Divide your confidence interval by 2. The confidence interval reflects how accurate you want your results to be: The higher the confidence interval, the more accurate your result. For example, if you want a 99 percent confidence interval, then 99/2 = 0.495.
Look in the center of the Z-Table (see Resources) for the figure you calculated in Step 1. In this example, the area 0.495 can be found at the intersection of row 2.5 and column 0.08.
Multiply Step 2 by the standard deviation of your population. For example, if the population standard deviation is 2.1, then 2.58 x 2.1 = 5.418.
Divide Step 3 by the margin of error. For example, if your margin of error is 0.5, then 5.418 / 0.5 = 10.836.
Square Step 4 to get the sample size you need. In this example, 10.836 x 10.836 = 117.418896.