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IQ Score Ranges: What the Bands on a Score Chart Mean

Score bands are cut points drawn across a continuous curve by a publisher, not natural categories. Here is how they are built and why the edges are unreliable.

By Published Updated 5 min read

Reign Creative LLC makes IQ Test & Brain Training Games, which this article links to, so we have a commercial interest in it. How we write and check articles.

Key takeaways

  • There is no natural boundary anywhere on an IQ scale. The distribution is continuous, and every band edge is a cut point a publisher chose.
  • Bands are defined in standard deviations from the mean. On a scale centred on 100 with a standard deviation of 15, one band is usually one standard deviation wide or two-thirds of one.
  • The percentages are fixed by the shape of the curve, not by the population. NIST gives 68.27 per cent within one standard deviation of the mean, 95.45 per cent within two and 99.73 per cent within three.
  • One band edge does carry formal weight: intellectual disability criteria have traditionally used approximately two standard deviations below the mean, a standard score of about 70 — and even there, measurement error means scores in the low 70s need careful handling.
  • Labels vary between publishers and between editions of the same instrument, so an identical number can attract different words depending on which chart is open.
  • IQ Test & Brain Training Games, our own app, includes an IQ-style test, and its listing describes the results as estimates based on app activities, not a clinical or professional assessment — so any figure it shows deserves more caution than a band on a publisher's chart, not less.

The bands on an IQ score chart are lines a publisher drew across a continuous curve. They are not discovered categories, there is no natural break anywhere along the scale, and the same number can carry different words depending on which chart you are reading. Understanding how the lines get placed is more useful than memorising any particular set of them.

Underneath every such chart is one distribution and two parameters, and once you can see those, the labels stop being mysterious.

The curve the bands are drawn on

Scores from the major individually administered batteries are expressed on a standard-score metric with a mean of 100 and a standard deviation of 15 — a convention stated explicitly in the psychometric literature, including a 2026 meta-analysis of Wechsler profiles that standardised its effect sizes against exactly that scale.

The shape is the normal distribution. NIST's statistical handbook gives the proportions that shape forces, and they are the real content of any score chart:

  • 68.27 per cent of the distribution lies within one standard deviation of the mean
  • 95.45 per cent lies within two
  • 99.73 per cent lies within three

Translate those onto a scale centred on 100 with a standard deviation of 15 and the familiar landmarks appear without anyone having to look anything up. About 68 per cent of the norm sample sits between 85 and 115. About 95 per cent sits between 70 and 130. About 99.7 per cent sits between 55 and 145.

Those percentages are properties of the curve, not facts about people. They would hold for any variable forced onto a normal metric.

Why a band is the width it is

Band widths are chosen for convenience. Ten points is a common choice; so is fifteen. On this scale, fifteen points is exactly one standard deviation and ten points is two-thirds of one. Nothing in the measurement requires either.

This has a consequence that charts rarely mention: bands near the middle of the scale contain enormously more people than bands of identical width near the ends. The stretch from 90 to 110 — two bands at ten-point spacing — covers close to half the comparison group. The stretch from 130 to 150, the same twenty points wide, covers a little over two per cent. Equal width, wildly unequal population. The relationship between a score's position and its percentile rank is worked through in percentile, standard score and raw score.

Who decides the labels

The descriptive words — the adjectives attached to each range — are editorial decisions by the publisher of a particular instrument. They differ between publishers, and they have been revised within the same instrument family across editions, generally away from older clinical terminology and towards neutral descriptions of position.

Three practical consequences follow.

A label is not portable. Two instruments can attach different words to the same number, so quoting a band without naming the test is close to meaningless.

A label is not a diagnosis. Descriptive classifications summarise where a score sits. They are not clinical conclusions, and no responsible report treats them as one.

A label is not a threshold effect. Nothing changes at 119 that does not change at 120. The boundary is a line on a chart.

The one boundary that does carry weight

There is a real exception, and it is worth stating carefully because it shows both how a cut point can matter and how fragile it is.

Criteria for intellectual disability have traditionally been anchored to performance approximately two standard deviations below the mean — in standard-score terms, roughly 70 on an individually administered measure. A study of Wechsler-based classification in Developmental Medicine & Child Neurology describes that convention directly, and then immediately complicates it: because of the standard error of measurement, the authors note that people scoring in the low 70s may in some cases be better conceptualised within the impaired range, and they treat a score around 75 as the upper end of the margin of error around the criterion.

That is the clearest available illustration of why a band edge should never be read as a wall. A cut point that carries clinical and legal consequences is still surrounded by measurement error wide enough to move an individual case across it.

The error bar the chart leaves out

ETS describes reliability as the extent to which scores are consistent across occasions of testing, across editions of a test, and across raters scoring the same responses. What is left over is measurement error, summarised as the standard error of measurement.

A score chart never shows it. That is the chart's central omission: it renders a number as a location with hard edges when the number is really an estimate with a range around it. A competent report gives that range, and a reader who only ever sees the point estimate is missing the part of the result that governs how much confidence it deserves.

Where our own app fits

IQ Test & Brain Training Games is our own app, built by Reign Creative, and the honest statement about it belongs here in plain words: it includes an IQ-style test, and its listing calls the results estimates based on app activities, not a clinical or professional assessment.

Its listing does not say whether, or how, a result is placed on a scale, and it documents no comparison group, so you cannot check a result against the standards this article describes. What the listing does describe is a record against your own history: personal bests, accuracy, streaks, response speed and an assessment history. Those numbers describe how you did on the app's own questions. They describe nothing else.

That distinction is the same one drawn throughout this series, and the listing's own disclaimer places the app's results on the practice side of it. Practice and entertainment produce feedback about a session. Standardised psychological assessment produces a position in a norm sample. A product that blurs the two is misrepresenting the second, not upgrading the first — a point developed at length in our guide to whether brain training apps work.

Reading a chart without being misled by it

Ask which instrument the chart belongs to, what the standard deviation is, and what confidence interval the report attaches. Treat the label as shorthand for a position, not as a category anyone belongs to. And remember that the most striking thing about the curve is how much of it is crowded into the middle.

For the arithmetic behind the number itself, see how an IQ score is calculated; for what the battery was sampling in the first place, what IQ tests actually measure. The rest of the series sits in the IQ tests and reasoning hub, with practice-side material in the education and brain training section.

Good to know

Frequently asked questions

What do the bands on an IQ score chart actually mean?
They are descriptive labels attached to ranges of standard scores by whoever published the chart. The underlying scale is continuous, so the boundaries are administrative conveniences rather than discovered categories. A band tells you roughly how far from the comparison group's average a score sits, in standard deviations, and nothing more than that.
How wide is a single IQ band?
It depends on the chart, which is the point. On a scale with a mean of 100 and a standard deviation of 15, bands are commonly ten points, fifteen points, or another convenient width. Ten points is two-thirds of a standard deviation; fifteen points is exactly one. Neither width is required by any property of the measurement.
Is a score near a band boundary reliable?
Less than the label implies. Every score carries a standard error of measurement, so a result a point or two from an edge can land in the neighbouring band on a retest without anything about the person changing. This is why reports give a confidence interval, and why a single number should never be treated as a category membership.
Does the IQ Test & Brain Training Games app place you in a band?
Its Google Play listing does not describe bands, a score chart or a classification. It does include an IQ-style test, and the listing states that IQ test and other results are estimates based on app activities, not medical, psychological, clinical or professional diagnostic assessments. Whatever result the app shows, read it as a record of how you did on its own questions, not as a position in a norm sample.

Sources

  1. 6.5.1. What do we mean by "Normal" data? (NIST/SEMATECH e-Handbook of Statistical Methods) (opens in a new tab)National Institute of Standards and Technology — accessed
  2. Classification of intellectual disability using the Wechsler Intelligence Scale for Children: Full Scale IQ or General Abilities Index? (opens in a new tab)Developmental Medicine & Child Neurology, via PubMed Central — accessed
  3. Test Reliability — Basic Concepts (Research Memorandum RM-18-01) (opens in a new tab)Educational Testing Service (Samuel A. Livingston, 2018) — accessed
  4. Cognitive Profile of Autism and Intellectual Disorder in Wechsler's Scales: Meta-Analysis (opens in a new tab)European Journal of Investigation in Health, Psychology and Education, via PubMed Central — accessed

About the author

— The independent app studio that builds the apps covered on this site and publishes every article here. On Google Play it publishes as Reign Collective Apps.

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