Bar Charts

What you will learn

• Read values from a bar chart.
• Choose a scale and draw a bar chart.
• Compare bars to find totals and differences.

Before you start: you should be able to read a table and count in twos, fives and tens. See Reading Tables and Pictograms.

The idea

A bar chart compares the size of different categories. Each bar stands for one category. The height of a bar shows its value, which you read from the scale on the vertical axis.

Reading a bar chart

1. Read the title and axis labels.
2. Find the step: how much each gap on the scale is worth.
3. Follow across from the top of the bar to the scale.
4. If the bar ends between two marks, estimate how far along it is.

Choosing a scale

The scale should start at $0$ and use equal steps. Pick a step that makes the biggest bar fit but does not leave a lot of empty space. For values up to $87$ steps of $10$ work well. For values up to $15$ steps of $5$ or $2$ both work.

Comparing bars

Use the bars to answer questions. The tallest bar is the biggest category. To find how many more, subtract. To find a total, add all the bar values.

Worked examples

Example 1: Reading values

Use the pets chart. How many more dogs than cats?

1. Dogs $12$, cats $8$.
2. $12 - 8 = 4$.

Answer: $4$ more dogs

Example 2: Choosing a scale

A class counts $18$, $42$, $27$ and $35$ votes. What step would you use?

1. The biggest value is $42$.
2. Steps of $10$ go to $50$ with only five marks, which is clear.

Answer: a scale from $0$ to $50$ in steps of $10$

Example 3: Estimating

A bar ends half way between $20$ and $25$. What is the value?

1. The gap is $5$, half of it is $2.5$.
2. $20 + 2.5 = 22.5$.

Answer: $22.5$

Tip: bars should be the same width with equal gaps. A scale that does not start at zero can make small differences look huge.

Why it matters

Bar charts show up in school reports, sports tables, news and surveys. They let you compare at a glance, which makes them the most common chart. Next, line graphs show how a quantity changes over time.