How to calculate the mean — a guide with examples

How do you calculate the mean?

The mean is the number that represents the "centre" of a set of values — a single score that sums up a whole term of tests, for example. The calculation is always the same: add up all the values, then divide by the number of values. In this guide we work through every common case together, including the weighted mean and finding a missing value.

The rule

Mean = sum of all the values ÷ number of values

The mean is the "shared height": if all the bars were levelled out, every one of them would stop at the dashed line.

Worked examples, step by step

A simple mean of test scores

Dana scored 70, 80, 90 and 100 in her tests. What is her mean score?

Add up all the scores: 70 + 80 + 90 + 100 = 340
Count how many scores there are: 4
Divide the total by the count: 340 ÷ 4 = 85
Answer: Dana's mean score is 85

Finding a missing value from a target mean

Joe's mean over three tests is 80, and he wants a mean of 82 after the fourth test. What score does he need?

If the mean of 3 tests is 80, their total is 80 × 3 = 240
For the mean of 4 tests to be 82, the total must be 82 × 4 = 328
The score needed is the difference: 328 − 240 = 88
Answer: Joe needs 88 in the fourth test

The weighted mean

A final grade is made of an exam (weight 70%), in which Noah scored 90, and coursework (weight 30%), in which he scored 80. What is the final grade?

Multiply each score by its weight: 90 × 0.7 = 63 and 80 × 0.3 = 24
Add the results: 63 + 24 = 87
The weights already add up to 100%, so there is nothing more to divide by
Answer: Noah's final grade is 87

How an outlier affects the mean

A set of measurements is 10, 12, 11 and 51. What is the mean, and what would it be without the 51?

With every value: 10 + 12 + 11 + 51 = 84, then 84 ÷ 4 = 21
Without the outlier: 10 + 12 + 11 = 33, then 33 ÷ 3 = 11
One extreme value almost doubled the mean — in cases like this it is worth checking the median too
Answer: the mean is 21, and without the outlier it is 11

Now try it yourself

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Frequently asked questions

What is the difference between the mean and the median?

The mean uses every value (total divided by count), so a single extreme value can move it a long way. The median is the middle value when the data is put in order, and it is barely affected by extreme values.

Does the mean have to be one of the numbers in the set?

No. The mean of 70 and 80 is 75 — a number that does not appear in the set. The mean is a balance point, not necessarily a value that was actually measured.

When do you use a weighted mean?

When the values do not all carry the same importance — for example a final grade where the exam is worth 70% and the coursework 30%. Multiply each value by its weight and add; if the weights do not add up to 1 (or 100%), divide by the sum of the weights.

How do you find the mean of percentages or of speeds?

Carefully: a mean of percentages is only valid when they refer to the same quantity. For speeds (for example there and back) the correct average is total distance divided by total time — not a simple mean of the two speeds.

✍️ Written by the MathsUK teamChecked against the National Curriculum and GCSE specificationsLast updated: 13.09.2026