Add the raw value of each sector in the circle graph to find the total. For example, if your circle graph shows the number of computers sold by brand and HP sold 200, Apple sold 260, IBM sold 240 and Dell sold 300, then add 200 plus 260 plus 240 plus 300 to get 1,000 computers sold.
Divide the value of the specified sector by the total to find the proportion of that sector. For this example, if you wanted to find the percentage of Dell computers, divide 300 by 1,000 to get 0.3.
Multiply the proportion by 100 to find the percentage. In this example, multiply 0.3 by 100 to find that Dell computers make up 30 percent of the total computers sold.