How the Rule of 72 works
At an 8% annual return, 72 divided by 8 equals about 9 years. At 6%, the estimate is about 12 years. The shortcut works because exponential growth creates a predictable relationship between rate and doubling time.
The result is approximate, not exact. It is most useful for quick intuition before using a more detailed compound interest calculator.
When to use the Rule of 72 Calculator
Use the Rule of 72 Calculator when you want a quick estimate of doubling time from a steady annual rate, or the rate needed to double over a target number of years.
It is useful for investments, inflation, and savings growth examples, but it assumes a constant rate and does not model contributions, fees, taxes, or volatility.
Using it with other investing tools
The Rule of 72 can sanity-check a projection. If a compound interest model shows money doubling much faster than the rule suggests, check the contribution assumptions or compounding inputs.
For purchasing power, the same shortcut can estimate how quickly inflation cuts value in half. At 3% inflation, prices roughly double in about 24 years.