The Rule of 72 and Other Mental Math Every Investor Should Know

You do not need a spreadsheet to sanity-check an investment. A handful of mental-math shortcuts, led by the famous Rule of 72, let you estimate doubling times and inflation effects in your head, which is often all you need to spot a good or bad deal.

The Rule of 72

To estimate how many years it takes money to double, divide 72 by the annual return. At 8%, money doubles in about 72 / 8 = 9 years. At 6%, about 12 years. At 4%, about 18 years. That is it, no calculator required.

It works in reverse too: to find the return needed to double in a given time, divide 72 by the years. Want to double in 6 years? You need about 72 / 6 = 12% a year.

How accurate is it?

Surprisingly accurate for typical rates. The Rule of 72 is most precise around 8%, and stays within a fraction of a year for rates between roughly 5% and 12%. For very high rates it drifts a little (some people use 69.3 or 70 for continuous compounding), but for everyday investing, 72 is both accurate and easy to divide.

The related rules

Rule of 114 estimates tripling time: divide 114 by the rate. At 8%, money triples in about 14 years. Rule of 144 estimates quadrupling: at 8%, about 18 years. Rule of 70 is the same idea applied to inflation, divide 70 by the inflation rate to see how fast prices double. At 3.5% inflation, prices double in about 20 years.

Why this matters for investors

These shortcuts make the cost of small differences vivid. The gap between a 6% and an 8% return is not minor: at 6% your money doubles every 12 years, at 8% every 9 years. Over a 36-year horizon, that is three doublings versus four, the difference between 8x and 16x your money. Fees work the same way in reverse, which is why a seemingly small 1 to 2% annual fee is so damaging over decades.

A couple more handy ones

The 4% rule offers a rough retirement-spending guide: a portfolio can sustainably support annual withdrawals of about 4% of its starting value, adjusted for inflation. And to estimate the real return on a bond or savings account quickly, just subtract the inflation rate from the yield.

When you want exact figures rather than estimates, our compound interest calculator and inflation calculator do the precise math.