# What Is the Rule of 72? Doubling Time in Plain English

> The Rule of 72 estimates how long a balance takes to double. Divide 72 by the annual rate in percent, then compare it with the exact years.

Source: MarketCapLens — https://www.marketcaplens.com/learn/what-is-the-rule-of-72
Updated: 2026-09-22

The Rule of 72 is a shortcut for how long a balance takes to double at a constant annual rate. Divide 72 by the rate in percent. At 8% a year, 72 ÷ 8 is 9 years.

**Key takeaways**

- Years to double ≈ 72 ÷ annual return in percent.
- The exact compound-interest time is the natural log of 2 ÷ the natural log of (1 + the rate).
- At **8%**, the shortcut is **9 years** and the exact time is about **9.01 years**.
- The shortcut is closest near 8%. It drifts as the rate moves away from that neighborhood.

## The formula

> **Years to double ≈ 72 ÷ annual return in percent**
>
> **Exact years = natural log of 2 ÷ natural log of (1 + annual rate)**

The rate stays constant, and the balance compounds once a year. No contributions are added along the way.

## A worked example

Take an **8%** annual return and a **$10,000** start.

- Rule of 72: 72 ÷ 8 = **9 years**.
- Exact doubling time ≈ **9.01 years**.
- The $10,000 becomes about **$20,000** after 9.01 years, **$40,000** after 18.01 years, and **$80,000** after 27.02 years.

Each step applies the exact doubling time again. The [Rule of 72 calculator](https://www.marketcaplens.com/tools/rule-of-72-calculator) prints the shortcut and the exact years side by side.

## What the Rule of 72 tells you

It turns a rate into a span of years you can check in your head. Use the exact line when the rate is far from 8%, or when you want the milestones on a starting amount.

## What the Rule of 72 leaves out

Contributions, withdrawals, taxes, and a rate that changes are all outside the shortcut. It also describes a smooth path. A balance that doubles over nine bumpy years still doubled. The rule does not show the bumps.

If you want the balance after a contribution schedule, use [compound growth](https://www.marketcaplens.com/learn/what-is-compound-growth). A company page such as [Apple](https://www.marketcaplens.com/company/AAPL) shows today's price. It does not show how long that price takes to double.

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*For general education only. Nothing here is investment advice.*

## Frequently asked questions

### How does the Rule of 72 work?

Divide 72 by the annual return in percent. At 8%, that is 9 years. The exact compound-interest doubling time at 8% is about 9.01 years. The MarketCapLens Rule of 72 calculator shows both.

### When is the Rule of 72 less accurate?

The shortcut is closest near 8%. It drifts as the rate moves away from that neighborhood. The exact line is the natural log of 2 divided by the natural log of 1 plus the rate.