You can put this solution on YOUR website! It depends on how frequently the money is compounded. The most frequent compounding is continuous compunding:
F = future value
P = present value
r = annual interest rate (expressed as a decimal)
t = number of years
or approx 0.0866 or
—
If the money were compounded quarterly (or some other period), use:
n = number of compounding periods per year, other variables are as above.
Continuing with the quarterly example:
Raise e to each side:
So not a huge difference (you need a slightly higher rate of return to double your money in 8 years if the interest is not continously compounded).
——
Finally, there is this "rule of 70" where you can get an estimate for doubling time (or rate):
70/r = 8 years
r = 70/8 = 8.75% <—<<< pretty close!
