Methodology

The cost of owning a car over a period is the sum of exactly these things, and nothing else:

depreciation + energy + maintenance + insurance + road tax + other yearly costs + one-off costs + loan interest

The purchase price is not in that list. It enters through depreciation, as the part of it you do not get back. Counting the price and the loss of value would count the same money twice.

Over the whole term, depreciation is simply:

purchase price − value at the end

Spreading that across the years needs a shape, and the calculator offers two. Both start at the purchase price and end at exactly the same residual value; they differ only in between, and which one is in use is always shown.

Faster at first is a geometric curve. With k as the share of the price retained at the end and n the number of years, the value after year t is:

value(t) = purchase price × k^(t ÷ n)

Evenly is a straight line: the same amount every year. It is easier to check by hand, and it is the honest choice when no residual curve has been researched for the model in question — which, today, is every model.

Each year’s figure is the difference between two already-rounded points on the curve, which is why the yearly numbers always add up to the total drop in value exactly, with no stray cent to explain.

Depreciation is never inflated. It already describes a change in value over time.

Borrowing money to buy a car does not make the car cost more. Only the interest is a cost. The principal is the purchase price arriving by a different route, and the purchase price is already accounted for as depreciation.

This is the commonest way a published cost-of-ownership figure goes wrong: monthly repayments get added on top of the car’s loss in value, and the same money is counted twice. Switch financing on at 0% interest and the total here does not move by a cent. That is the test, and it runs on every build.

The loan is a standard annuity — equal monthly payments, interest charged on the balance outstanding at one twelfth of the annual rate. The payment is rounded up to the next cent, as lenders do, and the final payment is reduced to clear the balance exactly.

If the loan runs past the end of the comparison, only the interest charged inside the period counts. The balance still owed at the end is reported separately, because it is something you need to know, but it is not a cost you have paid yet.

Fuel is straightforward:

litres = distance ÷ 100 × consumption per 100 km, then × price per litre

Electricity has a catch. A car’s published consumption is measured at the battery, but your meter charges for what leaves the wall, and some of it is lost as heat on the way. So:

energy billed = energy into the battery ÷ (1 − charging loss)

Note the division. A 10% loss means you pay for 11.1% more energy, not 10% more — multiplying by 1.1 instead understates the bill, and it is a mistake that is easy to make and hard to spot.

A plug-in hybrid splits the distance by the share driven on electricity and costs each part with its own figures. Charging loss applies only to the electric part; there is no equivalent at a fuel pump.

Running costs — energy, maintenance, insurance, road tax and anything else recurring — rise each year by the rate you set. Year one is unchanged; year t is multiplied by:

(1 + rate)^(t − 1)

Depreciation and loan interest are deliberately left out. The residual value is already a figure in today’s money, and loan interest is fixed by contract. Inflating either would count the same effect twice. One-off costs are paid in year one and are not inflated either.

The calculation itself is metric throughout: kilometres, litres per 100 km, kWh per 100 km, and a price per litre. Everything a US reader types is converted once, at the edge, before the arithmetic begins. That way a figure measured one way can never end up in a calculation that assumes another.

The constants are exact by definition, not approximations:

1 mile = 1.609344 km · 1 US gallon = 3.785411784 litres

Which gives 235.2146 as the constant linking l/100 km and US mpg, and 62.1371 for kWh/100 km and mi/kWh. Both are reciprocal relationships: a zero in either direction would mean infinite efficiency, so it is reported as “not stated” rather than converted.

Switching market does not convert your figures. It replaces them with that market’s defaults and says so, because silently reinterpreting 15,000 from kilometres to miles would be worse than starting again.

Every monetary value is held as a whole number of minor units — cents — never as a decimal fraction. Floating-point decimals do not represent money exactly, and a figure that drifts by a cent per line item is a figure nobody can reproduce.

The rounding policy is half away from zero: half a cent becomes a whole cent, and a credit of half a cent becomes a whole cent back. Rounding happens once per line item, when a rate meets an amount, so the error on any published figure is at most half a cent and errors do not accumulate down a column.

Unit prices stay fractional until the last moment. A US fuel price of $3.50 a gallon is 92.45 cents a litre, and rounding that to a whole cent before multiplying it by thousands of litres would move the answer by real money.

Every figure below is a placeholder, not a market observation. None of them has been researched, none is tied to a particular model, and all of them are editable. They exist so the calculator has somewhere to start. Real, cited figures arrive with the data imports, and until then the calculator will not pretend otherwise.

It does not know anything about your car. There is no catalogue yet, so no consumption figure, price or tax rate here is tied to a real model.

It does not model purchase subsidies, company-car taxation, VAT recovery, scrappage schemes, or the tax treatment of a private lease. It does not account for the time value of money: a euro in year ten is treated as a euro.

It assumes you keep the car for the whole period and sell it at the end for the residual value you entered. It does not model repairs outside normal maintenance, nor an accident, nor the cost of a home charger beyond whatever you enter as a one-off.

It does not compare two cars yet. That page is not built.

The calculation engine is a self-contained library with no dependency on the interface, and it is tested against the rules on this page: that the yearly depreciation figures sum to the exact drop in value, that an interest-free loan changes nothing, that charging loss divides rather than multiplies, that inflation never touches depreciation, and that a US form and a Dutch form describing the same journey produce the same fuel bill.

If you find a figure that does not follow from this page, it is a defect and we would like to know.