Rule of 72 Calculator - Investment Doubling Time Formula

Calculate how many years it takes for your investment to double in value using the Rule of 72, Rule of 70, Rule of 69.3, and exact natural logarithmic compounding.

⚙️ Configuration & Inputs

Annual Interest / Return Rate (%) Expected annual growth rate (e.g. S&P 500 ~7% - 10%)

Initial Investment ($) Starting principal to double

📊 Results & Analysis

Rule of 72 Doubling Time 10.0 Years $10,000 grows to $20,000

Exact Mathematical Compounding 9.97 Years Formula: ln(2) / ln(1 + r)

Quadrupling Time (4x Wealth) 20.0 Years $10,000 grows to $40,000

Octuping Time (8x Wealth) 30.0 Years $10,000 grows to $80,000

The Rule of 72 Shortcut for Compound Interest

Divide 72 by the annual rate of return to estimate how many years it will take for your money to double: Years = 72 / r.

Frequently Asked Questions

The exact natural logarithm of 2 is ln(2) ≈ 0.693 (69.3%). However, 72 is preferred for mental math because it has numerous clean divisors: 2, 3, 4, 6, 8, 9, 12, 18, 24, and 36.

If inflation is 4%, the purchasing power of your cash will be cut in half in 72 / 4 = 18 years.