Guide · Reading your score
Average IQ score: why 100 is the middle
The average IQ score is 100 by definition, not by measurement. That number is the center of the deviation IQ scale, and every other score simply describes how far above or below that center a person scored on a particular test.
The definition
Why the average is set to 100
IQ is not like height or weight. It is a relative ranking.
On the deviation IQ scale, the population mean is fixed at 100 and the standard deviation is fixed at 15. Test publishers norm their samples so that the average raw score maps to 100. This makes scores comparable across different tests and different editions.
A score of 100 does not mean someone answered half the questions correctly. It means they performed at the average level of the comparison group used to norm the test. A harder test produces lower raw scores, but the deviation score still centers on 100.
The comparison
How common scores compare to the average
Moving one standard deviation in either direction covers most people.
| IQ score | Percentile | Meaning |
|---|---|---|
| 115 | ~84th | One standard deviation above the mean |
| 100 | ~50th | The defined average |
| 85 | ~16th | One standard deviation below the mean |
About two thirds of people score between 85 and 115. Only about one in twenty scores above 130 or below 70. That is why small differences near the average are usually not meaningful.
National averages
Is the US average IQ different from 100?
Well-normed tests are designed to make the US average 100.
Major individually administered tests, such as the WAIS and SB5, are normed so that the reference group — typically a US census-matched sample — has a mean of 100. So a well-normed US average IQ is 100 by construction.
Cross-country comparisons are more complicated. Different nations use different tests, different norms, and different sampling methods. Online estimates also differ because the people who choose to take them are not a random sample. Treat any "national average IQ" list with skepticism unless it explains exactly how the numbers were produced.
Honest reporting
Why a range is better than a single number
Every score includes measurement error. A range makes that visible.
If you take the same IQ test twice, you will probably not get exactly the same score. Sleep, mood, practice, and chance all move the number. A responsible report gives a 95% confidence interval — for example, 95–110 — rather than pretending the score is precise.
Brain Rate's free short form reports a 95% range for exactly this reason. The paid full assessment uses more items, so the range is narrower, but it is still a range.
Keep reading
- IQ test: how scoring worksThe deviation scale, item scoring, and why the result is a range.
- Free IQ testA 16-question test with an honest score range, no account needed.
- Online IQ test: what to expectHow online testing differs from a supervised clinical assessment.
- Abstract reasoning testRule-finding puzzles with no words or numbers involved.
- Verbal reasoning testAnalogies and relationships between words and ideas.
- Matrix reasoning testPractice the grid-pattern items used on most modern tests.
- What is a good IQ score?What the numbers mean, and what they cannot tell you.
- IQ score range chartFrom below average to genius, with percentiles for each band.
- IQ percentile calculatorEnter a score and see its percentile on the deviation IQ scale.
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