What is the Rule of 72?

The Rule of 72 is a mental-math shortcut: divide 72 by your annual return rate (as a whole number) to estimate how many years it takes to double your money. At 6%, money doubles in roughly 72 ÷ 6 = 12 years. At 9%, roughly 8 years. It's an approximation — the exact answer uses logarithms — but it's accurate to within a year for rates between 4% and 12%.

Why 72 and not 70 or 69.3?

The mathematically exact divisor is 69.3 (100 × ln 2). But 72 is more divisible — it splits evenly by 1, 2, 3, 4, 6, 8, 9, and 12 — making mental math cleaner. For common rates like 6%, 8%, and 9%, 72 gives a very accurate answer. At high rates (15%+), the Rule of 72 slightly overestimates, and 69.3 or 70 is closer.

How does the Rule of 72 apply to debt?

It works the same way — in reverse. At 24% APR (a common credit card rate), 72 ÷ 24 = 3 years for your debt to double if you're only making minimum payments. A $5,000 balance becomes $10,000 in about 3 years without paydown. This is why high-rate debt is so destructive.

Does compounding frequency affect the result?

Yes, slightly. The Rule of 72 assumes annual compounding. With monthly compounding (as in most investment accounts), money doubles a little faster — the exact formula adjusts for this. The difference is small: at 7%, annual compounding doubles in 10.24 years; monthly compounding takes about 9.93 years.

Rule of 72 Calculator

Divide 72 by any annual return rate to instantly estimate how many years it takes your money to double — then see every subsequent doubling milestone.

Years to double

10.2

Rule of 72 estimate: 10.3 years. Exact answer: 10.24 years.

2× (first double)

10y — $20,000

4×

20y — $40,000

8×

31y — $80,000

16×

41y — $160,000

Doubling timeline

How the Rule of 72 works

The formula is simple: years to double ≈ 72 ÷ annual return rate. At 7%, your money doubles in about 10 years. At 4%, it takes 18 years. At 12%, just 6 years. No calculator required — it's designed to work in your head.

The mathematical basis is the natural log of 2 (≈ 0.693). The exact formula is ln(2) ÷ ln(1 + r), where r is the rate as a decimal. Multiplying by 100 gives 69.3 as the "true" divisor — but 72 divides more cleanly and is close enough that the error is under a year for rates between 4% and 12%.

Why it matters

The Rule of 72 makes compound interest intuitive. Without it, the difference between a 5% and 8% return can feel abstract — both seem "decent." But apply the rule: at 5%, money doubles every 14.4 years; at 8%, every 9 years. Over a 36-year career, 5% gives you two doublings while 8% gives you four. On a $10,000 investment, that's $40,000 vs $160,000 — a 4× gap from a 3-point rate difference.

The rule also cuts both ways. Credit card debt at 24% APR doubles in 3 years. A savings account at 0.01% takes 7,200 years to double. Seeing these numbers makes the cost of inaction — both on investing and on high-rate debt — concrete in a way abstract percentages don't.

Doubling at common rates

Annual return	Rule of 72	Exact years	Common example
2%	36 years	35.0 years	HYSA / I-bonds
4%	18 years	17.7 years	Conservative bonds
6%	12 years	11.9 years	Balanced portfolio
7%	10.3 years	10.2 years	S&P 500 (real)
10%	7.2 years	7.3 years	S&P 500 (nominal)
12%	6 years	6.1 years	Aggressive equity
24%	3 years	3.2 years	Credit card APR

Rules of thumb

At 7% (real S&P 500 average): money doubles roughly every 10 years. A 30-year investment triples its doublings compared to a 10-year one.
Starting a decade earlier: adds a full doubling. On $10,000 at 7%, waiting 10 years to start costs you roughly $10,000 in final balance — the equivalent of the original principal.
For debt: any interest rate above ~10% means your debt doubles within a decade without aggressive paydown. Prioritize high-rate debt before investing.

Frequently asked questions

What is the Rule of 72?: The Rule of 72 is a mental-math shortcut: divide 72 by your annual return rate (as a whole number) to estimate how many years it takes to double your money. At 6%, money doubles in roughly 72 ÷ 6 = 12 years. At 9%, roughly 8 years. It's an approximation — the exact answer uses logarithms — but it's accurate to within a year for rates between 4% and 12%.
Why 72 and not 70 or 69.3?: The mathematically exact divisor is 69.3 (100 × ln 2). But 72 is more divisible — it splits evenly by 1, 2, 3, 4, 6, 8, 9, and 12 — making mental math cleaner. For common rates like 6%, 8%, and 9%, 72 gives a very accurate answer. At high rates (15%+), the Rule of 72 slightly overestimates, and 69.3 or 70 is closer.
How does the Rule of 72 apply to debt?: It works the same way — in reverse. At 24% APR (a common credit card rate), 72 ÷ 24 = 3 years for your debt to double if you're only making minimum payments. A $5,000 balance becomes $10,000 in about 3 years without paydown. This is why high-rate debt is so destructive.
Does compounding frequency affect the result?: Yes, slightly. The Rule of 72 assumes annual compounding. With monthly compounding (as in most investment accounts), money doubles a little faster — the exact formula adjusts for this. The difference is small: at 7%, annual compounding doubles in 10.24 years; monthly compounding takes about 9.93 years.