Rule of 72 Calculator
The fastest mental-math trick in personal finance: 72 divided by your rate of return = years to double. Try it with different rates below.
Years to double at 7%
10.3
Rule of 72 estimate · exact calculation: 10.2 years
2× (double)
10.3y
3× (triple)
16.2y
4× (quadruple)
20.5y
Why this matters
The Rule of 72 turns abstract percentages into concrete time horizons. "I\'m getting 7%" feels arbitrary; "my money doubles every ~10 years" feels intuitive. That intuition shapes better decisions.
Compare two scenarios. Cash in a savings account at 1.5%: doubles every 48 years. Diversified equity portfolio at 7%: doubles every ~10 years. Same starting amount; over a 40-year horizon the equity portfolio multiplies 16× while the savings account barely doubles. That\'s the cost of staying in cash.
Or use it for inflation. At 3% inflation, purchasing power halves every 24 years. A $1M nest egg today buys what $500k buys in 24 years. Plan accordingly.
Frequently asked questions
What is the Rule of 72?
The Rule of 72 is a quick mental shortcut: divide 72 by your annual rate of return to estimate how many years it takes for an investment to double. At 7% it doubles in ~10.3 years (72/7). At 8% it's 9 years (72/8). The rule works because of how exponential growth approximates around typical investment rates.
How accurate is the Rule of 72?
Very accurate for rates between 6-12% — the typical range for stock-market returns. At lower rates (1-3% savings accounts) it slightly underestimates; at higher rates (>15%) it slightly overestimates. The exact formula is ln(2) / ln(1+r), which equals 69.3 / (rate as %) for low rates and rises to 72-73 for higher rates.
Where does "72" come from?
72 is a convenient approximation of 100 × ln(2) ≈ 69.3, adjusted upward slightly because most realistic investment rates are above 0%. 72 also has many divisors (1, 2, 3, 4, 6, 8, 9, 12), making the mental math easier than 69.3.
What's the inverse — Rule of 72 for required rate?
Same formula, rearranged. To double your money in 10 years, you need 72/10 = 7.2% return. To double in 20 years, 72/20 = 3.6%. Useful for thinking about realistic investment targets given a time horizon.
Does it work for negative returns / inflation?
Yes — divide 72 by the inflation rate to estimate how long until purchasing power halves. At 3% inflation, money loses half its value in ~24 years. At 5%, ~14.4 years. Useful for thinking about cash savings vs invested savings.