The 'Rule of 72': mental math that predicts your money's future

Imagine you could figure out how long it takes your savings to double — without a spreadsheet, calculator, or app. That's exactly what the Rule of 72 does. Just take the number 72 and divide it by your annual interest rate (as a whole number). The answer is roughly how many years it takes your money to double. At 6% interest, that's 72 ÷ 6 = 12 years. At 9%, just 8 years.

Why 72? It's a mathematical shortcut that approximates the real formula for compound growth, and it happens to be very accurate for the interest rates most people encounter — roughly 4% to 12%. The number 72 is also easy to divide by many rates (2, 3, 4, 6, 8, 9, 12), which is why it stuck around instead of the technically more precise 69.3.

The rule cuts both ways. It reveals how quickly debt compounds against you, too. A credit card charging 18% will double what you owe in just 72 ÷ 18 = 4 years if you ignore it. The same force that grows your investments silently multiplies your liabilities.

Use it as a gut-check, not a guarantee — it assumes a steady rate and doesn't account for taxes, fees, or market swings. But as a five-second sanity test, it turns abstract percentages into something you can actually feel.