Rule of 72 Calculator
Quickly estimate how long it takes to double your money at a given interest rate, or find out what rate you need to double in a specific time.
Calculate Years to Double
At 8% annual return, your money will double in:
(using Rule of 72)
Rule of 72
Most common
Rule of 70
Lower rates
Rule of 69.3
Continuous compound
Exact Calculation
Most accurate
Compound Growth Timeline
| Doublings | Years | Value (from $1) | Growth |
|---|---|---|---|
| 0x | 0.0 yrs | $1 | - |
| 1x | 9.0 yrs | $2 | +100% |
| 2x | 18.0 yrs | $4 | +300% |
| 3x | 27.0 yrs | $8 | +700% |
| 4x | 36.0 yrs | $16 | +1500% |
| 5x | 45.0 yrs | $32 | +3100% |
Starting with $1, your investment would grow to $32 after 45.0 years at 8% annual return.
Common Investment Doubling Times
4%
Savings Account
18.0 yrs
6%
Bonds
12.0 yrs
8%
Balanced Fund
9.0 yrs
10%
Stock Market (Historical)
7.2 yrs
12%
Growth Stocks
6.0 yrs
What is the Rule of 72?
The Rule of 72 is a simple formula used to estimate the number of years required to double an investment at a fixed annual rate of return. By dividing 72 by the annual rate of return, investors get a rough estimate of how many years it will take for the initial investment to duplicate itself.
Years to Double
Years = 72 / Interest Rate
Example: At 8% return, your money doubles in 72 / 8 = 9 years
Required Rate
Rate = 72 / Years
Example: To double in 6 years, you need 72 / 6 = 12% return
Why 72?
- +Mathematical convenience: 72 is divisible by many common numbers (2, 3, 4, 6, 8, 9, 12)
- +Good approximation: For interest rates between 6-10%, it's remarkably accurate
- +Easy mental math: No calculator needed for quick estimates
- +Historical use: The rule dates back to the 15th century mathematician Luca Pacioli
Rule Variations
| Rule | Best For | Formula |
|---|---|---|
| Rule of 72 | 8% interest rate | 72 / rate |
| Rule of 70 | Lower rates (2-4%) | 70 / rate |
| Rule of 69.3 | Continuous compounding | 69.3 / rate |
| Rule of 114 | Triple your money | 114 / rate |
Practical Applications
Retirement Planning
Estimate how many times your retirement savings will double before you retire.
Investment Comparison
Quickly compare different investments by their doubling times.
Debt Growth
See how fast credit card debt can double if left unpaid.
Inflation Impact
Calculate when prices will double due to inflation.
Limitations
- !Approximation only: The rule provides estimates, not exact calculations.
- !Assumes constant rate: Real returns fluctuate year to year.
- !Less accurate at extremes: Works best for rates between 6-10%.
- !Ignores taxes and fees: Actual returns may be lower after costs.