Financial literacy education for the modern era.
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Mental Math
Rule of 72
A quick mental math trick to estimate how long it takes for an investment to double. Divide 72 by the annual interest rate.
Try It Yourself
7%
72 ÷ 7 = 10.3 years
Exact calculation: 10.24 years (difference: 0.04 years)
10.3
years to 2x
Savings Account
2%
36.0 years
Bonds
5%
14.4 years
Index Funds (Historical)
7%
10.3 years
Growth Stocks
10%
7.2 years
Money Growth at 7% Return
$1
Year 0
$2
Year 10
$4
Year 21
$8
Year 31
$16
Year 41
Starting with $1, see how your money doubles repeatedly over time
When to Use
- Quick mental calculations during conversations
- Comparing different investment options
- Understanding the impact of fees (72 ÷ 2% fee = 36 years to halve)
- Estimating inflation's impact on purchasing power
Limitations
- Most accurate for rates between 6-10%
- Assumes constant returns (unrealistic for stocks)
- Doesn't account for taxes or fees
- For Rule of 69.3, use for continuous compounding