Pie charts (sometimes called circle graphs) are used to compare data. Slices of different size are marked on a circle (i.e. the pie) based on what part of the whole they represent. The pie chart example below illustrates this.
The pie chart below shows the Carbon Dioxide (CO_{2}) emissions in California.
CO_{2}emissions in California by consumption sector. Source: http://www.giss.nasa.gov/meetings/pollution2002/summaryd.html
Pie charts show a fraction of a circle that is the same fraction as is the quantity being represented is of the whole amount. Think of a group of 4 children where 1 is lefthanded and the other 3 are righthanded. The pie chart showing this would look like the second example below:
1 out of 4 students is lefthanded. This is onefourth of the whole group.
3 out of 4 students are righthanded. This is threefourths of the whole group.
This example is quite simple; a pie chart probably is not even needed to show the relationship between the number of lefthanded and the number of righthanded students. The example below is a better one.
The third example of a pie chart shown below shows time use on an average weekday for fulltime university and college students in the U.S.A.
The table below shows how the size of the slices and their angles are calculated for the above pie chart.
Steps  
1 
Find the total of all the parts. Note: In this example we are given the total but that is not always the case  8.3 + 3.6 + 3.3 + 3.0 + 2.5 + 1.5 + 1.0 + 0.8 = 24.0  
2 
Write the fraction of the total for the first part (Hours sleeping) 


3 
Write the fraction as a decimal. In other words, divide the numerator by the denominator  8.3 ÷ 24.0 = 0.3458  
4 
Multiply the decimal by 360° to find the angle for the slice that will show this part.  0.3458 x 360° = 124.5°  
Below you can see what we have just calculated. 

5 
Repeat Steps 2 to 4 for all the other parts

Leisure and sports


Educational activities

Working


Other

Traveling


Eating and drinking

Grooming


6  Check the angles add up to 360. (note: there could be rounding errors and the total might not be exactly 360.  124.5 + 54 + 49.5 + 45 + 37.5 + 22.5 + 15 + 12 = 360°  
7  Draw the slices on the pie chart. 
The three worksheets below will provide practice with calculating the angles that are used to create pie charts.
You will find more about measuring angles in degrees here and there is more about equivalent fractions here.
Check out this great web site where you can create many different pie charts and other types of chart too.
Remember that there are situations where pie charts are not a good way to show relationships between data. For example, if the quantities involved are quite similar and/or there a large number of sectors, the resulting chart can be a poor way of showing the information. In such cases, a table or other type of chart is often a better choice.