Rule of 72

When you roll a snowball down a snowy hill, it starts out small, but as it rolls, it grows dramatically faster. Compound interest—where the interest you earn begins earning interest of its own—works the exact same way. Given enough time, your money snowballs at an astonishing rate.

However, predicting when your initial deposit will actually double usually requires logarithms and a financial calculator. That is where the number 72 comes in. With this one figure, you can estimate the number of years needed to double your principal in just a few seconds.

For example, imagine you invest in an asset with an average annual return of 6%. Divide 72 by 6, and you get 12. That means it takes roughly 12 years for your money to double. If you take a slightly more aggressive approach and earn 8% per year, dividing 72 by 8 reveals that your investment doubles in just 9 years.

You can also work backward from a target timeline. If your goal is to double your savings in 6 years, dividing 72 by 6 tells you immediately that you need to target an annual return of about 12%.

This formula is not just for projecting investment gains. You can also turn it around to see how steady inflation quietly eats away at your purchasing power over time. Because rising prices mean the real value of your cash is falling, the math works in reverse.

Suppose the inflation rate averages 3% each year. Divide 72 by 3, and you get 24. That means it will take roughly 24 years for the purchasing power of $100,000 (around 100 million won) to drop by half to $50,000. Twenty-four years from now, keeping the same standard of living will cost twice as much cash.

Leaving cash idle in a regular bank account keeps the number on your statement steady, but the real goods and services that money can buy will constantly shrink. The Rule of 72 brings this invisible erosion into plain view.

This makes the rule far more than a fun wealth-building trick. It serves as a practical guide for setting the minimum return your investments must deliver simply to protect your hard-earned wealth from being drained by inflation.

Under continuous compounding, the mathematically precise number to double your principal is not 72, but approximately 69.3. This figure comes directly from the natural logarithm of 2 (ln 2 ≈ 0.693). Yet for standard interest compounded once a year at typical returns between 6% and 10%, 72 is actually closer to reality.

So why do finance professionals around the world prefer 72 over 69.3? Beyond its surprising accuracy in that typical range, 72 is cleanly divisible by many common numbers, such as 2, 3, 4, 6, 8, 9, and 12. That makes quick mental math effortless, freeing you from dealing with messy decimal points.

For standard investment returns between 5% and 12%, the formula produces answers that match rigorous logarithmic calculations with negligible error. It is more than accurate enough for everyday financial decisions.

Keep in mind that if returns climb well above 20% or drop below 2%, the gap between the shortcut and exact math widens. Therefore, the rule is best used as a quick mental compass to gauge compound growth, rather than a tool for decimal-point accounting.