Rule of 72: estimate how long money takes to double
Use the Rule of 72 to estimate doubling time under compound growth, and understand where this mental shortcut falls short.
What is the Rule of 72?
The Rule of 72 is a mental shortcut for estimating how many years an amount takes to double at a fixed annual compound rate:
years to double = 72 / annual rate as a percentageAt 8% a year:
72 / 8 = approximately 9 yearsThe rule illustrates a mathematical relationship; it does not promise that an investment will deliver the assumed rate.
Quick examples
| Annual rate | Approximate doubling time |
|---|---|
| 3% | 24 years |
| 6% | 12 years |
| 8% | 9 years |
| 9% | 8 years |
| 12% | 6 years |
This makes rates easier to compare without a spreadsheet. A seemingly modest difference can matter when it persists for many years.
Limits of the shortcut
The Rule of 72 assumes a constant rate and no deposits or withdrawals. It excludes inflation, taxes, fees and investment volatility, and becomes less precise at unusually low or high rates. Use it for orientation, then run a full calculation before making a decision.
It is not designed for monthly contributions. Once deposits are added, the result depends on their amount and timing as well as the rate and compounding schedule.
Applying the rule to inflation
The same shortcut can illustrate a loss of purchasing power. At 6% annual inflation, prices would roughly double in 12 years:
72 / 6 = approximately 12 yearsIn practical terms, the same sum of money would buy substantially less. Read inflation and compound interest for a calculation in today’s money.
Frequently asked questions
Is the Rule of 72 exact?
No. Exact doubling time depends on the compound-interest formula and compounding frequency.
What if the rate changes?
The estimate no longer describes the entire period. A lower rate generally extends doubling time; a higher rate shortens it.
Can it be used to choose an investment?
Not by itself. It says nothing about risk, liquidity, fees, taxes or whether the rate is sustainable.
Calculate more than doubling time
Use the compound interest calculator for an ending balance, regular deposits and different compounding frequencies. For the underlying mechanism, see what compound interest is.