What does an IQ of 55 mean?
An IQ of 55 is three standard deviations below the average of 100, so only about 1 person in 741 scores this low or lower. On a professional test it falls in the "extremely low" range. On its own, though, a number never diagnoses anything, and on an online test it usually says more about what went wrong on the day than about the person.
How far below average is 55?
An IQ of 55 is 45 points below the average of 100, or 3 standard deviations. About 0.13% of people of the same age score lower, the shaded sliver at the far left of the curve below.
On a test with a standard deviation of 16, such as older Stanford-Binet editions, the same result would be about 52, and on the Cattell scale, which uses 24, about 28. Why scales differ.
What an IQ of 55 tells you
55 is a mirror image of 145. Both sit exactly three standard deviations from the middle, and both are reached by roughly one person in 741. Professional tests such as the WAIS-IV can report scores well below this, down to 40, but at this level psychologists rely far more on how someone manages everyday life than on the exact number.
It is also the lowest score the Assessivo test can give, and it means zero correct answers out of 36. Every question has four options, so even picking at random would score about nine. Getting none right by chance happens about once in 31,000 attempts; a cat strolling across the keyboard would expect to do better. If you saw 55 on our test, the likeliest story is that the timer ran out or the answers never reached us, so please try again when you have 25 quiet minutes.
How an IQ of 55 is classified
Different tests and manuals use different words for the same score, and the words have changed over time. Here is where 55 falls on each of the systems you are most likely to meet.
Sources: American Psychiatric Association, DSM-5 fact sheet on intellectual disability; World Health Organization, ICD-11, 6A00 disorders of intellectual development; Peltopuro, M. and others (2014), "Borderline intellectual functioning: a systematic literature review", Intellectual and Developmental Disabilities, 52(6), 419–443. WISC-V descriptors from Pearson's interpretive reports, as on our good IQ score guide.
Does 55 mean an intellectual disability?
A score of 55 on its own never makes a diagnosis. The American Psychiatric Association's DSM-5 describes intellectual disability as "approximately two standard deviations or more below the population, which equals an IQ score of about 70 or below", and requires three things together:
- Low intellectual functioning, confirmed by a clinical assessment and individually given, standardised testing.
- Difficulties with everyday adaptive skills in at least one of three domains: conceptual (language, reading, maths, memory), social (communication, judgement, friendships) and practical (personal care, money, work and school tasks).
- A start during the developmental period, in childhood or adolescence.
Severity is set by those everyday skills, not by the IQ number. That is why only about 1% of people have an intellectual disability (a 2011 meta-analysis of 52 population studies found about 10 per 1,000), even though about 2.3% score below 70 on the bell curve. Because tests have a margin of error, DSM-5 treats roughly 65 to 75 as the uncertain zone around 70.
Sources: American Psychiatric Association, DSM-5 fact sheet on intellectual disability; Maulik, P. K. and others (2011), "Prevalence of intellectual disability: a meta-analysis of population-based studies", Research in Developmental Disabilities, 32(2), 419–436; Hall v. Florida, 572 U.S. 701 (2014), which cites the DSM-5 range of 65 to 75.
Is your 55 accurate?
Low scores are the most fragile results on any test, because almost everything that goes wrong on the day pulls a score down. Common reasons include:
- Running out of time. On timed tests, including ours, an unanswered question counts as wrong.
- Nerves. A review of 238 studies found test anxiety consistently linked with lower scores.
- Tiredness. Sleep loss hits attention and speed hardest, which costs points against the clock.
- Language and format. Taking a test in a second language, on a small screen or in an unfamiliar format can lower a result without saying anything about reasoning.
What a 55 means on our test
On the Assessivo test, 55 means no correct answers out of 36. Every question has four options, so someone guessing every answer at random averages nine correct, a score of 78. Pure guessing gets nothing right only about once in 31,463 attempts, so a cat walking across the keyboard would expect to do better. A result this low almost always means the timer ran out or the answers never reached us.
If any of this sounds like your session, retake the free IQ test when you are rested and have 25 quiet minutes. A score that moves a lot between attempts was never very solid.
Sources: von der Embse, N. and others (2018), "Test anxiety effects, predictors, and correlates: a 30-year meta-analytic review", Journal of Affective Disorders, 227, 483–493; Lim, J. and Dinges, D. F. (2010), "A meta-analysis of the impact of short-term sleep deprivation on cognitive variables", Psychological Bulletin, 136(3), 375–389. Guessing figures are worked out from the binomial distribution for 36 four-option questions.
What is the mental age for an IQ of 55?
The first IQ tests, for children, described performance as a "mental age": the age whose typical performance a child matched. IQ was mental age divided by real age, times 100. Run that old formula backwards and you get the figures below, for children only.
| Child's age | Performs like a typical child of about |
|---|---|
| 6 | 3 years 4 months |
| 8 | 4 years 5 months |
| 10 | 5 years 6 months |
| 12 | 6 years 7 months |
| 14 | 7 years 8 months |
For adults the idea doesn't work, which is why David Wechsler replaced it in 1939 with today's method: compare each person with others of the same age. An adult who scores 55 has an adult's knowledge, interests and experience. The score only says how their test performance compares with other adults'.
How common is an IQ of 55?
Line up 100 people of the same age and about 1 would score 55 or lower; the orange dot marks the place. In a city of a million, about 1,350 people would be at or below this level.
The bar shows the likely range on a professional test, about five points either way: 50 to 60, from the 0.04th to the 0.38th percentile.
How 55 compares with nearby scores
| IQ | Percentile | Classic label | Score this low or lower |
|---|---|---|---|
| 50 | 0.04th | Extremely low | 1 in 2,330 |
| 55 | 0.13th | Extremely low | 1 in 741 |
| 60 | 0.38th | Extremely low | 1 in 261 |
| 65 | 0.98th | Extremely low | 1 in 102 |
Questions about an IQ of 55
What percentile is an IQ of 55?
An IQ of 55 is at about the 0.13th percentile: it is higher than about 0.13% of people of the same age, and roughly 1 person in 741 scores this low or lower.
Is 55 a low IQ?
Yes. 55 is 45 points, or 3 standard deviations, below the average of 100. On the classic Wechsler labels it is extremely low. A single score is an estimate with a margin of error of about five points, so 55 is best read as roughly 50 to 60.
Does an IQ of 55 mean intellectual disability?
Not on its own. 55 is in the range where an intellectual disability (called a learning disability in the UK) is considered, but a diagnosis also needs significant difficulties with everyday skills that began during childhood, assessed by a qualified psychologist.
What is the mental age for an IQ of 55?
Mental age is an idea from the earliest tests for children. By the old formula, a 10-year-old scoring 55 would perform like a typical child of about 5 years 6 months. For adults it has no meaning: modern IQ tests compare adults with other adults, and an adult who scores 55 is not like a child.
Why is 55 the lowest score on the Assessivo test?
Our score is 100 plus the share of correct answers minus a half, times 90, so zero correct gives 55. A short online test can't measure reliably below that point, so we don't report lower scores. Professional tests, given one to one, can report full-scale IQs down to about 40.
Related: the IQ range chart, what counts as an average IQ, what IQ measures and misses and how to tell if an online IQ test is accurate.
Behind a single score of 55 sit several separate abilities. Read about fluid vs crystallized intelligence, abstract reasoning, spatial reasoning, verbal reasoning and processing speed, or try the free pattern recognition test.
Information and support
If a professional assessment has given you, your child or someone you care for a score like 55, these organisations explain what it means in practice and what help is available.
Want a second reading?
Take the free Assessivo IQ test when you are rested and have 25 quiet minutes: 36 questions, and your estimated score and percentile as soon as you finish.
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