Finance⏱ 5 min read
What Is Opportunity Cost and How Do You Calculate It?
Every financial decision involves a trade-off with the next best alternative. Opportunity cost makes those trade-offs explicit — and it changes how you think about spending, saving, and investing.
Opportunity cost is the value of the best alternative you give up when making any decision. It's one of the most important concepts in economics — and one of the most consistently ignored in personal finance.
The Basic Definition
Opportunity cost = Value of best alternative foregone
It's not the average of all alternatives you didn't choose —
it's the single best option you passed up.
Paying off a 3% mortgage vs investing at expected 7% return:
Opportunity cost of debt payoff = 7% - 3% = 4% per year
(You "lost" the difference between the two rates)
The True Cost of Spending
Every £1 spent today has a future opportunity cost — what that £1 would have grown to if invested instead. At 7% annual return over 30 years:
Future value = £1 x (1.07)^30 = £7.61
So a £30,000 car at age 25 "costs" not just £30,000
but £30,000 x 7.61 = £228,300 in retirement wealth
This doesn't mean never spend — it means understanding
the full cost of decisions, especially large irreversible ones.
Opportunity Cost of Holding Cash
Cash earning 0% in a current account while inflation = 4%:
Real return = 0% - 4% = -4% per year
£20,000 emergency fund, 5 years:
Nominal value: £20,000 (unchanged)
Real purchasing power: £20,000 / (1.04)^5 = £16,438
Opportunity cost: £3,562 in lost purchasing power
vs same £20,000 in a 5% easy-access savings account:
Real return = 5% - 4% = 1% per year
Real value after 5 years: £21,020
Difference: £4,582 better off just by moving the cash.
Opportunity Cost in Business: The Outsourcing Decision
Freelancer earns £80/hour consulting.
They spend 4 hours/month doing their own bookkeeping.
Explicit cost of DIY bookkeeping: £0 (they do it themselves)
Opportunity cost: 4 hours x £80 = £320/month foregone
Outsourcing bookkeeping costs £150/month.
Net saving from outsourcing: £320 - £150 = £170/month
Ignoring opportunity cost makes the DIY option look "free"
when it actually costs £320/month.
The Sunk Cost Fallacy (Opposite Error)
While opportunity cost looks forward at future alternatives, sunk costs look backward at past spending. A sunk cost is money already spent that cannot be recovered — it should never influence future decisions, because the money is gone regardless of what you do next.
Example: Paid £200 for a concert ticket (non-refundable).
You feel unwell on the night.
Sunk cost logic: "I must go — I paid £200."
Correct logic: The £200 is gone either way.
The real question: Is going to the concert better than staying home
given your health tonight?
The ticket price is irrelevant to that decision.
Applying Opportunity Cost to Overpaying Your Mortgage
Mortgage rate: 3.8%
ISA expected return: 7% (historical stock market average)
Opportunity cost of mortgage overpayment = 7% - 3.8% = 3.2%/year
On £10,000 overpayment:
Mortgage interest saved: £380/year (guaranteed)
Investment return foregone: £700/year (expected, not guaranteed)
Expected cost of overpaying: ~£320/year
For higher-rate mortgage (6%):
Guaranteed saving: £600/year
Expected foregone: £700/year
Much closer decision — many would prefer the guaranteed saving.