Rule of 72 Calculator

About Rule of 72

The Rule of 72 divides 72 by an annual interest rate (percent) to approximate years needed to double capital at compound growth. It is a mental math shortcut, not a precise actuarial formula.

Accuracy fades at very high rates or with contributions and taxes.

Educational illustration only—not financial advice.

How to use this tool

Enter annual interest rate percent. Calculate approximate years to double.

Formula & methodology

This rule of 72 calculator applies the standard relationship below. Intermediate values are kept at full precision; only the displayed result is rounded.

years to double ~ 72 / rate(%); exact doubling uses ln(2)/ln(1+r)

Tips for accurate results

• Works best for moderate rates (roughly 4–12%).
• Use compound interest calculator for dollar outcomes.
• Inflation erodes real doubling of purchasing power.
• Do not base investment decisions on Rule of 72 alone.

Frequently asked questions

Sources & references

The factors and formulas used by this tool follow these references:

• Consumer Financial Protection Bureau — consumer tools and explainers
• U.S. SEC Investor.gov — Compound Interest Calculator and definitions

Finance results are estimates based on the figures you enter and standard formulas. They are not tax, investment, or lending advice; actual bank terms, fees, and tax rules may differ.

Last reviewed: September 2026. Found an error? Tell us and we will correct it.