Savings Calculator

The Rule of 72

Mental Math for Compound Growth

The Rule of 72 gives you a quick way to estimate how long it takes for an investment to double at a fixed annual interest rate — without a calculator.

Formula: Years to Double = 72 / Interest Rate (%)

Example:
At 8% annual interest rate:
72 / 8 = 9 years

Tip: Works best between 4% and 15%. For higher rates, use 78 instead of 72 for better accuracy.