Rule of 72 Calculator
Estimate how long it takes to double your money at a given interest rate. A quick mental math trick for investors.
Calculate Doubling Time
Results
The Rule of 72
The Rule of 72 is a simple formula to estimate doubling time:
For example, at 8% interest: 72 ÷ 8 = 9 years to double your money.
Doubling Time at Common Rates
| Interest Rate | Years to Double | Example Investment |
|---|---|---|
| 2% | 36 years | Savings account |
| 4% | 18 years | Bonds |
| 6% | 12 years | Conservative portfolio |
| 8% | 9 years | Balanced portfolio |
| 10% | 7.2 years | Stock market average |
| 12% | 6 years | Growth portfolio |
The Power of Multiple Doublings
| Doubling | Years | Value |
|---|
How Accurate is the Rule of 72?
The Rule of 72 is most accurate for interest rates between 6% and 10%. For lower rates, use 70; for higher rates, use 73-75. The exact formula uses natural logarithms: Years = ln(2) ÷ ln(1 + r).
Related Rules
- Rule of 114: Triple your money (114 ÷ rate)
- Rule of 144: Quadruple your money (144 ÷ rate)
About the Rule Of 72 Calculator
The Rule of 72 is the simplest mental shortcut in finance: divide 72 by your annual return to estimate years to double your money. This page covers when it's accurate, when it's not, and how to extend it to other doubling/halving questions.
The Formula
Doubling years ≈ 72 ÷ Annual rate (as percentage, not decimal). Inverse: Required rate ≈ 72 ÷ Years to double.
Worked Example
At 6% return: 72 ÷ 6 = 12 years to double. At 9%: 8 years. At 4%: 18 years. At 12%: 6 years. To double in 10 years you need: 72 ÷ 10 = 7.2% return.
When the rule is accurate
Most accurate for rates between 5% and 12%. At 8%, actual doubling time is 9.0 years; rule says 9.0 — exact. At 2%: rule says 36, actual 35 — close. At 25%: rule says 2.88, actual 3.1 — diverging.
Reversing the rule for inflation
Use 72 ÷ inflation rate to estimate how long it takes purchasing power to halve. At 3% inflation: 24 years. At 5%: 14 years. Useful for retirement planning — your spending power can easily halve during a long retirement.
The Rule of 114 (tripling) and 144 (quadrupling)
72 doubles. 114 triples. 144 quadruples. At 8%: doubles in 9 years, triples in 14, quadruples in 18. Combined power of compounding.
Common Mistakes
- Using the rule for very high (30%+) or very low (under 2%) rates. Use the actual log formula instead.
- Forgetting taxes and inflation. The Rule of 72 calculates nominal pre-tax doubling.
- Confusing 'doubles in 10 years' with 'gives you 10% per year'. They're not the same.
Frequently Asked Questions
Why 72?
It's a mathematical approximation derived from the natural logarithm. Works because ln(2) ≈ 0.693, and 0.693/r approximates the doubling time for compound growth.
Is there a Rule of 70?
Yes — slightly more accurate at low rates. Use 70 for inflation calculations, 72 for everyday investment thinking.
This calculator is for informational purposes only and does not constitute financial, tax, or legal advice. Consult a licensed professional before making significant financial decisions.