A short definition
IQ stands for Intelligence Quotient. It is a single number that summarises how a person performs on a standardised test of reasoning, compared to a large reference population of the same age. By construction, the average IQ is 100, and about two thirds of people score between 85 and 115.
Crucially, an IQ score is not a measurement of a fixed quantity in your brain, the way a scale measures your weight. It is a statistical position: it tells you where you sit on a distribution at one moment in time, on one type of test, under one set of conditions. Change the test, the day, or your sleep the night before, and the number moves.
That distinction matters more than any other idea in this article. People argue about IQ as though it were a physical property like height. It is closer to a percentile rank in a race: informative about how you did relative to others on that course, on that day, and silent about everything the course did not ask you to do.
Where the number 100 comes from
The earliest scores were literally quotients. Alfred Binet's scale produced a 'mental age', and William Stern proposed dividing mental age by chronological age and multiplying by 100. A ten-year-old performing like an average twelve-year-old scored 120. That arithmetic gave us the name that stuck.
The quotient method broke down for adults, because cognitive performance does not keep climbing with age the way it does in childhood. David Wechsler replaced it with the deviation IQ that every serious test uses today: score the raw answers, compare them to a representative sample of the same age group, and convert the result to a scale with a mean of 100 and a standard deviation of 15.
So a score of 115 does not mean you have 15% more of anything. It means you are one standard deviation above the average of the reference sample — better than roughly 84 out of 100 people who took the same test under the same rules.
What IQ tests actually measure
Modern IQ tests probe several distinct abilities: fluid reasoning (solving genuinely new problems), crystallised knowledge (what you have learned and retained), working memory (holding and manipulating information in mind), processing speed, and visuospatial reasoning. The Cattell–Horn–Carroll model, the dominant framework in psychometrics, organises more than a dozen narrow abilities beneath these broad ones.
These abilities are separable, but they are not independent. People who do well on one tend to do somewhat better than average on the others. That shared variance is what psychologists call the general factor, or 'g'. A full-scale IQ score is essentially an estimate of g, weighted by whichever subtests the battery happens to include.
Most online IQ tests, including ours, focus on fluid reasoning through visual matrices. Matrices are useful because they are largely culture-free and language-free: you do not need a particular vocabulary or curriculum to see that a shape rotates ninety degrees each step. They are also the subtests that load most heavily on g, which is why a short matrix test can approximate a much longer battery.
What IQ does not measure
An IQ score does not capture creativity, emotional intelligence, social skill, motivation, grit, ethical judgement, or practical wisdom. People with very high IQs sometimes struggle badly in work and relationships; people with average scores routinely achieve extraordinary things. The number is informative, not a verdict.
It also does not measure your ceiling. Cognitive performance shifts with sleep, stress, medication, nutrition, mood, education, and practice. A score taken on a bad day, or after months of little mental stimulation, under-represents what the same person produces on a good day.
And it says nothing about why a score is what it is. Two people can arrive at 108 by completely different routes: one fast and error-prone, the other slow and accurate. That is why a good report breaks the total down by ability instead of stopping at a headline figure.
How to read a score sensibly
Translate the score into a percentile before you interpret it. A score of 115 means you outperformed about 84% of the reference population. A score of 130 sits near the top 2%. A score of 85 is one standard deviation below the mean and is still comfortably within the normal range that describes the majority of people.
Then attach an error bar. Well-standardised clinical tests report a confidence interval of roughly plus or minus 5 points; short online tests deserve a wider one. Treat any single number as the centre of a range rather than an exact value, and be sceptical of anyone who quotes an IQ to the decimal point.
Finally, ask what the score is for. If the goal is self-understanding, the ability breakdown is more useful than the total. If the goal is a clinical or educational decision, an online estimate is not the right instrument, and a licensed psychologist administering a full battery is.
Why the average keeps moving
Raw performance on IQ tests rose steadily through most of the twentieth century, roughly three points per decade in many countries. James Flynn documented the pattern so thoroughly that it carries his name. Because tests are periodically restandardised, the average is pushed back to 100 each time, which hides the underlying gain.
Explanations include better nutrition and health, longer schooling, smaller families, and a visual, abstraction-heavy environment that trains exactly the kind of thinking matrix tests reward. Several countries have since reported a plateau or slight reversal, which is itself evidence that these scores respond to environment rather than measuring something immovable.
The practical consequence is simple: an IQ measured against a 1985 norm set is not directly comparable to one measured against a 2020 norm set. Always ask which population a score is being compared to.
Where IQ genuinely predicts something
Across large samples, cognitive test scores correlate moderately with academic achievement, job performance in complex roles, and training success. The correlations are real and replicated, but they are correlations of about 0.4 to 0.5 in favourable conditions, which means a great deal of variation is left unexplained.
At the individual level, that unexplained variation is the entire story of a life. Knowing someone's IQ improves a prediction about a group's average outcome far more than it improves a prediction about one person's future. Treat the score as one modest input among many, and never as a filter for what someone should be allowed to attempt.