Opportunity cost in numbers
Every dollar spent today is a dollar that cannot grow. At 7% a year, money roughly doubles every ten years, so $500 today is about $1,000 in a decade, $3,800 in 30 years and $7,500 in 40. Economists call that the opportunity cost of spending.
Use the number as a pause button, not as guilt. Waiting 48 hours before non-essential purchases is a simple rule that filters out many impulse buys while still allowing the ones you really want.
Small impulses add up
One impulse buy rarely matters on its own. The habit is what adds up. At 7% a year, a single $500 purchase invested instead would be worth about $984 in ten years. But $100 a month of unplanned purchases, about the cost of a few small online orders, would grow to about $52,000 in 20 years if invested instead. Seeing the monthly figure is often more useful than agonising over each item.
A simple way to cut impulse spending without feeling deprived is a waiting rule: put anything over a set amount, say $50, on a list and buy it only if you still want it after 30 days. Many items quietly drop off the list. The cost of a habit calculator shows the long-run cost of any regular expense.
Common questions
What is the 30-day rule?
A saving habit where you wait 30 days before buying anything non-essential above a set price. If you still want it after the wait, you buy it; if not, the money stays saved.
What is opportunity cost?
The value of the best alternative you give up, here the future value of money you spend instead of investing.
What return should I use?
7% is a common long-term stock market planning figure after inflation is ignored; use 4% to 5% for a cautious view.
This calculator gives estimates for planning. Real returns, rates and prices change, so treat the result as a guide rather than a promise. Read how we build and check our calculators.