How to Calculate Petrol Money and Fuel Costs Per Mile

If you share your car with friends, family or co-workers, it’s important to ensure that everyone pays what they owe when it comes time to top up the petrol. It’s not enough to simply take the cost of refuelling and divide it by however many people use your car, because inevitably, some people will drive more than others. So it’s much better to work out, as best you can, just how much everyone owes in terms of the amount of petrol they consumed while using the car.

In this post, we’ll consider why it can be difficult to properly calculate petrol money, before sharing some hints on tips to ensure that everything stays fair and equitable for everyone.

These tips will also come in handy if you’ve ever given someone a lift and they’ve offered to pay you petrol money. This way, rather than asking them for an arbitrary figure, you can request they pay exactly what they owe!

Why Is It Hard to Calculate Petrol Money?

Calculating petrol money is hard for anyone, but it feels like it might be particularly difficult for UK drivers.

Gallons vs. Litres

When talking about fuel consumption, car manufacturers talk in terms of miles per gallon (MPG). Unfortunately, UK petrol stations charge by the litre. So before you even begin to calculate petrol money, you’ve already got to contend with two different units of measurement.

Motorways vs. City Driving

Plus, a car’s MPG is not set in stone. It’s really just an estimate. And on top of this, your car will return a different mileage when you’re driving in the city than it will when you’re driving on the motorway. On the motorway you can keep a pretty constant speed. Whereas in the city, you’ll repeatedly stop and start, your speed will fluctuate constantly, and you’ll spend a lot of time waiting in traffic. All of this can greatly ramp up your fuel consumption.

MPG Estimates

Do you even know what your car’s MPG is? Forget about the figure the manufacturers and the dealership told you. How much mileage are you actually getting? You can use this handy tool on the What Car site to find out. Just choose the make, model and specifications of your car, then answer a few questions about your driving habits. You’ll get a more-educated estimate of your actual MPG. And the figure you get might surprise you!

Some factors that affect your MPG are beyond your control. The weather, for example. Strong winds and wet surfaces can both increase your fuel consumption. You’ll also burn through more fuel if you’re carrying a heavy load.

All of these factors mean that it’s hard to work out exactly how much you spent on fuel for a given journey. Luckily, certain calculations can give you as good an idea as you’re going to get.

How to Calculate Petrol Money

So that whole gallon/litre discrepancy really isn’t as much of a problem as you think it is. There are 4.5 litres in a gallon. So your first step should be to multiply the cost of fuel per litre by 4.5. This will give you the cost of the fuel per gallon.

Then you need to divide the distance of the journey by your car’s actual MPG, which you can calculate using the What Car tool above. To calculate petrol money, it’s just a case of multiplying those two figures together:

Petrol Money for One Journey = (cost of fuel per litre x 4.5) x (trip distance / fuel consumption (mpg))

Other Costs You Might Want to Consider

But is it enough to simply charge for petrol? Because that’s not the only expense you incur while driving. You also have to take into consideration wear and tear to key components like the tyres and the transmission, and all of the ongoing maintenance and repairs this could lead to. And of course, you have to consider car insurance. The amount you pay for your car insurance is largely determined by the car you drive and your driving habits. So you should absolutely take this into consideration when calculating petrol money.

But thinking about the hidden costs of driving could just make your overall calculation considerably more complex. So why not use the same sort of formula that most companies use when calculating travel expenses? Rather than coming up with an exact figure (or an educated guess at an exact figure), they instead come up with a slightly exaggerated figure to account for the various hidden costs of driving. Here’s the formula most companies use:

Petrol Money = 45p per mile for the first 100 miles, then 25p per mile.


Really, calculating petrol money is a lot like working out the bill at the restaurant. Splitting it evenly means that some people pay more than they ought to, so you need to work out how much everyone owes. But unlike splitting a bill at a restaurant, you’ll never be able to get a precise figure for petrol money. There are far too many unknowns. So your best bet is to make things as fair as possible. And the two formulae in this post should help you to make things fair for everyone.


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