What is the Rule of 72?

The Rule of 72 is a simple formula for estimating how long an investment will take to double in value at a fixed annual rate of return. Divide 72 by the annual return rate to get the approximate number of years. For example, at 8% returns, your money doubles in approximately 72 ÷ 8 = 9 years.

How accurate is the Rule of 72?

The Rule of 72 is remarkably accurate for return rates between 2% and 15%. At 7% returns, the Rule of 72 gives 10.29 years vs the exact answer of 10.24 years — a 99.5% accuracy. It becomes less accurate at very high or very low rates. For rates outside 2–15%, the Rule of 69.3 is more mathematically precise.

How long does it take to double money at 7% return?

At a 7% annual return (the historical average real return of the S&P 500), your money doubles approximately every 10.3 years using the Rule of 72. Starting with $10,000 at age 25, you would have: $20,000 by 35, $40,000 by 45, $80,000 by 55, and $160,000 by 65 — just from that initial $10,000 with no additional contributions.

Rule of 72 Calculator: How Fast Will Your Money Double?

The Rule of 72 is the simplest mental math shortcut in investing: divide 72 by your annual return rate, and you get the approximate number of years to double your money.

$$\text{Years to Double} = \frac{72}{\text{Annual Return Rate (%)}}$$

At the stock market’s historical average of ~7% real returns, your money doubles roughly every 10 years. Understanding this makes compound growth intuitive.

⏱️ Rule of 72 Calculator

Enter your expected rate of return to see how long it takes to double your money — or enter a target number of years to find the required return rate.

Annual Return Rate (%)

Years to Double

Starting Amount ($)

Number of Doublings

10.3 years

Time to Double Your Money

Exact doubling time 10.24 years

Rule of 72 estimate 10.29 years

Rule of 72 accuracy 99.5%

How the Rule of 72 Works

The formula is brilliantly simple:

Annual Return	Rule of 72 (÷)	Years to Double	Exact Answer
2%	72 ÷ 2	36.0 years	35.0 years
4%	72 ÷ 4	18.0 years	17.7 years
6%	72 ÷ 6	12.0 years	11.9 years
7%	72 ÷ 7	10.3 years	10.2 years
8%	72 ÷ 8	9.0 years	9.0 years
10%	72 ÷ 10	7.2 years	7.3 years
12%	72 ÷ 12	6.0 years	6.1 years
15%	72 ÷ 15	4.8 years	4.96 years

The Power of Compound Doubling

What makes the Rule of 72 powerful isn’t a single doubling — it’s multiple doublings in sequence:

Starting Amount	After 1 Double (~10 yrs)	After 2 Doubles (~20 yrs)	After 3 Doubles (~30 yrs)	After 4 Doubles (~40 yrs)
$10,000	$20,000	$40,000	$80,000	$160,000
$50,000	$100,000	$200,000	$400,000	$800,000
$100,000	$200,000	$400,000	$800,000	$1,600,000

Assumes 7% real returns. Each doubling takes ~10.3 years.

Why This Matters for FIRE

If you save $100,000 by age 30, it becomes roughly $800,000 by age 60 even with zero additional contributions. This is exactly how Coast FIRE works — save aggressively early, then let compound doubling finish the job.

Rule of 72 Applications for FIRE

1. Estimate Your Coast FIRE Age

If you have $200,000 invested and need $1,600,000 to retire (25× $64K expenses), you need 3 doublings. At 7% returns, that’s ~31 years. If you’re 30, your investments alone would reach your FIRE number by 61 — no additional contributions needed.

2. Understand Inflation’s Impact

Inflation also doubles using the Rule of 72. At 3% inflation, prices double every 24 years. This is why using real returns (after inflation) gives you a clearer picture.

3. Compare Investment Options

Savings account at 4.5%: doubles in 16 years
Bond index at 5%: doubles in 14.4 years
Stock index at 7%: doubles in 10.3 years
Stock index at 10% (nominal): doubles in 7.2 years

4. Visualize the Cost of Fees

A fund charging 1% fee (6% net return) doubles your money in 12 years. A fund with 0.03% fee (6.97% net return) doubles in 10.3 years. Over 30 years, that “small” fee costs you an entire extra doubling — half your final wealth.

The Math Behind It

For the curious, the Rule of 72 approximates the exact compound interest formula:

$$t = \frac{\ln(2)}{\ln(1 + r)} \approx \frac{0.693}{r} \approx \frac{72}{100r}$$

The constant 72 is used instead of the mathematically exact 69.3 because it’s more divisible (factors: 2, 3, 4, 6, 8, 9, 12) which makes mental math easier. The slight overestimate at most common rates partially compensates for continuous vs. discrete compounding.

Try These Calculators Next

FIRE Number Calculator — How much you need to retire early
Coast FIRE Calculator — Have you saved enough to coast?
Savings Rate Guide — The #1 number for your FIRE timeline

Frequently Asked Questions

The Rule of 72 is a quick mental math formula: divide 72 by your expected annual return rate to estimate how many years it takes to double your investment. At 8% returns, money doubles in ~9 years (72 ÷ 8). It works for any mathematically compounding growth — investments, inflation, GDP, population.

Over 99% accurate for returns between 4% and 15%. At 7%, it gives 10.29 years vs. exact 10.24 — a 0.5% margin. It's less precise below 2% or above 20%, where the Rule of 69.3 or 70 may be better approximations.

Approximately 10.3 years. The 7% figure represents the S&P 500's historical average real (inflation-adjusted) return. Starting with $100,000, you'd have approximately $200,000 in 10 years, $400,000 in 20 years, and $800,000 in 30 years — with zero additional contributions.