Use Excel for the Rule of 72
 Create a table with a heading row B2:F2 that contains the rates in decimal .04 .06 .08 .09 .12
 Put the year number in A4:A21, from 1 to 18 inclusive.
 Into the range B4:F21 put the compounded values, beginning with year 1.
The formula for compound interest is = (1+rate)^years.
So, in B4 put =(1+$B$2)^$A$1:$A$21 and drag the cell down for Col. B
Then in C4 put the same formula as in B4 except change $B to $C
and drag the cell down for Col. C
Note : To save time , copy the formula in B4, select C4 then paste it into the Formula Bar and change $B to $C
 Set the cell format for these cells to display exactly two digits after
the decimal point.
 Set a yellow background color in the cell in each column where the initial value is closest to having doubled.

Get the spreadsheet Here
Make use of Excel's drag / copy formula feature.
Note further doublings ; for example,
note that at 8%, one doubling takes 9 years, and the next takes
another 9 years, that is, 18 years doubles twice, or a factor of four.
Your table should look like this: