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: 