'72 Divided by the Interest Rate' - A Compound Interest Party Trick You Can Do in Your Head

A Magic Calculation That Takes 3 Seconds

"If I invest at 6% annual returns, how many years until my money doubles?" You can answer this in 3 seconds. 72 ÷ 6 = 12. About 12 years. That is the Rule of 72. Just divide 72 by the interest rate and you instantly know how long it takes to double your money. No calculator, no smartphone - just your head.

Handy Numbers to Memorize

Here are some commonly encountered rates. At 1% annual interest: 72 years to double (savings deposit level). At 3%: 24 years (bond fund level). At 5%: about 14 years (balanced fund level). At 7%: about 10 years (stock index level). At 10%: about 7 years (a strong stock market).

At a dinner party, if someone says "I'm getting about 5% in my NISA account," you can instantly reply: "So your money doubles in about 14 years." "Wait, how did you know that?" "72 divided by 5 is 14.4. It's called the Rule of 72." Guaranteed to spark a conversation.

It Works in Reverse Too - Finding the Required Rate from a Target Year

The Rule of 72 works both ways. "I want to double my money in 10 years - what rate do I need?" 72 ÷ 10 = 7.2%. You need about 7% annual returns. "Double in 20 years?" 72 ÷ 20 = 3.6%. About 4% will do. Working backward from your goal instantly tells you the return rate you need to aim for.books on mental math techniques introduce plenty of everyday calculation shortcuts beyond the Rule of 72.

The Scary Side: It Works on Debt Too

The Rule of 72 applies to debt as well. If revolving credit charges 15% annual interest, 72 ÷ 15 = 4.8 years. Your debt doubles in about 5 years. Consumer finance at 18%? 72 ÷ 18 = 4 years to double. Drop this at a dinner party - "Did you know revolving credit doubles your debt in 5 years?" - and you might save a friend from a debt spiral. The Rule of 72 is a universal tool for both building wealth and avoiding financial traps.