Bottom 1%
What IQ is the bottom 1 percent?
The 1st percentile on the bell curve, on three common scales, with what a score in that range does and does not mean.
Short answer
The bottom 1% of IQ scores is about 65 and below on the standard scale (mean 100, standard deviation 15); the exact cut is 65.1. That is the 1st percentile, about 1 in 100 people. On a scale with standard deviation 16 the cut is about 63, and on the SD 24 scale about 44.
Specimen · Bottom 1%
1%Bottom
- Cut at
- IQ 65.1
- Classification
- Extremely Low
- z-score
- −2.33
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| Scale | IQ at the 1st percentile |
|---|---|
| Standard deviation 15 (Wechsler, SB5) | 65.1 |
| Standard deviation 16 (older Stanford-Binet) | 62.8 |
| Standard deviation 24 (Cattell Culture Fair III) | 44.2 |
How the cut is calculated
IQ scores are standardized to a normal curve with mean 100 (Wikipedia: Intelligence quotient). To find the score at a percentile, take the z score that leaves 1% of the curve below it, multiply by the scale's standard deviation and add 100. For the 1st percentile the z score is -2.326, so the cut is 65.1 on SD 15. Older Stanford-Binet forms used SD 16 and the Cattell Culture Fair III scale uses SD 24 (Wikipedia: IQ classification), which is why the same percentile reads as 62.8 and 44.2 there. The percentile to IQ converter does this for any percentile.
What a score in the bottom 1% means
A score of 65 is labeled Extremely Low on the WAIS-IV scale (Wikipedia: IQ classification). Labels describe score ranges on one test; they are not diagnoses. A low score on any single session can reflect tiredness, an unfamiliar format, distraction or a small screen as much as ability.
Clinical scores carry measurement error of around plus or minus 5 points at 95% (Wechsler, 2008), so a measured 65 is consistent with a true score from 60 to 70, about the 0.38 to 2.3 percentile. Questions about a learning or intellectual disability need a full assessment by a licensed psychologist, not an online estimate.
A percentile is not a percentage of questions
The 1st percentile means scoring higher than 1% of people the same age, not answering that share of questions correctly. See whether an IQ score is a percentage for the difference.
Neighboring percentiles
| Percentile | IQ (SD 15) | Gap from 65 |
|---|---|---|
| Bottom 2% | 69.2 | +4.1 |
See the score itself on IQ 65, every cut on the IQ by percentile page, or any score on the IQ score chart.
Sources
- Intelligence quotient. Wikipedia. en.wikipedia.org/wiki/Intelligence_quotient
- IQ classification. Wikipedia. en.wikipedia.org/wiki/IQ_classification
- 68-95-99.7 rule. Wikipedia. en.wikipedia.org/wiki/68%E2%80%9395%E2%80%9399.7_rule
- Wechsler, D. (2008). WAIS-IV Technical and Interpretive Manual. Pearson.