The rule of what? Maybe you have heard of the Rule of 72. We heard about it back in high school or college. If you avoided upper-level math or finance classes, it’s probably new to you.
How long will an investment take to double in value? If I invest $100,000 at 8% interest, compounded monthly, it will double in 9 years. 72 / 8 = 9.
The rule is a simple shortcut for figuring how long your investment will take to double in value, avoiding a more complex calculation involving taking the logs of numbers. If you’re a math nerd, you can perform the proof as a homework assignment! It is most accurate for more common rates from 4% to 15%. Using rates higher or lower may skew your results.
It could also be used to find the negative effects. A stable value fund (for argument’s sake, let’s say it earns 0%) would be worth half its value (in purchasing power) in just 12 years if the economy is subject to 6% inflation. Or, more simply, if it were charged a 3% brokerage fee, its real value would be $50,000 in 24 years.
https://www.investopedia.com/terms/r/ruleof72.asp
If you need a more specific return, you can use Excel or a similar spreadsheet application to calculate the Internal Rate of Return (IRR)
https://corporatefinanceinstitute.com/resources/excel/functions/irr-function/
