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To calculate based on a lower interest rate, like 2 percent, drop the 72 to 71. To calculate based on a higher interest rate, add one to 72 for every 3 percentage point increase.
For example, if you take out a five-year loan for $20,000 and the interest rate on the loan is 5 percent, the simple interest formula would be $20,000 x .05 x 5 = $5,000 in interest. Who benefits ...
The rule number (e.g., 72) is divided by the interest percentage per period (usually years) to obtain the approximate number of periods required for doubling. Although scientific calculators and spreadsheet programs have functions to find the accurate doubling time, the rules are useful for mental calculations and when only a basic calculator ...
Note that the interest rate is commonly referred to as an annual percentage rate (e.g. 8% APR), but in the above formula, since the payments are monthly, the rate must be in terms of a monthly percent. Converting an annual interest rate (that is to say, annual percentage yield or APY) to the monthly rate is not as simple as dividing by 12; see ...
The notion of doubling time dates to interest on loans in Babylonian mathematics. Clay tablets from circa 2000 BCE include the exercise "Given an interest rate of 1/60 per month (no compounding), come the doubling time." This yields an annual interest rate of 12/60 = 20%, and hence a doubling time of 100% growth/20% growth per year = 5 years.
The rule of 25 vs. 4% rule. The rule of 25 is just a different way to look at another popular retirement rule, the 4% rule. It flips the equation (100/4% = 25) to emphasize a different part of the ...
For example, compounding at an annual interest rate of 6 percent, it will take 72/6 = 12 years for the money to double. The rule provides a good indication for interest rates up to 10%. In the case of an interest rate of 18 percent, the rule of 72 predicts that money will double after 72/18 = 4 years.
r - the monthly interest rate. Since the quoted yearly percentage rate is not a compounded rate, the monthly percentage rate is simply the yearly percentage rate divided by 12. For example, if the yearly percentage rate was 6% (i.e. 0.06), then r would be / or 0.5% (i.e. 0.005).