Measure the angle of the sector of the pie chart using a compass or protractor. For example, if six of the 24 students represented in the pie chart liked gym class best and you are solving for the decimal they represent of the whole group, measure the gym class sector with a protractor to find that it spans 90 degrees.
Divide the number of degrees in the sector by 360, the total number of degrees in the pie chart. In this example, divide 90 by 360 to get 0.25.
Check to make sure your answer makes sense. It can be easy to measure wrong or to divide the wrong numbers, so look at your answer logically. 0.25 is the same as one quarter, and the gym class sector of the pie chart in the example would look like one quarter even if you didn't carefully measure it. This means that 0.25 makes sense as an answer.