The Rule of 72: A Simple Trick to Know When Your Money Will Double
Have you ever wondered, "If I invest this money at 10% interest, how long will it take to double?"
You don't need a calculator or complex Excel formulas to answer this. There is a famous mental math shortcut used by investors worldwide called the Rule of 72.
How it Works
The formula is very simple: 72 ÷ Interest Rate = Years to Double
Examples
Let’s say you are lending money or investing in a Fixed Deposit (FD):
At 6% Interest:
72 ÷ 6 = 12 Years. Your money will double in 12 years.At 12% Interest:
72 ÷ 12 = 6 Years. Your money doubles in just 6 years.At 24% Interest (Typical local lending rate):
72 ÷ 24 = 3 Years.
Why is this useful?
This rule helps you quickly compare investment opportunities. If someone promises to double your money in 2 years, the Rule of 72 tells you they must be generating 36% returns (72 ÷ 2), which implies very high risk!
Does it work for monthly interest?
Yes! If you calculate in months, the answer will be in months.
If you charge 2% per month:
72 ÷ 2 = 36 Months. The principal doubles in 3 years.
Conclusion
The Rule of 72 gives you a quick estimate, but for precise daily calculations—especially when partial payments are made—you need a proper tool.
Need Precise Calculations? Don't rely on estimates for your real dealings. Use the Byaj Khata Bookcalculator to get exact figures down to the last rupee.