14.87% per annum, compounded for 5 years would give back very slightly more than double (2.000014).
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Nine years at 8%
The "Rule of 72" gives a good approximation of 72/4=18%.
Use the "rule of 72"...simply put, using compound interest you take the number 72 and divide it by the interest rate. Thus, at 5% the time to double is 14.4 years. This formula can be used for calculating a "double" for any interest rate using the same mathematical procedure.
The same time that it will take for any other amount to double. However, for the actual calculations you need to know the interest rate.
7% compound interest means that the amount of money increases, every year, by a factor of 1.07. After 4 years, you have 300 x 1.07^4.