00 a gallon (use whatever your receipt says):
Cost of the tank 11.6 × $4.00 = $46.40 Cost per mile $4.00 ÷ 26.8966 = $0.1487 → 14.9¢ a mile Check $46.40 ÷ 312 = $0.1487
That is the number to use when you split the cost of a road trip by distance rather than by heads.
Where the decimals go wrong
312 ÷ 11.6 looks harmless. As a fraction it is 3120/116, which reduces to 780/29, and a denominator of 29 gives a repeating decimal with a 28 digit cycle: 26.8965517241379310344827586206 and around again. Any calculator has to stop somewhere, so round once, at the end, to 26.9.
Adding up receipts has a subtler problem. Two fills of 10.1 and 10.2 gallons are 20.3 gallons. A machine doing binary floating point gets 20.299999999999997, the same fault behind 0.1 + 0.2 = 0.30000000000000004, because neither 10.1 nor 10.2 has an exact binary form. Prism works in exact decimals, so the receipts add to 20.3, 0.1 + 0.2 is 0.3, and the mpg is computed from the real totals.
Divide it, then convert it
The miles over gallons division works in Prism's free calculator, with history to keep each tank's figures. The fuel economy category of the unit converter (US mpg, UK mpg, km/L and L/100km) is part of the one-time $3.99 unlock. No ads, no subscription. See the app.