MPG calculator for vans and cars
Work out what your van or car really does to the gallon, in UK gallons, from the miles on the trip meter and either the litres on the pump or the receipt total. You also get litres per 100km, what each mile costs in fuel and the cost of the fill. Free, no account, and your last entries are kept in your browser for next time.
- UK gallons of 4.54609 litres, not US
- Works from litres or the £ on the receipt
- Litres per 100km, cost per mile and the cost of the fill
Enter the miles since the last fill-up and the fuel used, as litres on the pump or the receipt total. Add the price per litre for the costs.
The working
| Litres used | 60 L |
|---|---|
| Gallons: ÷ 4.54609 | 13.198 |
| 412 miles ÷ gallons | 31.22 mpg |
UK gallons, not US (a US gallon is smaller, so US mpg figures look lower). Use your figure in the fuel cost calculator to price travel into quotes.
How to calculate mpg: the sum
Divide the litres by 4.54609 to get UK gallons, then divide the miles by the gallons. That is your miles per gallon. Multiply the litres by the price per litre and divide by the miles for the fuel cost of each mile.
The calculator opens on a working week in a diesel van: 412 miles on the trip meter between two brim fills, 60 litres to fill it the second time, at 199.3p a litre. That is the government's UK average diesel price for the week starting 19 October 2026, 199.33p in its weekly road fuel prices, rounded to one decimal place.
- Gallons: 60 ÷ 4.54609 = 13.20 UK gallons.
- MPG: 412 ÷ 13.20 = 31.2mpg.
- Litres per 100km: 412 miles is 663.05km, and 60 ÷ 663.05 × 100 = 9.05L/100km.
- Fuel cost: 60 × £1.993 = £119.58.
- Cost per mile: £119.58 ÷ 412 = 29.0p a mile.
Working from the fuel receipt
If you only kept the amount you paid, set Fuel used, from to The receipt total, then enter that total and the price per litre. It divides one by the other to get the litres. In the example, £119.58 at 199.3p a litre is 60 litres, and every other figure comes out the same.
Receipts are worth keeping for this. A run of them, with the mileage written on each, gives you an mpg figure for every fill. One bad week in traffic shows up as a dip, not as your van's true figure.
Brim to brim
Fill until the pump clicks off, zero the trip meter, then drive as normal. At the next fill, the litres it takes to refill are what those miles used. The first fill only sets the start point, so its litres never go in the sum.
Several tanks
Add up the miles and the litres over three or four fills, then put the totals in. Pumps click off at slightly different levels, and a longer run smooths that out.
Same pump if you can
Filling at the same forecourt, on level ground, makes the brim more consistent from one fill to the next. It matters most when you are measuring over a single tank.
L/100km, US mpg and UK gallons
To turn litres per 100km into UK mpg, divide 282.481 by the L/100km figure. The same sum works the other way. The 282.481 comes from two exact definitions: the UK gallon is 4.54609 litres and the mile is 1.609344km, both in NIST's conversion tables and in the Weights and Measures Act 1985. 100 × 4.54609 ÷ 1.609344 is 282.481.
American figures use a smaller gallon, 3.785412 litres on NIST's list. So the same van does about a fifth more miles to the UK gallon than to the US one. Multiply US mpg by 1.201 for UK mpg. In the example, 31.2 UK mpg is 26.0 US mpg. If a figure online looks oddly low, check which gallon it is in.
- 4.4L/100km = 64.2mpg
- 5.6L/100km = 50.4mpg
- 6.9L/100km = 40.9mpg
- 7.4L/100km = 38.2mpg
- 7.6L/100km = 37.2mpg
- 9.0L/100km = 31.4mpg
What your mpg costs per mile
A few mpg either way looks small but adds up over a year of driving to jobs. At 199.3p a litre, a van that does 25mpg costs 36.2p a mile in diesel, and one that does 35mpg costs 25.9p. Over 12,000 miles that gap is about £1,240 a year. Run your own mileage through the fuel cost calculator to see the cost per job, per week and per year.
Fuel is only one part of running a van. HMRC's flat rate for self-employed people using simplified expenses allows 55p per business mile up to 10,000 miles in the 2026/27 tax year and 25p beyond that. You claim it in place of what the van really costs you, so it stands in for fuel, servicing, repairs and insurance together. A van that has had capital allowances claimed on it cannot switch to the flat rate. Set your own cost per mile beside it to see how much of that rate the fuel takes up.
- 25mpg: 11.3L/100km, 36.2p a mile at 199.3p a litre.
- 30mpg: 9.4L/100km, 30.2p a mile.
- 35mpg: 8.1L/100km, 25.9p a mile.
- 40mpg: 7.1L/100km, 22.7p a mile.
- 45mpg: 6.3L/100km, 20.1p a mile.
Why your mpg is below the official figure
Official fuel figures come from a laboratory test. The Vehicle Certification Agency explains that WLTP tests are run in a laboratory so cars can be compared fairly, and that driving style, accessories and extra weight carried all change what drivers actually get. A van with a full load of kit, a roof rack and a day of short hops between jobs is a long way from the test.
That is why the number that matters for pricing is the one you measure. Use the official figure to compare vans when you are buying. Use your own brim-to-brim figure for quotes and budgets, and re-measure when your work changes, such as a winter of short local jobs after a summer of motorway runs.
Where the mpg figure can mislead
It works out fuel economy from the numbers you give it. A misread trip meter or a fill that did not reach the brim will skew the answer, and it cannot tell.
- Electric and plug-in hybrid vans are not covered: their running cost is in kWh and pence per kWh, and a hybrid's petrol figure alone is misleading.
- Fuel only. Tyres, servicing, insurance, finance and depreciation are not in the cost per mile.
- No live prices. Enter the price on your receipt. The default is a national average, and your forecourt may differ.
- Not a fault check. A sudden drop in mpg can point to a problem with the vehicle. Get it looked at by a garage rather than relying on this.
Common questions
How do I calculate mpg in the UK?
Divide the litres you used by 4.54609 to get UK gallons, then divide the miles by that. 400 miles on 55 litres is 400 ÷ 12.10 = 33.1mpg.
How many litres are in a gallon in the UK?
4.54609 litres, exactly. That is the imperial gallon. The US gallon is smaller, at about 3.785 litres, so make sure any mpg figure you compare against is in UK gallons.
How do I convert L/100km to mpg?
Divide 282.481 by the L/100km figure for UK mpg. 6.9L/100km is 40.9mpg, 7.4L/100km is 38.2mpg and 5.6L/100km is 50.4mpg. The same division turns mpg back into L/100km.
How do I work out my mpg from a fuel receipt?
Divide the amount paid by the price per litre to get the litres, then use the miles since your last full tank. £80 at 160p a litre is 50 litres. Over 360 miles that is 32.7mpg.
How do I work out cost per mile?
Divide what you spent on fuel by the miles it covered. Or multiply the price per litre by 4.54609 and divide by your mpg: at 199.3p and 31.2mpg, that is 29.0p a mile.
Is the trip computer's mpg accurate?
It is an estimate from the vehicle's own sensors, and it can read differently from a measured figure. Check it against a brim-to-brim measurement over a few tanks before you price work from it.
What is good mpg for a van?
That varies with the size of the van, the load and the driving, so compare like with like. The most useful comparison is your own van over time, measured the same way, and the official figures of the vans you are choosing between.
Why is US mpg lower than UK mpg for the same car?
Because the US gallon is smaller. A car doing 40 miles to the UK gallon does about 33.3 to the US gallon. The fuel use is the same, only the unit changes.
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