Correlation Calculator (Pearson r)

Where it is used

• Teachers: A teacher checks whether the number of hours pupils spend studying is linked to the marks they get on a test.
• Café and shop owners: A café owner sees whether hotter days go together with higher cold-drink sales across the past month.
• HR staff: An HR officer looks at whether staff satisfaction scores rise or fall with the number of years people have worked at the company.

Interpreting r

Pearson’s r ranges from −1 to +1. Values near +1 mean the two variables rise together, near −1 mean one rises as the other falls, and near 0 mean little linear relationship. Correlation does not imply causation.

When should you use it?

Use this calculator when you have two numbers recorded for each person or item and you want to know if they are linked. For example, for each pupil you might have hours studied and a test score, or for each day you might have the temperature and the number of drinks sold. It only works with numbers that can go up or down, like age, price, score, or count. It is not the right tool for yes or no answers, categories, or a single list of numbers on its own.

What does the result mean?

The result is one number between -1 and +1, often called r. A value near +1 means a strong link where both rise together. A value near -1 means a strong link where one rises as the other falls. A value near 0 means almost no straight-line link. A common guideline treats about 0.1 as a weak link, about 0.3 as moderate, and 0.5 or higher as strong. The plus or minus sign tells you the direction, and the size tells you the strength.

Mistakes to avoid

Do not assume a link means one thing causes the other. Ice cream sales and sunburn rise together, but ice cream does not cause sunburn; hot weather drives both. Keep your two lists in the same order so each pair belongs to the same person or item, and make sure both lists are the same length. Watch out for one very unusual value, as a single odd number can push the result up or down. Finally, a result from only a handful of pairs is not very reliable.

How to use this calculator

1. Collect your paired numbers, so you have two measurements for each person or item, such as hours studied and test score.
2. Type or paste the first measurement into the first list and the matching second measurement into the second list, keeping them in the same order.
3. Separate each number with a comma or a new line, and check that both lists have the same number of values.
4. Read the result: look at the plus or minus sign for the direction, and how close it is to 1 or -1 for the strength of the link.

Worked example

Imagine five pupils. Their hours studied were 1, 2, 3, 4, and 5, and their test scores were 50, 55, 65, 70, and 80. Enter the hours in one list and the scores in the other, in the same order. The calculator returns about 0.99, a very strong positive link, meaning pupils who studied more tended to score higher.

Frequently asked questions

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