How Are IQ Tests Scored? The Science Behind the Numbers

IQ scores follow a bell curve with an average of 100 and a standard deviation of 15. Here's how modern IQ tests are scored, what the ranges mean, and where the numbers come from.

IQ testing is one of the most widely used—and widely misunderstood—tools in psychology. Millions of people have taken intelligence tests, yet very few understand how the scores are actually calculated. What does an IQ score of 115 really mean? Why is 100 the average? And how do testers arrive at a single number that supposedly captures something as complex as human intelligence? In this article, we'll walk through the science of IQ scoring, from its origins in early twentieth-century France to the modern statistical methods used today.

The Origins of IQ Testing

The intelligence quotient test was born out of a practical problem. In 1904, the French government commissioned the psychologist Alfred Binet to develop a method for identifying children who needed extra help in school. Binet and his colleague Théodore Simon created a series of tasks—arranging blocks, repeating numbers, solving simple puzzles—designed to assess a child's mental age relative to their chronological age. The result was the Binet-Simon scale, the first practical intelligence test.

The concept of "intelligence quotient" itself was introduced a few years later by the German psychologist William Stern. Stern proposed dividing a child's mental age by their chronological age and multiplying by 100. A child with a mental age of 10 and a chronological age of 8 would have an IQ of 125 (10/8 × 100). This ratio-based approach worked reasonably well for children but was problematic for adults, because mental age doesn't increase indefinitely—most adults reach a mental age equivalent of about 16 to 18, which would make everyone's IQ decline as they got older.

The Modern Approach: Deviation IQ

Modern IQ tests abandoned the mental-age approach in favor of a statistical method called deviation IQ. Instead of comparing mental age to chronological age, deviation IQ compares an individual's performance to the performance of others in their age group. The scores are arranged on a bell curve—a normal distribution—with the average score set at 100 and a standard deviation of 15 points.

Here's what that means in practice. The bell curve is symmetric, with most scores clustered around the average and fewer scores at the extremes. About 68 percent of all people score between 85 and 115—within one standard deviation of the mean. About 95 percent score between 70 and 130—within two standard deviations. Only about 2 percent score above 130, and only about 0.5 percent score above 140. These percentages are not arbitrary; they are mathematical properties of the normal distribution, the same bell curve that describes many natural phenomena, from human height to measurement error.

The Standard IQ Ranges

Using the deviation IQ framework with a mean of 100 and a standard deviation of 15, psychologists have established standard classification ranges. Scores below 70 are classified as extremely low, achieved by about 2 percent of the population. Scores from 70 to 84 are borderline or low average. The vast majority of people—about 68 percent—score between 85 and 115, the average range. Scores from 115 to 129 are above average or superior, achieved by about 14 percent of the population. Scores from 130 to 144 are classified as gifted or very superior, and scores above 145 are considered very gifted, achieved by fewer than 0.2 percent of people.

These classifications can vary slightly depending on the specific test used. The two most widely used individual IQ tests are the Wechsler Adult Intelligence Scale (WAIS) for adults and the Stanford-Binet Intelligence Scales, which can be used with both children and adults. Both tests use the deviation IQ approach, but they assess slightly different combinations of cognitive abilities and may produce slightly different scores for the same individual.

What Do IQ Tests Actually Measure?

This is where the science gets more complicated. IQ tests do not measure a single, unified thing called "intelligence." They measure a set of specific cognitive abilities, and different tests emphasize different abilities. The WAIS, for example, is divided into four indexes: Verbal Comprehension (the ability to understand and use language), Perceptual Reasoning (the ability to solve nonverbal, visual problems), Working Memory (the ability to hold and manipulate information in the short term), and Processing Speed (the ability to quickly and accurately process simple information).

These abilities are correlated—people who perform well on one subtest tend to perform well on others—and this correlation is what allows testers to produce a single overall IQ score. The idea is that there is a general intelligence factor, often called "g," that underlies performance across all cognitive tasks. The overall IQ score is intended as an estimate of this general factor. But critics of IQ testing argue that the concept of "g" is reductive—that intelligence is too complex and varied to be captured by a single number.

The Einstein IQ assessment takes a different approach. Rather than producing a single score, it measures five distinct dimensions of intelligence: Logical Reasoning, Pattern Recognition, Verbal Intelligence, Spatial Reasoning, and Creative Intuition. This multidimensional approach recognizes that intelligence is a profile, not a number. Someone might have exceptional pattern recognition but average verbal intelligence, or extraordinary creative intuition alongside solid logical reasoning. By presenting intelligence as a profile rather than a single score, the assessment captures a richer picture of cognitive ability.

The Limits of IQ Testing

It's important to understand what IQ tests do not measure. They do not measure creativity in the artistic sense, emotional intelligence, motivation, curiosity, wisdom, or practical problem-solving ability. They do not predict success in life with any precision—while IQ is correlated with academic achievement and certain job performance measures, the correlations are modest, and many other factors contribute to real-world success.

Consider Marilyn Monroe, profiled in the Einstein IQ assessment with an estimated IQ of 105 to 115—above average but not extraordinary. Monroe's cultural impact was arguably greater than that of many people with much higher IQs, because her strengths—creative intuition, emotional perception, charisma—are not well captured by traditional intelligence tests. Or consider Andy Warhol, estimated at 100 to 110—perfectly average—whose influence on twentieth-century art and culture was profound. These examples illustrate why IQ should be understood as one measure of cognitive ability among many, not as a definitive assessment of a person's worth or potential.

How Historical IQ Estimates Work

The Einstein IQ assessment uses the IQ ranges described above to map quiz performance to historical figures. But as we've discussed in our article on famous IQ scores, the IQ numbers assigned to historical figures like Leonardo da Vinci, Nikola Tesla, and Albert Einstein are not based on actual test results—they are retrospective estimates based on documented achievements and cognitive abilities. These estimates use the same deviation IQ framework (mean of 100, standard deviation of 15) but are inherently approximate. The Einstein IQ assessment frames all historical IQ figures as stylized estimations for entertainment purposes, not clinical measurements.

Discover Your Own Cognitive Profile

If you're curious about how your own cognitive abilities compare to the standard IQ ranges, the Einstein IQ assessment offers a free 12-question evaluation across five dimensions of intelligence. After completing the test, you'll receive an estimated IQ range, a match with a comparable historical figure, and a detailed cognitive trait profile showing your strengths. Whether you're average, above average, or gifted, the assessment is designed to be informative and engaging—helping you understand your own mind while being honest about the limits of IQ as a measure of human intelligence.

Take the free Einstein IQ assessment and discover your cognitive profile today. The test takes about five minutes, and you might be surprised by what you learn about yourself.