Maths
What is a pie chart?
Quick answer
A pie chart is a circle divided into slices that show parts of a whole. A bigger slice means a bigger share, and all the slices together make 100 percent, or 360 degrees.
In one sentence
A pie chart shows how a whole is split into parts by cutting a circle into slices of different sizes.
Explained
Imagine a round pizza that stands for everything you counted. Each slice is a category. The bigger the slice, the larger the share. If half the circle is blue, then half of the group fits that category. A quarter of the circle is 25%, and a tenth is 10%. All the slices together are 100% of the data.
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Pie charts are best for showing parts of a whole, like how a class chooses a favourite pet. They are not good for showing change over time (use a line graph) or for comparing many small amounts (a bar chart is clearer).
To draw a pie chart you work out an angle for each slice, because a full circle has 360 degrees.
Example: 24 children chose a pet: 12 dogs, 6 cats, 4 fish and 2 rabbits.
Step by step
- Find each fraction.Dogs are 12/24, which is 1/2.
- Multiply the fraction by 360.1/2 × 360 = 180 degrees for dogs.
- Do the same for the others.Cats: 6/24 × 360 = 90. Fish: 4/24 × 360 = 60. Rabbits: 2/24 × 360 = 30.
- Check.180 + 90 + 60 + 30 = 360.
- Draw with a protractor.Mark each angle in turn from the centre.
To read a pie chart, estimate each slice as a fraction or percentage. Half the circle is 50%. You can also use percentages if you know the total.
Examples
| Pet | Children | Fraction | Angle |
|---|---|---|---|
| Dogs | 12 | 1/2 | 180° |
| Cats | 6 | 1/4 | 90° |
| Fish | 4 | 1/6 | 60° |
| Rabbits | 2 | 1/12 | 30° |
Common mistakes
- Angles that do not add to 360. Always check your total.
- Reading size from a flat picture. Compare slices by angle, not by guessing.
- Using a pie chart for one category. A pie chart needs a whole split into at least two parts.
Quick quiz
Pie charts
Check what you know with 4 quick questions.
Question 1 of 4
How many degrees are there in a full circle?
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Related topics
Bar charts
A bar chart shows data using bars of different lengths. The longer the bar, the bigger the number. In the US it is usually called a bar graph.
Fractions
A fraction is part of a whole that has been split into equal parts. In ¾, the bottom number says how many equal parts and the top says how many you have.
Percentages
A percentage is a number out of 100, written with the sign %. So 25% means 25 out of every 100, which is the same as one quarter.
Line graphs
A line graph shows how something changes over time. Points are plotted on a grid and joined by a line, so you can see rises, falls and patterns at a glance.
Tally charts
A tally chart is a table that counts things with little marks. Each mark is one, and every fifth mark crosses the four before it, so you can count in fives.
Venn diagrams
A Venn diagram uses overlapping circles to sort things into groups. The overlap shows what belongs to both groups, and the outside shows what belongs to neither.