What is a pie chart?
A pie chart is a circle divided into several sectors (slices). Each sector represents the proportion of one category compared to the whole set of data. A pie chart is useful when we want to show parts of a single whole, such as the percentage of land use.
Calculating a sector's angle
A full circle equals 360° and represents 100% of the data. The angle for each category is worked out with the formula:
Key idea
Sector angle = (category value ÷ total) × 360°. A full circle = 360° = 100%.
For example, if a category has a value of 50 out of a total of 200, its angle is (50 ÷ 200) × 360° = 90°. A category taking 25% of the data equals 90°, while 50% equals 180°. To find the percentage of a category we use (value ÷ total) × 100.
Constructing and interpreting
To construct a pie chart we first draw a circle and mark its centre, then use a protractor to measure each sector's angle accurately, starting from a vertical radius. Each sector is labelled with the category name and its percentage, shaded a different colour, and the chart is given a clear title. When interpreting, the largest sector represents the largest category and the smallest sector the smallest. We can also convert an angle back to a percentage with (angle ÷ 360°) × 100. Because all the sectors add up to the whole circle, a pie chart lets us compare parts to the whole quickly and clearly. However, a pie chart is not suitable when there are too many categories or when we need to show change over time.