Pie Charts (Sector Graphs)

In short: A pie chart is a circle cut into slices called sectors, and the whole circle is 360°. A slice's angle is its fraction of the total times 360°. To go back, the number in a slice is its angle out of 360 times the total.

Before you start

• A full turn is 360° (see Types of Angles).
• Fractions of a quantity and percentages (see Fractions of a Quantity and Fractions, Decimals and Percentages).

Slices are fractions of 360°

Finding the angle for each slice

To draw a pie chart, draw a circle and one radius. Put the centre of a protractor on the middle, measure the first angle and draw the next radius. Carry on from that line for each slice.

From an angle back to a number

Percentage pie charts

Some pie charts give percentages. 100% is the whole circle, so 1% is 3.6°.

• 25% is $\frac{1}{4}$ of 360°, which is 90°.
• 10% is 36°.
• If the slices are 35%, 25% and 15%, the last slice is $100\%-75\%=25\%$, which is 90°.

Quick summary

• A pie chart is a circle of 360°, cut into sectors.
• Angle of a slice $=\frac{\text{number}}{\text{total}}\times 360°$.
• Number in a slice $=\frac{\text{angle}}{360°}\times \text{total}$.
• All the angles add to 360°, and all the percentages add to 100%.
• In a percentage pie chart, 1% is 3.6°, so 25% is 90°.

Quiz