The Rule of 72: A Simple Formula for Estimating Investment Growth

Last editorial review: September 22, 2026

U.S. scope: This article discusses U.S. institutions, financial products, tax rules, and dollar examples unless stated otherwise. Rules and product terms may change; verify current official guidance for your situation. The Rule of 72 is a quick mental shortcut: divide 72 by an annual interest rate (as a whole percent) to estimate how many years it will take for an investment to double. Use it for fast comparisons and intuition; verify decisions with precise models before acting. Finelo provides financial education, not financial or investment advice. This content is educational; investing involves risk, including loss. For an explanation of this shortcut, see Investor.gov’s compound-interest guide.

What is the Rule of 72?

The Rule of 72 is a simple heuristic that estimates how long a sum of money will take to double at a given annual compound rate. The basic formula is:

• Years to double ≈ 72 ÷ annual interest rate (percent).

People use the Rule because it’s fast, easy to do mentally, and gives a directional answer without a calculator. It’s commonly taught as an introductory way to demonstrate how compounding accelerates growth and to compare different rates quickly; financial educators often present it as a rule‑of‑thumb rather than a precise tool.

The Rule also works in reverse:

• Required annual rate ≈ 72 ÷ desired years to double.

Keep in mind this is an approximation. When you need exact timing or tax‑aware projections, use a financial calculator, spreadsheet, or exact logarithmic formulas.

How to Use the Rule of 72

Step-by-step application

1. Express the expected annual rate as a whole percent (for example, 6%).
2. Divide 72 by that percent: 72 ÷ 6 = 12.
3. Read the result as approximately 12 years to double at 6% annual compounding.

To find a required rate:

• If you want money to double in 9 years, compute 72 ÷ 9 = 8% per year (approximate).

This is useful for mental comparisons — for instance, seeing how a 1–2 percentage point difference in returns changes doubling time.

Worked example — comparing two options

• Account A: 3% → 72 ÷ 3 = 24 years to double.
• Account B: 8% → 72 ÷ 8 = 9 years to double.

If your horizon is 18 years, Account B will double twice (about 18 years ≈ two doublings at 8%), while Account A will not double in that period. Use the Rule to spot such differences quickly, then model exact cash flows before committing.

Practical tips for using the Rule

• Use the Rule for quick, back‑of‑envelope thinking when comparing savings rates, bond yields, or required returns.
• For low rates (under ~3%) or very high rates, the approximation becomes less accurate; run precise calculations in those cases.
• When fees, taxes, periodic contributions, or variable returns matter, build a detailed model rather than relying on the Rule.

Examples of the Rule of 72 in Action

Below are illustrative computations using the Rule of 72.

Quick reference table (approximate years to double)

These are mental‑math approximations; exact compound interest timing uses logarithms.

Stocks vs. bonds — illustrative scenario

• Hypothetical bond fund averaging 4% → 72 ÷ 4 = 18 years to double.
• Hypothetical stock portfolio averaging 8% → 72 ÷ 8 = 9 years to double.

This example shows how higher average returns shorten doubling time. It does not imply future performance and does not account for volatility, sequence of returns, fees, or taxes.

Applying the Rule to debt

The Rule works for compounding costs as well. If unpaid interest causes a balance to grow at 18% APR, then 72 ÷ 18 ≈ 4 years gives a rough estimate of doubling time; the exact result depends on the compounding frequency and assumes no payments or fees. Using the Rule this way highlights how quickly high‑rate debt can balloon; follow up with an exact amortization schedule for repayment planning.

Case study (hypothetical) — retirement planning

Imagine a 30‑year‑old who expects a portfolio return of 6%: each doubling ≈ 12 years. Over 36 years (three doublings), their savings would grow roughly eightfold (2³). The Rule gives immediate intuition: three doublings at 6% ≈ 36 years. Use that to test scenarios, then confirm with precise retirement models that include contributions, taxes, and expected inflation.