What does an IQ test measure?

An IQ test measures how well you solve a set of reasoning problems compared with other people who took the same test. The number at the end is not a reading from a gauge inside your head. It is a rank, rescaled so that the average person scores 100.

That short answer hides a lot of interesting detail, so let’s unpack it.

Reasoning skills that travel together

Psychologists noticed more than a century ago that people who do well on one kind of mental test tend to do at least a little better on others: vocabulary, arithmetic, spatial puzzles, memory tasks. In 1904 Charles Spearman proposed that a single general factor, usually written g, sits behind this pattern.

Modern research still finds that general factor in almost every large set of mental tests. It does not mean everyone is good or bad at everything. It means that across thousands of people, scores on different reasoning tasks are positively related. An IQ score is mostly an estimate of that shared part.

Fluid and crystallised abilities

A useful split, introduced by Raymond Cattell, separates two broad kinds of ability:

• Fluid reasoning: solving new problems you have not seen before, spotting patterns and rules, thinking on the spot.
• Crystallised knowledge: what you have learned, such as vocabulary, facts and familiar methods.

Full clinical batteries, given one to one by a psychologist, sample both. They typically include verbal tasks, visual puzzles, working-memory tasks and speed tasks, and combine them into one overall score plus several index scores.

What a short online test can measure

A short test cannot cover all of that. The test on this site focuses on fluid reasoning, with three kinds of puzzle that need no language and can be generated fresh every time:

• Matrices: find the figure that completes a 3×3 grid of shapes. See how to solve matrix puzzles.
• Number series: work out the rule and choose the next number. See common number series patterns.
• Mental rotation: pick the shape that is the same as a target, only turned. See mental rotation explained.

Matrices and series are among the item types most closely related to general reasoning, and rotation adds spatial ability. Mixing them is what makes it reasonable to put the result on the IQ scale at all. It is still a narrow slice of what a full battery measures.

Why the score is a rank

Raw scores mean little on their own: 12 correct answers out of 16 could be excellent on a hard test and ordinary on an easy one. So every IQ test is norm-referenced. Your raw score is compared with the scores of a reference group and turned into a position on a scale where:

• the average is set to 100,
• the standard deviation is set to 15,
• about two thirds of people score between 85 and 115,
• about 2% score above 130.

The IQ scale explained goes through what each part of that scale means.

What IQ tests do not measure

An IQ score leaves out a great deal that matters in real life:

• Creativity and original ideas.
• Knowledge and expertise built over years.
• Judgement, wisdom and common sense.
• Motivation, persistence and curiosity.
• Social and emotional skills.
• Practical skills: making, fixing, leading, caring.

Scores also move with sleep, stress, illness, the device you use and how familiar you are with the puzzle types. That is why a single result, especially from a short test, should be treated as an estimate.

Where this test fits

This site gives a free, instant estimated range after 16 or 32 questions, with every question explained afterwards. It is built for curiosity and entertainment, not diagnosis, and the methodology page explains exactly how the range is calculated and where its limits are.

Ready to see the puzzles for yourself? Take the free quick test: 16 questions, about 12 minutes, no email and no payment.