Rule of 72 Calculator

What Is the Rule of 72?

The Rule of 72 is a simple mental shortcut for estimating how long it takes an investment to double in value at a given fixed annual rate of return, without needing a calculator or spreadsheet. Divide 72 by the annual growth rate (as a whole number, not a decimal), and the result is approximately the number of years to double.

For example, at 8% annual return, 72 ÷ 8 = 9 years to double. At 6%, it takes 12 years. At 12%, only 6 years. The relationship also works in reverse: divide 72 by the number of years you want, and you get the approximate annual return needed.

Why 72 Specifically?

The number 72 works well as an approximation because it has many small whole-number divisors (1, 2, 3, 4, 6, 8, 9, 12...), making the mental math clean for common interest rates, and because it closely approximates the more precise mathematical relationship (based on the natural logarithm of 2) across the range of rates most relevant to real-world investing, roughly 6–10%. Outside that range the approximation drifts slightly, and the true doubling time is technically ln(2) ÷ ln(1 + rate) — but the Rule of 72 stays impressively accurate for practical purposes across the range investors actually encounter.

Why This Matters for Value Investors

The Rule of 72 is a useful gut-check when comparing investment opportunities or when evaluating your own return assumptions. If you're building a DCF model assuming a business grows earnings at 15% annually, the Rule of 72 instantly tells you that implies earnings doubling roughly every 4.8 years — a useful sanity check on whether that growth assumption is actually realistic for the business in question.

Frequently Asked Questions

Does the Rule of 72 work for any rate of return?

It's most accurate in the 6–10% range typical of long-run stock market and balanced investment returns. For very high rates (above ~20%) or very low rates (below ~2%), the approximation becomes noticeably less precise, and the exact logarithmic formula gives a more accurate answer.

Does this account for inflation?

No — the Rule of 72 uses whatever rate you input. If you want a real (inflation-adjusted) doubling time, use a real rate of return (nominal return minus inflation) as your input rather than the nominal rate.

Can I use this for debt as well as investments?

Yes — the same math applies to any compounding growth, including how quickly debt balances grow at a given interest rate if left unpaid, which makes it a useful quick check on credit card or loan costs too.