Rule of 72: When Will Your Money Double?

How long does it take for $10,000 to double at 7% returns? The Rule of 72 gives you the answer in seconds — no calculator needed. This compound interest rule is one of the most useful tools in personal finance. Divide 72 by the interest rate, and you know when your money will double.

What Is the Rule of 72?

The Rule of 72 is a shortcut that estimates how many years it takes for an investment to double at a fixed annual rate of return. The formula:

Doubling time ≈ 72 ÷ Interest rate (in percent)

At 6% interest: 72 ÷ 6 = 12 years. After 12 years, your $10,000 grows to roughly $20,000 — purely through compound interest, without adding a single dollar.

The rule works in reverse too: want to double your money in 10 years? You need 72 ÷ 10 = 7.2% annual returns.

Why 72?

The exact doubling formula is: t = ln(2) / ln(1 + r), where r is the interest rate as a decimal. For small rates, this simplifies to t ≈ 0.693 / r. Multiply numerator and denominator by 100 and you get t ≈ 69.3 / interest rate.

Why 72 instead of 69.3? Because 72 has many divisors: 1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 36, 72. That makes mental math easy. The slight overestimate also compensates for the approximation error at higher interest rates. Between 2% and 12%, the Rule of 72 delivers remarkably accurate results.

Practical Examples

ETF Portfolio at 7% Returns

A broad market ETF tracking the MSCI World has historically returned about 7% per year (before inflation). Using the Rule of 72: 72 ÷ 7 ≈ 10.3 years. $10,000 becomes $20,000 after about 10 years. $40,000 after 20 years. $80,000 after 30 years. Each doubling builds on the previous one — that is the power of compound interest.

Savings Account at 2%

A typical savings account pays around 2%. 72 ÷ 2 = 36 years. It takes over three decades to double your money. For wealth building, savings accounts are too slow — but they are perfect for emergency funds.

Inflation at 3%

The Rule of 72 works in the other direction too: at 3% inflation, your purchasing power halves in 72 ÷ 3 = 24 years. $10,000 in a checking account will only buy $5,000 worth of goods in 24 years. A powerful argument against letting your money sit idle.

Comparison Table: Doubling Time at Different Interest Rates

The table below shows the estimated doubling time using the Rule of 72 compared to the exact calculation:

Interest Rate	Rule of 72	Exact	Typical Example
1%	72.0 years	69.7 years	Savings account (low)
2%	36.0 years	35.0 years	High-yield savings
3%	24.0 years	23.4 years	Inflation / bonds
4%	18.0 years	17.7 years	Balanced fund
5%	14.4 years	14.2 years	Dividend ETF
6%	12.0 years	11.9 years	Global equity ETF (after fees)
7%	10.3 years	10.2 years	MSCI World (historical)
8%	9.0 years	9.0 years	S&P 500 (historical)
10%	7.2 years	7.3 years	Growth stocks
12%	6.0 years	6.1 years	Aggressive growth

Between 4% and 10%, the error is less than one year. At very low or very high rates, the estimate drifts a bit further — but for everyday financial decisions, the Rule of 72 is accurate enough.

What the Rule of 72 Means for Building Wealth

The Rule of 72 makes compound interest tangible. Three key takeaways:

1. Every Percent Matters

The difference between 5% and 7% sounds small. But at 5%, your money doubles every 14.4 years. At 7%, every 10.3 years. Over 30 years, that is the difference between tripling and eight-folding your wealth. Low fees — which often account for 1–2 percentage points — are critical.

2. Time Is the Strongest Lever

Start investing at 25 with 7% returns and you get four doublings by retirement at 65: $10,000 → $20,000 → $40,000 → $80,000 → $160,000. Start at 35 and you only get three doublings, ending at $80,000. Ten years earlier means double the capital — same rate of return, same initial investment.

3. Inflation Eats Silently

At 3% inflation, your purchasing power halves every 24 years. Money in a checking account loses value quietly. The Rule of 72 shows: not investing means getting poorer automatically. Real returns (after inflation) are what count.

Limitations of the Rule of 72

The Rule of 72 is an approximation — not a precision tool. Know its limits:

• Assumes constant returns: The formula requires a fixed interest rate over the entire period. Stock markets fluctuate — the Rule of 72 reflects the long-term average, not the reality of any single year.
• Less accurate at extreme rates: Below 2% or above 15%, the deviation becomes noticeable. For normal investment decisions, this does not matter.
• Ignores taxes and fees: The formula uses the gross interest rate. Capital gains taxes, fund expenses and account fees reduce the effective return. Use the net rate for more realistic results.
• Lump sums only: The Rule of 72 applies to a single investment compounding over time. For regular contributions (dollar-cost averaging), use the Savings Plan Calculator.

Variants: Rule of 69 and Rule of 70

Two related rules exist alongside the Rule of 72:

• Rule of 69 (69.3): Mathematically more precise, since ln(2) ≈ 0.693. Used in academic finance. Harder to divide mentally.
• Rule of 70: A compromise between accuracy and ease of mental math. Slightly better than 72 for rates below 4%.

For everyday use, the Rule of 72 remains the best choice: accurate enough, easy to compute, simple to remember.

Conclusion

The Rule of 72 is one of the simplest and most useful rules of thumb in finance. Divide 72 by your interest rate — done. You instantly know when your money will double. Key takeaways:

1. Formula: 72 ÷ interest rate = years to double.
2. Accuracy: Between 2% and 12%, the error is less than one year.
3. Dual use: Works for returns (money doubling) and inflation (purchasing power halving).
4. Core message: Every percent of return and every year of investing matter — exponentially.

Compound interest is the eighth wonder of the world. He who understands it, earns it. He who doesn't, pays it. — attributed to Albert Einstein

More useful calculators: Compound Interest Calculator · Savings Plan Calculator · ROI Calculator