Rule of 72
The Rule of 72 is a mental shortcut for compound growth: divide 72 by your annual rate of return to estimate how many years it takes money to double. At 7% a year, money doubles in roughly 72 ÷ 7 ≈ 10 years; at 9%, about 8 years.
At 7% the rule says 10.3 years against a true 10.24 — accurate enough for mental arithmetic. It drifts at the extremes: at 3% it says 24 years where the answer is 23.4, and at 12% it says 6 where the answer is 6.1, so treat it as reliable roughly between 5% and 12%, which covers most long-run investment assumptions. It works in reverse, too — 72 divided by the years available gives the return you would need. The most useful application is not optimism but its opposite: at 3% inflation, prices double in 24 years, so a retirement plan built on today's costs is roughly half the size it needs to be.
Put it to work
Compound Interest Calculator
See how your savings grow with compound interest and monthly contributions — final balance, interest earned, and a year-by-year growth table.
Investment Calculator
Project an investment portfolio's growth with monthly contributions — final value, your money vs market growth, and the year-by-year path.