Rule of 72 — How Long to Double Your Money

What Is the Rule of 72?

The Rule of 72 is a simple formula to estimate how long an investment takes to double: divide 72 by the annual interest rate. No calculator needed.

Years to double = 72 ÷ interest rate

At 7% annual returns, your money doubles in approximately 10.3 years (72 ÷ 7 = 10.3).

Rule of 72 Quick Reference Table

Interest Rate	Years to Double	After 2 Doublings	After 3 Doublings
3%	24 years	48 years	72 years
4%	18 years	36 years	54 years
5%	14.4 years	28.8 years	43.2 years
6%	12 years	24 years	36 years
7%	10.3 years	20.6 years	30.9 years
8%	9 years	18 years	27 years
10%	7.2 years	14.4 years	21.6 years
12%	6 years	12 years	18 years

Why It Works

The Rule of 72 is derived from the compound interest formula. Solving for t when A = 2P:

2 = (1 + r)^t → t = ln(2) / ln(1 + r) ≈ 0.693 / r

Since 0.693 ≈ 69.3, why not the “Rule of 69”? Because 72 has more divisors (2, 3, 4, 6, 8, 9, 12) making mental math easier, and it’s slightly more accurate for typical investment rates due to continuous vs. discrete compounding.

Practical Applications

Investing

• S&P 500 averages ~10% historically → doubles every 7.2 years
• A high-yield savings account at 5% → doubles every 14.4 years
• Government bonds at 3% → doubles every 24 years

Debt (the Rule works against you)

• Credit card at 20% → debt doubles in 3.6 years
• Student loan at 6% → debt doubles in 12 years
• Mortgage at 7% → outstanding balance doubles in 10.3 years (if unpaid)

Inflation

• At 3% inflation, your purchasing power halves every 24 years
• At 7% inflation, prices double every 10.3 years

Rule of 72 vs. Exact Calculation

Rate	Rule of 72	Exact Answer	Error
2%	36.0 years	35.0 years	+2.9%
6%	12.0 years	11.9 years	+0.9%
8%	9.0 years	9.01 years	-0.1%
10%	7.2 years	7.27 years	-1.0%
15%	4.8 years	4.96 years	-3.2%
20%	3.6 years	3.80 years	-5.3%

The rule is most accurate between 6-10%. For rates above 12%, use the “Rule of 69.3” for better precision.

→ Try exact doubling calculations

Related Tools

• Compound Interest Calculator — Calculate exact growth over any period
• Compound Interest Formula — The math behind compounding
• Simple vs Compound Interest — Why compounding matters