What Is a Percentile? How to Read Your IQ Result
The most common mix-up when reading a test result is thinking a percentile is the share of questions you got right. It is not. You can answer only half of a very hard exam correctly and still land in the 90th percentile.
What it means to be in the 80th percentile
A percentile is the percentage of people in the comparison group who scored the same as or lower than you. Being in the 80th means you matched or beat 80% of the group, and 20% scored higher. It says nothing about how many questions you answered correctly, only where you stand in line.
You already use them without noticing. The growth charts your pediatrician pulls up, SAT score reports and school performance summaries all speak in percentiles. It is the short way of saying "where you stand compared with everyone else," and the same percentile can mean very different things depending on who that "everyone else" is.
From IQ to percentile: a quick table
With a mean of 100 and a standard deviation of 15, the approximate equivalents are:
- IQ 70: 2nd percentile.
- IQ 85: 16th percentile.
- IQ 90: 25th percentile.
- IQ 100: 50th percentile.
- IQ 110: 75th percentile.
- IQ 115: 84th percentile.
- IQ 120: 91st percentile.
- IQ 130: 98th percentile.
These values come straight from the bell curve, which we explain in another article. That is why the 50th percentile always matches an IQ of 100.
Why percentiles mislead at the extremes
Percentiles do not advance at the same pace as IQ. Between the 40th and 60th percentile there are only about 8 IQ points, because that is where people crowd together. Between the 90th and 99th there are about 16, because the bell stretches out in the tails. Moving from the 50th to the 55th is a tiny difference; moving from the 97th to the 99th is a big jump in performance.
That is why two people at the 48th and 53rd percentiles are practically indistinguishable, even if on paper "one beats the other." There is little point obsessing over a couple of percentile points in the middle of the range.
A percentile of which group
A percentile always depends on its reference group. Being in the 90th among college students is not the same as being there in the general population. Professional tests use samples standardized by age and country; informal ones use whatever group they happen to have, and that changes what the number means. Before you celebrate or worry, ask: compared with whom?
Percentile is not percent correct
A percentage correct (say, 70 of 100 questions) describes your exam. A percentile describes you against other people. On a very demanding test you can have a low percent correct and a high percentile, and on a very easy one the reverse.
Three examples to practice
First: you get an estimated IQ of 112. On the bell, that falls between the 75th percentile (IQ 110) and the 84th (IQ 115), so you would be near the 79th. That means about 79 out of 100 people in your group would score the same or lower.
Second: your child is at the 30th percentile on a reading test. That does not mean they read at "30%" of the expected level. It means 30 out of 100 children in the reference group read the same or worse, and 70 read better. It is a result within the normal range, in the lower-middle part of the distribution.
Third: a test puts you at the 99th percentile. Before celebrating, remember that measurement is less precise in the tails and a one-point difference there rests on very few comparison people. The 99th is genuinely high, but it should not be read with millimeter precision.
A handy trick: to turn a percentile into "one in every N people," subtract it from 100 and divide 100 by the result. At the 98th percentile, 2% do better, or one in 50. That makes a high score easier to picture: 1 in 50 is roughly one student in a big lecture class, or one person on a full city bus.
If you want to see your own percentile, our free test shows your estimate and where you land on the curve. Remember that it is a guide, not a diagnosis.
