The cost of fuel for a journey is not a price to look up; it is a calculation from three figures you already know or can measure. How far you are going, how much fuel your vehicle uses per unit of distance, and what you pay for the fuel. Multiply the distance by the consumption to get the fuel, multiply the fuel by the price to get the cost, and the answer is exact for the figures entered. This guide works the calculation in miles and gallons, in kilometres and litres, and per mile and per kilometre, and it explains the US-gallon and UK-gallon trap that silently moves every answer by a fifth. No live fuel price appears anywhere here, because the price is always the figure you enter.

The method: distance, consumption, price

Every journey fuel calculation has the same three inputs. Distance: how far the journey is, in miles or kilometres. Consumption: how much fuel the vehicle uses per unit of distance, in miles per gallon, litres per 100 kilometres, or kilometres per litre. Price: what one unit of fuel costs, per gallon or per litre, at the station you actually use. The fuel for the journey is the distance divided by the efficiency, and the cost is the fuel multiplied by the price. That is the whole method, and every complication in this guide is a unit conversion sitting on top of it: miles to kilometres, gallons to litres, and per-gallon figures to per-litre figures. Get the units consistent first, and the arithmetic is two divisions and a multiplication.

There are two ways to know the consumption, and they suit different journeys. The trip method measures a finished journey: divide the distance driven by the fuel used, from a full tank to a full tank, and the result is the consumption for that driving, on those roads, in that weather. The rating method uses the vehicle published consumption figure, and the result is an estimate for a journey not yet driven, as good as the rating is representative of the driving. The trip method is more accurate for the next similar journey; the rating method is the only option for a journey with no history. Either way, the price must be entered fresh: fuel prices move, and a calculation that reuses last month price answers last month question.

Miles and gallons: the US calculation

In miles per gallon, the fuel for a journey is the distance in miles divided by the miles-per-gallon figure. A 300-mile journey at 30 mpg needs 300 divided by 30, which is 10.0 gallons. The cost is the gallons multiplied by the price per gallon: at the hub page own example price of $3.50 per gallon, 10.0 gallons cost $35.00. The per-mile cost is the total cost divided by the distance, $35.00 divided by 300, which is about 11.7 cents per mile, and it is the figure that compares most directly with other travel: a per-mile cost lets a car journey be compared with a train fare, a flight, or a second car, on the same basis, without reworking the whole calculation.

The mpg figure itself is worth understanding, because it is not linear. Improving from 20 mpg to 25 mpg saves more fuel than improving from 40 mpg to 50 mpg, even though the second improvement looks larger, because fuel used is distance divided by mpg, and the division punishes low figures disproportionately. On a 300-mile journey, 20 mpg uses 15.0 gallons, 25 mpg uses 12.0 gallons, a saving of 3.0 gallons, while 40 mpg uses 7.5 gallons and 50 mpg uses 6.0 gallons, a saving of only 1.5 gallons. That is why L/100 km, which is linear in fuel used, is the clearer scale for comparing efficiency, and why the metric section below converts every mpg figure into it.

Kilometres and litres: L/100 km and km/L

In metric units, consumption is usually stated as litres per 100 kilometres, which is the fuel for a fixed distance, and the journey fuel is the distance in kilometres divided by 100, multiplied by the L/100 km figure. A 500-kilometre journey at 7.84 L/100 km needs 5 times 7.84, which is 39.2 litres. The cost is the litres multiplied by the price per litre, entered by you. The alternative metric figure, kilometres per litre, is the distance divided by the fuel: 500 kilometres on 39.2 litres is 12.75 km/L, and the journey fuel is the distance divided by the km/L figure, 500 divided by 12.75, which is the same 39.2 litres. Both figures describe the same efficiency; L/100 km is better for comparing cars, because a smaller number always means less fuel, while km/L is better for estimating range, because multiplying it by the tank size gives the distance directly.

The conversion between mpg and L/100 km is a single constant, because both units measure the same thing in different scales: L/100 km equals 235.214 divided by US mpg. The constant 235.214 is a unit conversion, not a fudge factor, and it only applies to US gallons: a UK gallon gives a different constant, 282.481, which is the next section subject. The hub calculator states the same formula, and its default tank, 300 miles on 10.0 gallons, reads 30.0 mpg, 7.84 L/100 km, and 12.75 km/L, which are three statements of one efficiency. Learn one conversion, and the other two figures follow.

L/100 km = 235.214 / mpg (US gallons). Fuel = distance / mpg. Lower L/100 km is better.

Per mile and per kilometre costs

Per-unit costs turn a journey total into a budgeting figure. Cost per mile equals the price per gallon divided by mpg: at $3.50 per gallon and 30 mpg, the cost is $3.50 divided by 30, which is about 11.7 cents per mile. Cost per kilometre equals the price per litre multiplied by the L/100 km figure divided by 100: at a price you enter, say 1.60 per litre, and 7.84 L/100 km, the cost is 1.60 times 7.84 divided by 100, which is about 12.5 cents per kilometre. The per-unit figure is the one to carry in the head for budgeting, because it scales to any distance without rework: a 40-mile commute at 11.7 cents a mile costs about $4.67 a day in fuel, and a 64-kilometre commute at the metric figure costs the equivalent in litres.

Per-unit costs also expose the effect of price changes cleanly. If the price per gallon rises by 50 cents, the per-mile cost rises by 50 cents divided by mpg, which is 1.7 cents a mile at 30 mpg and 2.5 cents a mile at 20 mpg: the less efficient car feels every price rise more, because it burns more fuel per mile. That is the sense in which efficiency is insurance against price rises, and it is a better reason to know the per-mile figure than any single journey total. For a household budget, multiply the per-mile cost by the monthly mileage, and the fuel line in the budget is done, with the price as the only figure that needs updating.

US gallons, UK gallons, and the 20 percent trap

A US gallon is 3.785 litres. A UK gallon, the imperial gallon, is 4.546 litres, about 20% larger. The same car, on the same fuel, therefore reports a higher mpg figure in the UK than in the US, because each UK gallon carries it further: 30 US mpg is about 36 UK mpg, since 30 multiplied by the ratio 4.546 divided by 3.785, which is 1.201, gives 36.0. The car has not become more efficient; the gallon has become bigger. Any comparison of mpg figures across the Atlantic that does not state the gallon is meaningless, and any fuel-cost calculation that mixes a UK mpg figure with a US price per gallon is wrong by a fifth before it starts.

The safe practice is to convert to litres at the first step, whenever the figures come from different systems. Convert the distance to kilometres, 1 mile is 1.609344 kilometres, convert the fuel to litres, and work in L/100 km or km/L from there. The hub guide states the same warning, and its conversion constant, 235.214, is explicitly the US-gallon constant. If a figure is labelled UK mpg, divide 282.481 by it to get L/100 km, and the two systems reconcile. State the gallon in every note, every spreadsheet, and every comparison, and the trap never opens.

Worked example: 300 miles and 500 km, both ways

Work one journey in both systems, end to end, with the price entered, not assumed. Journey: 300 miles, which is 482.8 kilometres. Vehicle: 30.0 US mpg, which is 7.84 L/100 km and 12.75 km/L. Fuel: 300 divided by 30 is 10.0 US gallons, which is 37.9 litres, and 482.8 kilometres at 7.84 L/100 km is 37.9 litres, the same fuel by the other route, as it must be. Cost, at the hub example price of $3.50 per US gallon: 10.0 times $3.50 is $35.00, and per mile that is $35.00 divided by 300, about 11.7 cents. Cost, at an entered price of 0.92 per litre, which is the same $3.50 per gallon converted at 3.785 litres: 37.9 times 0.92 is about $34.87, the small difference being rounding in the litre price. The two routes agree, which is the check that the conversions were done right.

Now the metric journey. Journey: 500 kilometres. Vehicle: 7.84 L/100 km. Fuel: 5 times 7.84 is 39.2 litres. At an entered price of 1.60 per litre, the cost is 39.2 times 1.60, which is 62.72 in the same currency as the price, and the per-kilometre cost is 62.72 divided by 500, about 12.5 cents. Change the price and only the cost changes; change the efficiency and the fuel changes; change the distance and everything scales. Keep the three inputs separate in any spreadsheet, price in its own cell, and the calculation stays honest when any one of them moves.

Why your car beats the rating, or misses it

A consumption figure measured on one journey rarely matches the published rating, and the reasons are physical, not mysterious. Speed dominates: aerodynamic drag rises with the square of speed, so motorway cruising at high speed burns disproportionately more fuel than the same distance at moderate speed. Load, roof boxes, and open windows add drag and weight. Cold starts burn extra fuel while the engine warms. Short trips never reach efficient operating temperature. Hilly terrain, headwinds, air conditioning, and under-inflated tyres each add their share. A trip figure measured on a long, steady motorway run will beat the rating; a trip figure measured on short urban runs in winter will miss it, and both figures are correct for the driving they describe.

The useful response is to measure the consumption for the driving you actually do, by the full-tank method, over several tanks, and to use that figure for budgeting, keeping the published rating only for comparing vehicles before purchase. Record the distance and the fuel at each fill, divide, and average over at least three tanks, because a single tank is noisy: a fill that stops early, a pump that clicks off differently, or a journey mix that changes, all move one tank figure. The average over several tanks is the consumption to budget with, and the calculator trip mode does the division for each tank as it is entered.

Mistakes: price, gallon, and rounding

First, using a remembered price instead of the price at the pump: the calculation is only as current as the price entered, and prices move weekly. Second, mixing gallons: a UK mpg with a US price, or a US mpg with a UK price, is wrong by about 20%, in either direction. Third, reading mpg as linear, and expecting a 10 mpg improvement to save the same fuel at every level, when the saving shrinks as mpg rises. Fourth, dividing the wrong way round: fuel is distance divided by efficiency in mpg or km/L, but distance multiplied by efficiency in L/100 km, and swapping the two gives a nonsense figure that looks plausible. Fifth, rounding the fuel to whole gallons before multiplying by the price, which throws away the part-gallon that the journey actually needs. Keep one decimal place in the fuel, keep the price exact, and round only the final cost, to the cent.

Frequently asked questions

How do I calculate fuel cost for a trip?

Divide the distance by the efficiency to get the fuel, then multiply the fuel by the price. For example, 300 miles at 30 mpg needs 10.0 gallons, and at $3.50 per gallon the cost is $35.00. In metric, 500 km at 7.84 L/100 km needs 39.2 litres, multiplied by the price per litre you enter.

How do you convert mpg to L/100 km?

Divide 235.214 by the US mpg figure: 235.214 divided by 30 is 7.84 L/100 km. The constant 235.214 applies to US gallons only. For UK gallons, divide 282.481 by the mpg figure instead, because a UK gallon is about 20% larger than a US gallon.

What is the difference between US mpg and UK mpg?

The gallon. A US gallon is 3.785 litres and a UK gallon is 4.546 litres, so the same car reports about 20% higher mpg in the UK. 30 US mpg is about 36 UK mpg for the same efficiency. Always state which gallon a figure uses, or convert to litres per 100 km before comparing.

How much does it cost per mile in fuel?

Divide the price per gallon by the mpg figure. At $3.50 per gallon and 30 mpg, the cost is about 11.7 cents per mile. Per kilometre, multiply the price per litre by the L/100 km figure and divide by 100. The per-unit figure scales to any distance and is the right figure for budgeting.

Why is my actual fuel consumption worse than the rating?

Speed, load, cold starts, short trips, hills, wind, air conditioning, and tyre pressure all raise consumption above a standardised rating, which is measured under controlled conditions. Measure consumption by the full-tank method over several tanks for the driving you actually do, and budget with that figure.

Does this calculator use live fuel prices?

No. No live fuel price appears anywhere in the guide or the calculator, because prices vary by place and day. The price is always the figure you enter, and the cost examples use the hub page own $3.50 per gallon example price. Enter the price you actually pay for a current answer.