The cost of a choice is not limited to the money spent: it is the value of the best option it makes you give up.

Opportunity Cost

Definition

An hour free: spend it reading, or sleeping. Choosing to read means giving up sleep — and that renunciation is the real cost of the hour of reading, even though no money changed hands.

The opportunity cost of a choice is the value of the best option that the choice makes you give up.

It is not the total of the options forgone, but the most highly valued among them — the one ranked second when you have taken the first. The reason is that these options exclude one another: you could only ever have taken a single one, and adding them up would amount to believing yourself deprived of what you could never have obtained together. Giving up ten possibilities of which the best was worth 100 € costs 100 €, not the sum of the ten.

This cost arises from scarcity. A resource available without limit would force no trade-off: choosing would then cost nothing. But time remains bounded — and it is because resources are bounded that to choose is necessarily to give something up.

Cost does not reduce to expenditure

The cost of a decision is not limited to the sums paid out. Those form part of it — money disbursed also had other uses — but they do not exhaust it: what accounting does not record counts just as much.

	What accounting records	What opportunity cost adds to it
Nature	Sums actually paid out	Value of the best option forgone, never invoiced
Where found	On an invoice	Not booked, but often estimable from a market price
Can be zero	Yes	Yes, if nothing of value is forgone
Unpaid time	Ignored	Counted

An hour spent studying involves no expenditure. If the best other use of that hour was to sleep, its opportunity cost is the value that hour of sleep would have had.

Likewise, a resource already owned is not free. Premises you own appear on no rent invoice, but occupying them rules out letting them: if the best other option is to let them, their opportunity cost is the rent you could draw from them, net of the costs of putting them on the market.

What it excludes: sunk cost

A sunk cost is an expenditure already incurred and impossible to recover, whatever decision is taken next.

Being identical across every branch of the choice, it cannot separate them: it cancels out in the calculation and should therefore weigh on no decision. Taking it into account — pursuing a mediocre commitment because too much has already been put into it — distorts the trade-off, since no present decision can change the past.

Only the consequences that differ from one option to another count.

Accounting profit and economic profit

Accounting profit is revenue minus the sums paid out. Economic profit further subtracts the value of the best alternative forgone.

An activity can therefore show a positive accounting profit and a negative economic profit: it brings in money and still loses in the economic sense, because the same resources were bringing in more elsewhere.

Formulation

For a set of at least two mutually exclusive options, with values $V_1, \dots, V_n$ net of the sums they require and measured on a single scale, the opportunity cost of choice $i$ is written:

$C_i = \max_{j \neq i} V_j$

— that is, the greatest value among all options other than the one retained.

The choice is not disadvantageous when $V_i - C_i \geq 0$ , and strictly advantageous when $V_i - C_i > 0$ ; at equality, the option retained and the best one forgone are worth the same. When these values are monetary amounts, this difference is the economic profit.

The formula assumes three things: that the options are genuinely exclusive, that their values are comparable on a single scale, and that they are known — under uncertainty, one compares values weighted by their probability. Outside these conditions, the formula no longer applies as it stands.

Opportunity cost is first of all a norm of reasoning: it says what ought to be compared. Whether decision-makers conform to it is a separate, empirical question.

Do real decisions conform to it?

Two departures have been described. Opportunity cost neglect: alternatives that are not presented explicitly are often left out of the comparison. The sunk cost bias: a past expenditure weighs on the decision when it ought to be absent from it.

These departures are observed repeatedly, but their magnitude varies with protocols and populations, and their reach outside the laboratory remains debated. They therefore do not stand at the same level of certainty as the definition itself.

Summary

The opportunity cost of a choice is the value of the best option it makes you give up — neither the money spent, nor the total of the options forgone.