The Doubling time formula is used in finance to calculate the length of time needed to double an investment or money in an interest-bearing account. As per the formula, to calculate doubling time just divide constant growth rate by 100 and add it with 1 and find the log of the answer and then divide log(2) by the answer. In the Doubling time formula, 'r' is the rate of the growth period. The monthly rate can be found by dividing the annual rate by 12.