In short: The rule of 72 is a quick way to estimate how long it takes for an amount to double with compound growth. Divide 72 by the yearly growth rate in percent. At 8% a year: 72 ÷ 8 = 9 years. It is an estimate, not a promise: real returns go up and down.
How the rule works
Take the yearly rate in percent and divide 72 by it. The answer is roughly the number of years until the amount has doubled, as long as the growth stays in the pot and keeps growing too. At 4% a year that is 72 ÷ 4 = 18 years; at 9% it is 72 ÷ 9 = 8 years.
The rule works in both directions. It also tells you how fast something halves: if costs or rising prices take 3% a year, the value of an amount halves in roughly 72 ÷ 3 = 24 years. That 3% is just an example, not a figure for any real year.
How accurate is it?
Close enough for mental arithmetic between about 4% and 12%. The table compares the rule with the exact doubling time, which you get with logarithms: ln 2 ÷ ln(1 + rate).
| Growth per year | Rule of 72 | Exact |
|---|---|---|
| 2% | 36 years | 35.0 years |
| 4% | 18 years | 17.7 years |
| 6% | 12 years | 11.9 years |
| 8% | 9 years | 9.0 years |
| 10% | 7.2 years | 7.3 years |
| 12% | 6 years | 6.1 years |
At low rates the rule overestimates a little; there, 70 or 69.3 works better (69.3 is roughly 100 × ln 2). People use 72 because it divides neatly by 2, 3, 4, 6, 8, 9 and 12, which makes it easy to use in your head.
The rule and the ETF calculator
Stazora’s calculators use 6% a year as an illustrative assumption, not a forecast. 6% a year is applied as 0.5% per month, ≈6.17% a year. Growing monthly at that rate, an amount doubles in about 11.6 years; the rule of 72 says 12. The ETF calculator shows the full path, including monthly deposits and costs.
Real investments do not grow at a steady rate. Shares can fall sharply in some years and rise in others, and the order of those years matters. The rule of 72 says how a steady rate works out; it says nothing about what an investment will do.
Costs work the same way
Because the rule is about rates, it also shows why small yearly costs matter. A fund that costs 0.5% a year more lowers the growth rate by about that much. At an illustrative 6% instead of 6.5%, doubling takes about 12 years instead of about 11. Over several decades, that gap adds up. The ETF cost comparison works it out in euros.
Investing involves risk: you can lose money. The rates on this page are examples to explain the arithmetic, not expected returns, and this page is not personal advice.
Investing involves risk of loss. The figures presented by this calculator are for illustrative purposes only and do not constitute financial advice. Past performance does not guarantee future results.
This is general information, not tax or investment advice. For your own situation, consult a qualified tax adviser, a financial adviser licensed by the AFM (the Dutch financial markets authority) or the Belastingdienst.
Frequently asked questions
Why 72 and not 70?
Both are approximations. 69.3 is the most accurate for growth that compounds continuously, 70 is easier at low rates, and 72 has the most whole divisors, so it is the easiest for mental arithmetic at common rates.
Does the rule work with monthly deposits?
No. It is about a single amount that is left to grow. With regular deposits your total grows faster, because you keep adding money. A calculator handles that.
Can the rule tell me what my investment will be worth?
No. It describes a steady rate, which real investments do not have. Shares can fall sharply in some years and rise in others; the rule says nothing about what an investment will do.
How long does it take money to double at 4%, 6% and 8%?
Divide 72 by the rate: about 18 years at 4%, 12 years at 6% and 9 years at 8%. These are arithmetic examples of the rule; real returns vary from year to year and are not promised.
Does the rule work for costs too?
Yes. At a net rate of 5% instead of 6%, doubling takes about 14.4 years instead of 12. Small differences in yearly costs add up over time.
Is the rule of 72 exact?
No, it is an approximation. It is closest for rates of about 6% to 10% and drifts for very low or very high rates.
Related pages
Sources
The figures on this page are arithmetic, not data from a source: the exact doubling time is ln 2 / ln(1 + r), computed by Stazora on 24 October 2026. The rates are examples.