One of the initial and the most general questions regarding an investment optional is the time period needed to double the investment. One clear way is to consider to the table of compound factor from that this period can be computed.  For illustration the doubling period at 3 percent, 4 percent, 5 percent, 6 percent, 7 percent, 8 percent, 9 percent, 10 percent, 12 percent would be approximately 23 years, 18 years, 14 years, 12 years, 10 years, 9 years, 8 years, 7 years, and 6 years correspondingly.

If one is not inclined to utilize future value interest factor tables there is an option, termed as rule of 72. As per to this rule of thumb the doubling period is acquired by dividing 72 through the interest rate.  For illustration, at the interest rate of 8 percent the approximate time for doubling an amount would be as 72/8 = 9 years.

A vary accurate rule of thumb is rule of 69. According to this rule the doubling period is equivalent to:

.35 + (69/ Interest rate)

By using this rule the doubling period used for an amount fetching 10 % and 15% interest would be as given:

35 +  69/10 = .35 + 6.9 = 7.25 years

35 +  69/15 =.35 + 4.6 = 4.95 years