Maths
What is a bar chart?
Quick answer
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.
In one sentence
A bar chart is a picture of data where each bar's length shows how many.
Explained
Imagine asking 25 classmates about their favourite pet. You could write a list of answers, but a bar chart shows the result at a glance:
| Pet | Children |
|---|---|
| Dog | 9 |
| Cat | 6 |
| Fish | 4 |
| Rabbit | 6 |
dog
cat
fish
rabbit
To draw one:
- Draw two axes. Along the bottom go the categories (dog, cat, fish, rabbit). Up the side goes the scale (the number of children).
- Choose a sensible scale that starts at 0 and goes up in equal steps, like 1s, 2s or 5s, high enough for your biggest number.
- Draw one bar for each category, all the same width with equal gaps.
- Give the chart a title and label both axes.
Reading a bar chart is like reading a ruler. Find the top of the bar, follow it across to the scale, and read the number. If the bar stops between two marks, work out what each small step is worth. On a scale going up in 2s, a bar halfway between 8 and 10 shows 9.
Bar charts are great for comparing. Which is most popular? How many more chose dogs than fish (9 − 4 = 5)? How many children altogether (9 + 6 + 4 + 6 = 25)? You can even find the mode: it's the tallest bar.
Bars can go up (vertical) or across (horizontal). With two sets of data, like boys' and girls' choices, you can draw a dual bar chart with two bars side by side for each category. Younger children often start with a pictogram, which uses pictures instead of bars.
Examples
- A bar reaching 12 on a scale of 0, 2, 4… 14 shows 12.
- Rainfall bar chart: March 40 mm, April 55 mm, so April had 15 mm more.
- Sunny days: 18 in July and 6 in December. July had 3 times as many.
Common mistakes
- Uneven scales, like 0, 1, 2, 5, 10. Fix: every step must be the same size.
- Starting the scale above 0, which makes small differences look dramatic.
- Bars of different widths. Only the length should carry the information.
- Mixing up bar charts and histograms. In a histogram (for continuous data, like heights) the bars touch; in a bar chart they usually have gaps.
Quick quiz
Bar charts
Check what you know with 4 quick questions.
Question 1 of 4
In a bar chart of favourite pets, the dog bar reaches 9 and the cat bar reaches 6. How many more children chose dogs?
Practise it
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Browse maths worksheets for charts and data handling.
For grown-ups
Related topics
Pictograms
A pictogram shows data using small pictures or symbols. A key tells you what each picture is worth, such as one apple standing for 2 apples. In the US it is often called a picture graph.
Mean, median and mode
Mean, median and mode are three kinds of average. The mean shares the total out equally, the median is the middle value in order, and the mode is the most common value.
Counting
Counting means saying one number for each thing in a group, in order, to find out how many there are. The last number you say is the total.
Probability
Probability measures how likely something is to happen, on a scale from 0 (impossible) to 1 (certain). A fair coin landing heads has a probability of 1/2.
2D shapes
2D shapes are flat shapes with length and width but no thickness, like circles, triangles, squares and hexagons. You can draw them on paper.
3D shapes
3D shapes are solid shapes with length, width and height, like cubes, spheres, cylinders and pyramids. They have faces, edges and vertices.