The Rule of 72: the fastest way to estimate how fast your money doubles

You don't need a calculator to estimate how fast your money will double — you need one number. The Rule of 72 is the mental shortcut investors have used for decades, and it takes ten seconds to learn.

The rule, in one line

Divide 72 by your annual interest rate, and you get roughly how many years it takes your money to double. That's it. At 8% annual return, 72 ÷ 8 = 9 years to double. At 6%, it's 12 years. At 12%, just 6 years.

Why it works

The Rule of 72 is an approximation of the compound interest formula, tuned to be accurate in the range most people actually invest at (roughly 6% to 10%). The math behind compound growth isn't linear, but 72 happens to be a number with a lot of divisors (2, 3, 4, 6, 8, 9, 12...), which makes the mental division clean for most common rates.

A few real examples

• Savings account at 2%: 72 ÷ 2 = 36 years to double. This is exactly why cash sitting in a low-yield account barely grows.
• A CD or bond at 5%: 72 ÷ 5 ≈ 14.4 years.
• Stock market historical average around 10%: 72 ÷ 10 ≈ 7.2 years.
• Credit card debt at 24%: 72 ÷ 24 = 3 years for the balance to double if you're only paying interest — the same rule that helps your savings grow works against you just as fast on debt.

The Rule of 72 isn't just about growing money — it's a warning about debt too. The same math that doubles your savings doubles what you owe.

When to use the real calculator instead

The Rule of 72 is a mental estimate — it gets less accurate at very high rates (above ~20%) or very low ones (below ~2%), and it doesn't account for regular contributions, only a lump sum left alone to grow. For an exact projection that includes monthly contributions, use a real compound interest calculator instead.

In short

72 ÷ rate = years to double. It's not precise, but it's fast enough to compare options in your head — a savings account, a CD, an index fund — before you ever open a spreadsheet.