If you invest ₹100,000 today, how long will it take to turn into ₹200,000? What about ₹400,000?
Answering these questions mathematically requires complex logarithmic formulas. But in the world of personal finance, we use a simple, brilliantly effective mental model known as the Rule of 72.
The Rule of 72 is a quick formula that estimates the number of years required to double your invested money at a given annual rate of return. You simply divide 72 by your expected interest rate.
The Doubling Calculator
Use the interactive tool below. You can either input your expected interest rate to see how many years it will take to double your money, or input your target timeframe to see what interest rate you need to achieve it.
Why This Rule is a Game Changer
The Rule of 72 isn't just a party trick; it fundamentally changes how you perceive risk, inflation, and debt.
If a standard savings account offers 2% interest, the Rule of 72 tells you it will take a staggering 36 years (72 / 2) for your money to double. Suddenly, that "safe" investment doesn't look so appealing when you realize you'll be nearing retirement before it pays off.
The rule applies to debt just as easily. If you carry a credit card balance with an 18% APR, the Rule of 72 (72 / 18) reveals that your debt will double in exactly 4 years if left unpaid. It is the ultimate warning sign against high-interest loans.
Historically, broad market index funds return around 10% before inflation. That means your invested wealth will double roughly every 7.2 years without you lifting a finger. If you invest ₹100,000 at age 25, it doubles 5 times by age 61—turning into ₹3,200,000.