What is a Pie Chart?
A pie chart is a circular graph that shows how a whole is divided into parts. Imagine slicing a pizza — each slice represents a different category, and the size of each slice tells you how much of the total that category takes up.
A circular statistical graphic divided into sectors (slices), where each sector's size is proportional to the quantity it represents as part of the whole.
Pie charts are especially useful when you want to:
- Show proportions or percentages of a whole
- Compare parts of a dataset to each other and to the total
- Communicate data visually in a way that's easy to understand at a glance
Think of a pie chart like a clock face. The full circle represents 360° — just like a clock goes all the way around. If something takes up half the data, it gets half the circle (180°), just like the minute hand sweeping from 12 to 6.
In science, pie charts are commonly used to display things like:
- The composition of Earth's atmosphere (78% nitrogen, 21% oxygen, 1% other gases)
- The proportion of different blood types in a population
- How students in a class chose their favourite type of renewable energy
Key Vocabulary for Pie Charts
Before we start constructing pie charts, let's make sure you know the key terms:
A "slice" of the pie chart. Each sector represents one category of data.
The angle at the centre of the pie chart that determines the size of each sector. All central angles in a pie chart add up to 360°.
The number of times a particular value or category occurs in a dataset.
The proportion of the total that a category represents, often expressed as a fraction, decimal, or percentage.
Key facts to remember:
- A full circle = 360°
- All sectors must add up to 360° (or 100%)
- Each sector's angle is calculated from its share of the total
Always check that your angles add up to exactly 360°. If they don't, you've made a calculation error somewhere — go back and check!
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