What Does an IQ Score Mean? The 100 and 15 Scale Explained

The earliest intelligence tests, developed by Alfred Binet in France in the early 1900s, compared a child's performance with what was typical at each age. The phrase 'intelligence quotient' comes from that era, when a score was calculated as mental age divided by actual age, multiplied by 100. That formula stopped making sense for adults, so modern tests use a different method called the deviation IQ.

In a deviation IQ, your raw result is compared with a large reference group, usually people of your age. The scale is then set so that the average of that group is 100 and the standard deviation is 15. The standard deviation is simply a measure of spread: it tells you how far a typical person sits from the average. An IQ of 115 means one standard deviation above the reference group's average, and 85 means one standard deviation below.

Because the scale is defined relative to a group, the same raw performance can lead to slightly different scores on different tests or in different decades. Average scores on many tests rose steadily during the twentieth century in many countries, a pattern known as the Flynn effect, which is one reason professional tests are re-normed from time to time.

Scores on well-built tests roughly follow a bell-shaped curve. That makes it possible to translate an IQ score into a percentile, the share of the reference group who scored at or below you. A score of 100 sits at the 50th percentile. A score of 115 sits around the 84th percentile, and 130 around the 98th. Going the other way, 85 is around the 16th percentile.

About two in three people score between 85 and 115, and about 95 out of 100 score between 70 and 130. This is why a difference of a few points near the middle of the scale means very little in practice: many people are clustered there, and small differences are within the normal ups and downs of testing.

Percentiles are often easier to understand than the raw IQ number. Saying that a result is higher than about 84 percent of the comparison group is clearer than quoting 115, and it reminds you that the score is always a comparison with a particular group of people.

Most IQ-style tests sample several kinds of thinking. Fluid reasoning is the ability to spot patterns and solve unfamiliar problems, often measured with matrix puzzles where you find the rule that links a grid of shapes. Working memory is the ability to hold information in mind and work with it, such as repeating numbers backwards. Processing speed reflects how quickly and accurately you can handle simple visual tasks. Many batteries also include verbal knowledge and spatial reasoning.

Professional batteries, such as the Wechsler scales, report separate index scores for areas like these alongside an overall score. The different abilities tend to be positively related, which is why a single summary number is useful, but most people show a mix of relative strengths and weaker areas. Looking at that profile often tells you more than the headline score does.

IQ scores are linked on average with outcomes such as school performance, but they leave a great deal out. Creativity, practical judgment, motivation, emotional skills, specialist knowledge and the habits that help people learn and persist are only partly captured, if at all. Two people with the same score can have very different lives and talents.

A score also reflects the day you took the test. Sleep, stress, unfamiliarity with the question format, language and test anxiety can all pull a result down. Even professionally administered tests report a range of likely values rather than a single exact number, because every measurement contains some error.

An official IQ assessment is given one to one by a trained professional, uses a test that has been standardized on a large representative sample, and takes into account your background and circumstances. A free online test cannot do all of that. You take it on your own device, with your own distractions, and the comparison group is built in a different way.

That does not make online tests useless. A carefully designed online test can give a reasonable estimate of your reasoning skills and a sense of where your strengths lie. It just should not be treated as a diagnosis or used for any official purpose. If you need an assessment for school, work or health reasons, a qualified psychologist is the right route.

The free Witprism cognitive test has 30 timed questions in 30 minutes, led by matrix reasoning and joined by numerical, logical and spatial items. Your result is reported on the familiar 100 and 15 scale with a percentile and a score for each ability, so you can see your overall standing and your profile at the same time.

Treat the number as a snapshot: read the percentile first, then look at which abilities stood out, and remember that a few points either way is normal. If you are curious, take the test when you are rested and can focus for the full half hour, and you will get the fairest estimate of what you can do.