Rule of 72 Calculator
Estimate how many years it takes for money to double at a given annual return, using the classic Rule of 72.
A doubling time you can check in your head
This is the quickest tool in the set. It answers only one question, roughly how many years a steady rate takes to double something, and it needs only the rate. Use it as a sanity check before reaching for a full projection.
The calculator shows three lines:
- Years to double:
72 ÷ rate - Years to quadruple: twice that, since quadrupling is two doublings
- More exact (ln 2):
ln 2 ÷ ln(1 + rate ÷ 100), the true doubling time with growth compounded once a year
At the default 8%, money doubles in about 9 years, quadruples in 18, and the exact figure is 9.01 years.
How close the shortcut gets
| Rate | Rule of 72 | Exact (annual compounding) |
|---|---|---|
| 1% | 72 | 69.66 |
| 2% | 36 | 35 |
| 4% | 18 | 17.67 |
| 6% | 12 | 11.9 |
| 8% | 9 | 9.01 |
| 10% | 7.2 | 7.27 |
| 12% | 6 | 6.12 |
| 15% | 4.8 | 4.96 |
| 20% | 3.6 | 3.8 |
| 24% | 3 | 3.22 |
The rule is closest around 8%. Below that it slightly overstates the time, and above it the rule says you double sooner than you actually do. At 24% it is off by under three months. At 1% it is off by more than two years, but that is still small next to a doubling time of about 70 years.
Running it backwards to find a rate
Divide 72 by the number of years instead and you get the rate needed to double in that time. To double in 6 years you need roughly 72 ÷ 6 = 12% a year. The exact answer is 12.25%.
The calculator only takes a rate, so to check a target, type rates until the doubling time matches. This is a quick way to test whether a promised outcome, such as doubling in five years, implies a return you consider realistic.
Doubling works against you too
Any steady percentage compounds, including the ones you pay:
- Prices. If inflation runs at 3%, prices double in about 24 years by the rule (23.45 exact). The same money buys half as much.
- Debt. An unpaid balance growing at 24% a year doubles in about 3 years (3.22 with annual compounding).
- Decline. For something shrinking by a steady rate, enter the rate as a positive number and read the result as an approximate halving time. Falling 5% a year halves a value in about 14.4 years by the rule; the exact figure is 13.51 years. The exact line on the calculator assumes growth, so it won't match for a decline.
Frequently asked questions
Why 72 and not 70 or 69?+
The precise constant for continuous compounding is about 69.3, from 100 × ln 2. People use 72 because it divides evenly by 2, 3, 4, 6, 8, 9 and 12, and because it happens to be more accurate for typical annual rates. Some prefer 70 for very low rates.
Does the result change with monthly compounding?+
Yes, slightly. The exact line here assumes the rate is applied once a year. At 8% compounded monthly, money doubles in about 8.69 years instead of 9.01.
Can I use it to see how long money takes to triple?+
Not with 72. A common rough rule for tripling uses 114 instead of 72. At 8% it gives about 14.25 years, while the exact figure with annual compounding is about 14.27.
Does the rule of 72 work with regular contributions?+
No. It assumes one sum growing at a steady rate with nothing added. If you keep adding money, your balance doubles much sooner than the rule suggests, and you need the investment calculator to see by how much.
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