Sunk Cost vs Opportunity Cost
Sunk cost is money or effort already spent and unrecoverable; opportunity cost is the value of the best alternative you give up by choosing one path. Rational decisions ignore the first and weigh the second — yet people reliably do the reverse.
| Dimension | Sunk Cost | Opportunity Cost |
|---|---|---|
| What it is | A past, unrecoverable expense | The value of the best foregone alternative |
| Type | A cognitive bias (a trap) | A decision-making model (a tool) |
| Time direction | Backward-looking — already paid | Forward-looking — what you give up next |
| Rational role | Should be ignored in the decision | Should be weighed in every decision |
| Failure mode | Throwing good money after bad | Ignoring the cost of the road not taken |
Two costs facing opposite directions
A sunk cost points into the past: the non-refundable ticket, the two years already poured into a doomed project, the money spent on a course you no longer attend. An opportunity cost points into the future: of all the things you could do next, the value of the best one you must forgo. One is history; the other is a live trade-off.
Why they get confused
Both wear the word "cost," and both feel like losses. But a sunk cost is not a real input to any good decision — it is gone regardless of what you choose now. An opportunity cost is the only cost that actually matters for the choice in front of you, because it measures what you are truly giving up by saying yes to one option.
The decision rule
The discipline is one sentence: ignore sunk costs, weigh opportunity costs. The right question is never "how much have I already put in?" but "given where I am now, what is the best use of my next hour or euro — and what does each option cost me in foregone alternatives?" The past spend is irrelevant; only the forward comparison counts.
A worked example
You are 90 minutes into a three-hour film you dislike. The sunk-cost voice says "I paid for it, I should finish." The opportunity-cost voice asks "what is the best thing I could do with the next 90 minutes?" The ticket is spent either way. The only live cost is the better evening you are trading away by staying in your seat.
The verdict
They are not rivals — they are a trap and its antidote. The sunk-cost fallacy keeps you anchored to irrecoverable past spend; opportunity-cost thinking is the corrective that re-centres every decision on the live, forward trade-off. Master the second and the first loses its grip: once you habitually ask "what is the best alternative use?", "but I already paid" stops sounding like a reason.
Frequently asked
- Is opportunity cost a bias like sunk cost?
- No. Opportunity cost is a sound decision-making model — a tool for comparing options. The sunk-cost fallacy is a cognitive bias — an error. Confusing the two is itself part of the trap.
- Why do we fall for the sunk-cost fallacy?
- Loss aversion: abandoning a project makes the past spend feel "wasted," and we hate realising losses. Continuing lets us avoid that feeling — even when continuing costs more. The discomfort is emotional, not rational.
- How do I use opportunity cost in daily decisions?
- Before committing time or money, name the single best alternative you would give up. If your chosen option is not clearly better than that foregone one, reconsider. The comparison — not the past spend — is the real decision.
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Editorial synthesis © ReadGlobe 2026, drawing on the behavioural-economics and decision-theory literature (Kahneman, Thaler) and the mental-models tradition. · Last reviewed 2026-05-29.