Here’s a mental math trick that professional investors actually use: divide 72 by your expected annual rate of return. The answer tells you roughly how many years it will take your money to double.
It’s called the Rule of 72, and it works because of how compound interest math behaves. At an 8% annual return, 72 divided by 8 equals 9, so your money doubles roughly every 9 years. At 6%, it takes about 12 years. At 4%, about 18 years. At 12%, just 6 years. You can flip the formula around too: if you want your money to double in 10 years, divide 72 by 10, and you’ll see you need roughly a 7.2% annual return to get there.
Why 72 specifically, and not some rounder number like 70 or 100? It comes down to a mathematical approximation. The actual formula for doubling time uses natural logarithms (ln(2) divided by the growth rate), which works out to about 69.3 for continuously compounded interest. But 72 is a much more convenient number to do mental math with, since it divides evenly by 2, 3, 4, 6, 8, 9, and 12, all common interest rates. That convenience costs you very little accuracy: the Rule of 72 stays impressively close to the real answer for any rate between roughly 6% and 10%, which covers most realistic long-term investment assumptions.
The Rule of 72 isn’t just a trivia fact; it’s genuinely useful for quick, gut-check comparisons. If someone tells you a fund has historically returned 10% a year, you can instantly estimate it doubles roughly every 7.2 years, meaning $10,000 invested today could become $20,000 in about 7 years, $40,000 in about 14, and $80,000 in about 21, assuming that rate holds. Seeing the doubling pattern laid out like that makes the power of compounding feel a lot more concrete than staring at a percentage.
It also works in reverse for inflation, which is really just negative compounding on your purchasing power. If inflation runs at 3% a year, the Rule of 72 tells you your money’s purchasing power will be cut in half in about 24 years (72 divided by 3), a useful reminder for anyone planning decades ahead for retirement.
The Rule of 72 is great for a rough mental estimate, but when you want the exact number for your specific rate, timeframe, and starting amount, our Compound Interest Calculator will do the precise math for you in seconds.