Bill has 3,435 euro in a current account earning nothing. A colleague mentions that she puts everything above 2,000 euro into an index fund. A friend says he keeps six months of expenses in cash and invests the rest. Bill reads three articles and ends up more confused than when he started.
The question he is actually asking is not which investment to choose. The question is what he gives up by not choosing.
Opportunity cost is the value of the best alternative foregone when a decision is made. In Bill's case, leaving 3,435 euro in a zero-interest current account has a cost: the return he could have earned elsewhere. This cost is invisible on the bank statement. It does not appear as an outflow. It accumulates silently as the difference between what he has and what he could have had.
He runs a simple calculation. If he invested 2,000 euro at an average annual return of 5 percent, he would have 2,100 at the end of the year instead of 2,000. The opportunity cost of not investing is approximately 88 euro in year one. Small in absolute terms. Significant as a habit over ten years.
He keeps 1,500 as a liquidity buffer. He decides to invest the remaining 1,770. He does not do it today. He tells himself he will research the options this weekend.
The ledger records the decision. It does not record the weekend.
The tip
The cost of not deciding is not zero. Calculate the opportunity cost of the default option, which is usually doing nothing, before comparing alternatives. Inaction is a choice with a price.
Opportunity cost evaluation, February 2025: 3,435 in current account, opportunity cost of zero-return estimated at 97 per year on investable portion. Decision deferred. No ledger impact this period.
Bill is 24. Every episode, we read his personal ledger through accounting eyes. / FY 2025 ledger