Rule of 72
A shortcut to estimate how long money takes to double: divide 72 by the yearly growth rate.
The rule of 72 gives a quick estimate, not an exact answer, and works best for rates between about 4% and 12%. It works for anything that compounds, including debt and inflation.
Use it to see why rate differences matter. At 8%, money doubles in about 9 years. At 4%, it takes about 18. On a card charging 24%, a balance you never pay down could double in about 3 years.
Example
At an assumed 6% a year, 72 ÷ 6 = 12 years to double. The exact answer is about 11.9 years. At 3% inflation, prices double in about 72 ÷ 3 = 24 years.