Opportunity Cost in One Line
Opportunity cost is the value of the best alternative you give up when you make a choice.
If you spend Saturday doing your own bookkeeping, the opportunity cost is whatever else you would have done with that Saturday. If that was a paid job worth £400, the bookkeeping cost you £400, even though no money left your bank account.
That last point is the whole reason the idea matters. Your profit and loss account records what you spent. It never records what you missed. A business can look profitable on paper and still be making a poor return, because the accounts have no line for the better option that was turned down.
The Opportunity Cost Formula
The usual formula is:
Opportunity cost = return on the best option not chosen − return on the option chosen
Say you have £50,000 to put to work. Option A, a new van, should add £6,000 a year of profit. Option B, paying off a loan, would save £4,000 a year of interest. You choose the van.
Opportunity cost = £4,000 − £6,000 = −£2,000
A negative answer means you chose the better option. A positive answer means the road not taken was worth more than the one you took.
Two rules make the formula useful rather than decorative:
- Only the best alternative counts. You do not add up every option you rejected. You compare against the next best one.
- The alternative must be real. "I could have put it all in a fund that went up 30%" is hindsight, not an alternative. Use options that were actually available, at the risk level you would actually accept.
Opportunity Cost in Economics
Most people searching this term are studying economics, so here is the textbook version briefly.
Economics treats every resource as scarce. A worker, an hour, a machine or a pound can only be used once, so every use has a cost in terms of the next best use. The classic illustration is a production possibility frontier: a bakery with one oven can make 200 loaves or 400 croissants a day. Each loaf therefore has an opportunity cost of two croissants, and each croissant costs half a loaf.
Opportunity cost ratio = what you give up ÷ what you gain
Economists also split costs into two kinds:
- Explicit costs, which involve a payment. Rent, wages, materials.
- Implicit costs, which do not. The salary an owner could earn elsewhere, or the interest their money could earn elsewhere.
Accounting profit deducts only explicit costs. Economic profit deducts both. That gap is where most of the practical lessons below come from.
Why Accountants Care About a Cost That Is Not in the Accounts
Financial accounts are built to record what happened, under rules that need evidence: invoices, bank statements, contracts. An option you did not take leaves no evidence, so it cannot go in. That is correct for statutory accounts. It is a problem when owners use those same accounts to make decisions.
Our view, after years of preparing accounts for owner-managed businesses, is that opportunity cost is the most important number that never appears in a set of accounts. It shows up in three places more than anywhere else: the owner's own time, cash sitting idle, and money tied up in stock and unpaid invoices. We put a rough figure on each in the management accounts we prepare, because a number on the page gets discussed and a number in someone's head does not.
Example 1: The Owner's Own Time
Priya is a self-employed graphic designer in Finchley. Her accounts show a profit of £42,000 for the year. That looks like a decent result.
She has been offered an in-house role at a London agency for £48,000, with a 5% employer pension contribution of £2,400.
| Amount | |
|---|---|
| Accounting profit from self-employment | £42,000 |
| Salary given up | (£48,000) |
| Employer pension given up | (£2,400) |
| Economic profit | (£8,400) |
In economic terms Priya's business is losing £8,400 a year compared with her best alternative. That is a pre-tax comparison and it ignores plenty that matters: flexibility, the value of a client base she owns, the chance to grow, and the tax differences between employment and self-employment. We would never tell a client to shut their business on this sum alone.
What we do tell them is to know the number. Priya's real question is not "am I profitable?" It is "am I paid fairly for my hours?" Once she sees the gap, the answers are usually about pricing: raising day rates, dropping low-margin clients, or charging properly for revisions.
The same logic applies to admin. If you spend six hours a month on bookkeeping and could bill those hours at £60, the opportunity cost is £4,320 a year. But only if you would genuinely fill those hours with paid work. If the alternative is watching television, the opportunity cost of your Sunday bookkeeping is close to zero, and doing it yourself may be perfectly sensible. Be honest with yourself about which one it is.
Example 2: Cash Sitting in a Current Account
Ashby Design Ltd, a small studio in Barnet, has £180,000 in its business current account earning nothing. It needs about £40,000 as a buffer, roughly two months of costs. That leaves £140,000 of surplus cash. It also has a £60,000 bank loan at 8%.
| Option | Annual effect before tax |
|---|---|
| Leave it in the current account | £0 |
| Move £140,000 to a business deposit account at, say, 3.5% | +£4,900 interest |
| Repay the £60,000 loan, put £80,000 on deposit | +£4,800 interest saved, +£2,800 interest earned = +£7,600 |
Doing nothing costs the company about £7,600 a year compared with its best realistic option. Over five years that is close to £38,000, which would never appear anywhere in the accounts as a cost.
The reason repaying the loan wins is simple. Every pound used to clear the loan "earns" 8% by not paying interest, while a pound on deposit earns 3.5%. Paying off the expensive debt first has the lower opportunity cost.
A few cautions from real cases. Do not repay debt with the buffer money, because a business that clears its loan and then cannot pay VAT is worse off. Check for early repayment charges. Interest earned inside a company is taxed as profit, so the net gain is smaller than the gross figure. And if the cash is building up because profits are not being extracted, the better question may be dividends, pension contributions or retained earnings planning, which is a tax decision as much as a cash one.
Example 3: Taking on a Job When You Are Busy
Opportunity cost matters most when capacity is tight. Here is a decision we see in some form most months.
Hartley Joinery Ltd in Muswell Hill is offered a one-off job fitting bespoke shelving in a restaurant for £9,000. The workshop's job costing sheet prices it like this:
| Cost line (full costing) | Amount |
|---|---|
| Oak already in stock, bought last year for a cancelled job | £2,800 |
| Extra materials to buy | £1,200 |
| Labour, 60 hours at £22 | £1,320 |
| Depreciation on the CNC machine | £600 |
| Share of workshop overheads | £1,400 |
| Total cost | £7,320 |
| Profit | £1,680 |
A thin but positive margin. Take the job? The full costing sheet answers the wrong question. For a decision you need relevant costs: future, cash, and only those that change because of the decision.
- The oak. The £2,800 has already been spent. It is a sunk cost and is irrelevant. What matters is what Hartley gives up by using it. Nobody else needs it, but another workshop would buy it for £1,500. The relevant cost is £1,500.
- Extra materials. New spending caused by the job. Relevant cost £1,200.
- Labour. The two joiners are salaried and paid whether or not this job happens. The relevant cost depends entirely on what else they would be doing.
- Depreciation. No extra cash moves because of this job. Depreciation is an allocation of a past purchase. Not relevant.
- Overheads. Rent and insurance are the same either way. These are fixed costs and not relevant.
Now look at two scenarios.
Scenario A, a quiet month. The joiners have spare hours. Labour has no opportunity cost.
Relevant cost = £1,500 + £1,200 + £0 = £2,700. The job adds £6,300 to the company's position. A much better deal than the costing sheet suggested.
Scenario B, a full order book. Taking the restaurant job means turning away a kitchen fit-out that would have contributed £7,000 after its own materials. Now the labour hours have an opportunity cost of £7,000.
Relevant cost = £1,500 + £1,200 + £7,000 = £9,700. The job makes Hartley £700 worse off.
The full costing sheet says yes in both cases. Relevant costing says take it gladly in a quiet month and politely decline in a busy one. Same job, same price, opposite answers, and the only thing that changed was the opportunity cost of the joiners' time.
Opportunity Cost vs Sunk Cost
These two get confused because both are "costs" that do not behave like normal costs. They point in opposite directions.
| Opportunity cost | Sunk cost | |
|---|---|---|
| Timing | Future | Past |
| Can you still change it? | Yes, by choosing differently | No, it is already spent |
| Shows in the accounts? | Never | Yes, as an expense or an asset |
| Relevant to a decision? | Always | Never |
| Example | Interest you could earn on idle cash | The £2,800 already paid for the oak |
What Is a Sunk Cost?
A sunk cost is money already spent that you cannot get back whatever you decide next. Market research for a product you then drop, a deposit you cannot recover, fees for a planning application, a fit-out on a lease you are now leaving. Accounting treats some of these as assets on the balance sheet for a while, but that does not change their nature. The cash is gone.
The Sunk Cost Fallacy
The sunk cost fallacy is the habit of continuing with something because of what you have already put in, rather than because of what it will return from here. "We have spent £30,000 on this software project, we cannot stop now" is the classic version. The £30,000 is gone whether you stop or carry on. The only question is whether the next £20,000 will earn more than £20,000 spent anywhere else.
Psychologists Hal Arkes and Catherine Blumer showed in a 1985 study that people are more likely to keep going with a failing project the more they have already spent on it, which is the reverse of what sound decision making requires.
In our experience the fallacy is strongest in three places: second premises that never found their trade, stock bought in bulk that will not sell at full price, and in-house software or systems that are always "nearly finished". The test we suggest is simple. Imagine you had not spent a penny yet and someone offered you the project as it stands today, for the remaining cost. Would you buy it? If not, stop.
Opportunity Cost Hiding in Your Working Capital
Every pound tied up in working capital has an opportunity cost. It could be repaying debt, earning interest, or funding growth.
- Slow-paying customers. If your debtor days are 70 when your terms are 30, the extra 40 days of sales are effectively an interest-free loan to your customers. On £1 million of turnover that is roughly £110,000 of cash you are funding. At 8% borrowing cost, that is close to £8,800 a year.
- Slow-moving stock. Our guide to stock turnover shows how a few extra weeks of stock ties up cash. The opportunity cost is the return that cash could earn, plus storage, insurance and the risk of it going out of date.
- Paying HMRC late. Some owners hold on to cash by paying tax late. HMRC charges late payment interest at Bank of England base rate plus 4%, and for individuals that interest is not tax deductible. Using HMRC as a lender is usually more expensive than an overdraft. Going the other way, HMRC pays repayment interest at only base rate minus 1%, with a 0.5% floor, so paying tax very early rarely beats a deposit account either.
How to Use Opportunity Cost Without Overthinking It
Opportunity cost can become an excuse never to decide, because there is always some alternative. These are the rules we use with clients:
- Name one realistic alternative. Not the best case, not a dream. The option you would actually take.
- Put a rough number on it. A range is fine. "Somewhere between £3,000 and £5,000 a year" is enough to make a decision.
- Ignore sunk costs completely. Past spending only matters if it can be recovered or sold, in which case use the recoverable value.
- Ignore allocated costs that do not change. Depreciation and overhead shares are for pricing and reporting, not for one-off decisions.
- Do not count it twice. If you have already deducted the interest you are paying on a loan, do not also deduct the interest you could have earned on the same money.
- Remember non-financial factors. Risk, stress, customer relationships and flexibility have value. The sum informs the decision, it does not make it for you.
Frequently Asked Questions
Is opportunity cost a real cost?
Yes in economics, no in accounting. It does not involve a payment, so it never appears in a profit and loss account or on a tax return. But it is real in the sense that choosing badly leaves you with less money than you could have had.
Can opportunity cost be zero?
Yes. If a resource has no alternative use, its opportunity cost is zero. Spare machine time in a quiet month, staff who are paid anyway with nothing else to do, or materials with no resale value all have an opportunity cost of nothing.
Can opportunity cost be negative?
Using the formula above, a negative result means the option you chose earned more than the best alternative, so you made the right call. Some people describe opportunity cost only as a positive value, in which case it is simply the value of the next best option.
Is depreciation a sunk cost?
The purchase of the asset is a sunk cost. Depreciation is the accounting method of spreading that past purchase over its useful life. That is why depreciation is ignored in relevant costing: deciding to take a job or not does not change what you already paid for the machine.
What is the difference between opportunity cost and trade-off?
A trade-off is the choice itself, giving up some of one thing to get more of another. Opportunity cost is the value of what you give up in that trade-off.
How IAK Can Help
We work with small businesses, limited companies and construction companies across North London and Hertfordshire. Most of the decisions that matter to them, from taking on a big contract to buying a van or leaving cash in the bank, turn on a cost that their year-end accounts never show.
Our management reporting puts those numbers in front of you every month, alongside break-even analysis, a cash flow forecast and your working capital days. Our tax planning work looks at the cash building up in your company and the most efficient way to use it. If you are weighing up a decision and want a second pair of eyes on the numbers, get in touch.
Sources
- Arkes, H. R. and Blumer, C. (1985), The psychology of sunk cost, Organizational Behavior and Human Decision Processes 35(1), the original experimental study of the sunk cost effect.
- Rates and allowances: HMRC interest rates for late and early payments, GOV.UK, for late payment interest at base rate plus 4% and repayment interest at base rate minus 1%.
- Bank Rate, Bank of England, the base rate that HMRC interest and many business deposit and loan rates track.
- Opportunity cost, Simple English Wikipedia, for the general economics definition.