The Magic Number Every Investor Should Know: 72

Here's a trick most people never learn in school: divide 72 by any interest rate, and you'll roughly know how many years it takes your money to double. It's called the Rule of 72. Quick example: money growing at 4% (a savings account) takes 72 ÷ 4 = 18 years to double. Money growing at 24% takes just 3 years. Here's a real one to make it click.

(Page Industries, the Jockey brand license holder) listed in 2007 at around ₹480. Over the next 19 years, it delivered a CAGR of roughly 25.9% — meaning, by the Rule of 72, your money would have doubled almost every 2.8 years. ₹10,000 invested in 2007 would be worth close to ₹7.95 lakh today. That's the power of compounding at a high rate, sustained over a long stretch. But here's the honest part worth adding — this isn't a straight line up. Page Industries is actually down close to 19% over the past year, and well off its 2022 highs. Even a genuinely strong long-term compounder goes through real multi-year stretches of underperformance. The 25.9% CAGR is a 19-year average, not a guarantee of every single year looking the same. The real takeaway: the Rule of 72 shows you why even a few extra percentage points of return matter enormously over decades. But it also only works if you actually stay invested through the flat and rough years — the compounding happens in hindsight, not in a straight line while you're living through it.