Estimating numbers on a number line

Estimating using a number line

When you estimate a number you decide approximately, or roughly, how big that number is.

To estimate accurately, you need to have a good understanding of the value and order of numbers.

A number line is a useful way of showing the order of numbers and can help you compare and estimate the value of numbers.

The arrow on this number line is between 60 and 70.

It is less than 65, so a good estimate is approximately 62.

Try this estimating numbers on a number line quiz. Then work through the page and see if you can beat your score.

Number lines are a great way to compare and order numbers.

They also help you to count, understand place value and estimate where a number is.

This number line goes from 0 to 1000 in intervals of 100.

It is just like a 0 to 10 number line, with two extra zeros for each number to show it is going up in hundreds.

Let's look at another number line.

Even if some of the numbers on a number line have been removed, you can still use it to estimate the value of a number.

Let's work out what number is shown by the arrow.

Step 1 - Look at the range. This starts at 500 and ends at 1500.

Step 2 - Work out what each interval represents.

The difference between 500 and 1500 is 1000. Then, count the number of intervals. There are 10 in total.

How many 10s go into 1000? 100! So each interval increases by 100 each time.

Step 3 - Finally, count the number of intervals up to the arrow.

Starting at 500, there are 6 intervals. Each interval represents 100, so the calculation you need is:

100 × 6 = 600

The number line starts at 500, so:

500 + 600 = 1100

You can double check by counting 6 intervals from 500 on the number line. The arrow reaches the number 1100.

You can use the halfway position on a number line to help you estimate.

For example, to find out the halfway number on this number line, you find the difference between 1500 and 500.

1500 − 500 = 1000

Then halve it.

1000 ÷ 2 = 500

Now you can add 500 to the start number to work out that the middle number is 1000.

Once you know the halfway point, it is easier to work out that the arrow is pointing to 1100.

Some number lines are unmarked, but they are still useful for estimating the approximate value of a number.

Find out how to estimate the value of the arrow on this number line in the slideshow below.

Image caption,
Step 1
First work out the halfway point. The number line goes from 0 to 1000. It is useful to mark the halfway point, 500. This will help to estimate the value of the arrow.
Image caption,
Step 2
The arrow is closer to 500, so the number must be between 0 and 500.
Image caption,
Step 3
Split the section into equal intervals. If you split it into 5, the intervals will represent 100. The arrow is pointing to the 4th interval. As each interval is 100, you can estimate that the arrow is pointing to 400.