# What Is the Average IQ? The Basics of Standard Deviation and Distribution - URL: https://brainrank.jp/en/blog/iq-average - Published: 2026-05-01 - Publisher: BrainRank Editorial Team (https://brainrank.jp) The average IQ is 100 and the standard deviation is 15. We explain in plain terms why 100 is the average, the shape of the distribution, and how to think about "top X%." The average IQ is 100. That is not a value that happened to come out of measurement — modern intelligence tests are designed so that the average of an age group lands exactly at 100 with a standard deviation of 15. In other words, an IQ of 100 is the middle by definition. This article explains the mechanism and how to read 'top X%.' ## Why the average is 100: IQ is a relative value IQ is not an absolute quantity like height or weight — it measures your relative position within your own age group. Raw scores (number correct, and so on) from large numbers of test takers are collected, and the distribution is then refitted onto a normal distribution with a mean of 100 and a standard deviation of 15, a process called standardization. Because of this design, the same person's IQ changes if the reference group changes. The single most important premise when reading any IQ figure is that a reference group is always attached to it. ## What a standard deviation of 15 means The standard deviation (SD) of 15 describes the spread of scores. On a normal distribution with a mean of 100 and an SD of 15, the share of people in each range works out as follows: - IQ 85–115 (mean ±1 SD): about 68% of people - IQ 70–130 (mean ±2 SD): about 95% of people - IQ 130 and above (+2 SD and up): the top 2% or so (about 1 in 44) - Below IQ 70 (under −2 SD): the bottom 2% or so All of these follow arithmetically from the properties of the normal distribution and require no additional survey data. Detailed population shares for each band are collected in our [IQ distribution data](https://brainrank.jp/en/data/iq-distribution). ## Converting to a deviation score makes it concrete The deviation score familiar from school exams uses a scale with a mean of 50 and a standard deviation of 10. Because IQ and deviation scores express the same normal distribution on different rulers, you can convert between them directly. Deviation score = 50 + (IQ − 100) ÷ 15 × 10. By this formula, IQ 115 maps to a deviation score of 60 and [IQ 130](https://brainrank.jp/en/iq/130) maps to 70. The full conversion procedure is covered in [converting between IQ and deviation scores](https://brainrank.jp/en/blog/iq-deviation-conversion). ## Does the 'average of 100' move across eras and countries? The average comes out to 100 only relative to the reference group used when the test was built. Raw scores rose throughout the 20th century in many countries — the [Flynn effect](https://brainrank.jp/en/blog/flynn-effect) — and tests are re-standardized every few decades to pull the average back to 100. So 'IQ 100 back then' and 'IQ 100 today' do not correspond to the same raw performance. Comparisons of national averages are likewise heavily influenced by which test was used, when it was administered, and how the sample was drawn, so simple cross-country rankings warrant caution (see [how to read average IQ by country](https://brainrank.jp/en/blog/iq-by-country)). ## 'Average' is not a bad result About 50% of people fall within IQ 90–109. Scoring near the average is therefore the most common, most standard outcome there is; we cover this in [is an IQ of 100 'just normal'?](https://brainrank.jp/en/blog/iq-100-meaning). What matters in daily life is far less a few points of overall IQ than the profile of strengths and weaknesses across domains. ## How to find out your own IQ To learn your true relative position, a test that measures multiple domains in a balanced way is most effective. [BrainRank's free IQ test](https://brainrank.jp/en/start) scores 20 questions across spatial reasoning, pattern recognition, logical reasoning, and classification (about 10 minutes) using item response theory (IRT), producing an estimated IQ, deviation score, and percentile on the same mean-100, SD-15 scale used throughout this article. --- Statistics and percentages are calculated from a normal distribution model with a mean of 100 and a standard deviation of 15. The tests on this site provide estimates for entertainment and self-understanding; they are not medical or clinical assessments, nor official intelligence tests. When citing, credit "BrainRank (brainrank.jp)" and link the article URL.