Standard score to percentile calculator
Assessment reports mix five kinds of score: standard scores, scaled scores, T-scores, percentile ranks and stanines. Type any one and see all the others, the words the report probably used, and what the number means in a sentence.
Mean 10, SD 3, from 1 to 19. Wechsler subtests.
1 to 19
A scaled score of 13 is 1 standard deviation above average: higher than about 84 in 100 people in the norm group. On the IQ-style scale that is 115.
- Standard score (IQ, index)
- 115
- T-score
- 60
- z-score
- +1.00
- Percentile rank
- 84.1th
- Stanine
- 7
How reports word it: High average on the WISC-V, High average on the WAIS-IV.
The five kinds of score on a report
Every one of these is the same idea: how far a person sits from the average of a comparison group of the same age, in standard deviations. Only the ruler changes.
| Score | Average | 1 SD | Where you see it |
|---|---|---|---|
| Standard score | 100 | 15 | Full Scale IQ, index scores, achievement tests, CAT4 standard age score |
| Scaled score | 10 | 3 | Wechsler subtests (1 to 19) |
| T-score | 50 | 10 | Behaviour and symptom rating scales, some cognitive tests |
| z-score | 0 | 1 | Research papers and some neuropsychology reports |
| Percentile rank | 50th | not linear | Almost every report, next to the score |
| Stanine | 5 | 2 | Group tests in schools, often with a percentile rank |
Equivalents at a glance
Each row is one position on the bell curve. Labels are the publishers' wording for the standard score in that row.
| z | Standard | Scaled | T | Percentile | Stanine | WISC-V label | WAIS-IV label |
|---|---|---|---|---|---|---|---|
| -2.5 | 63 | 3 | 25 | 0.6 | 1 | Extremely low | Extremely low |
| -2 | 70 | 4 | 30 | 2.3 | 1 | Very low | Borderline |
| -1.5 | 78 | 6 | 35 | 6.7 | 2 | Very low | Borderline |
| -1 | 85 | 7 | 40 | 15.9 | 3 | Low average | Low average |
| -0.5 | 93 | 9 | 45 | 30.9 | 4 | Average | Average |
| 0 | 100 | 10 | 50 | 50 | 5 | Average | Average |
| +0.5 | 108 | 12 | 55 | 69.1 | 6 | Average | Average |
| +1 | 115 | 13 | 60 | 84.1 | 7 | High average | High average |
| +1.5 | 123 | 15 | 65 | 93.3 | 8 | Very high | Superior |
| +2 | 130 | 16 | 70 | 97.7 | 9 | Extremely high | Very superior |
| +2.5 | 138 | 18 | 75 | 99.4 | 9 | Extremely high | Very superior |
| +3 | 145 | 19 | 80 | 99.9 | 9 | Extremely high | Very superior |
Why percentiles stretch in the middle and squeeze at the ends
Percentile ranks are not evenly spaced. Most people score near the average, so a small move there passes a lot of people, and a big move in the tails passes very few.
- From 100 to 115 (one standard deviation) a score climbs from the 50th to the 84th percentile: 34 places.
- From 115 to 130 (another standard deviation) it climbs from the 84th to the 98th: 14 places.
- From 130 to 145 it climbs from the 98th to the 99.9th: under 2 places.
That is why reports lead with the standard score and use the percentile as a translation. It is also why a jump from the 30th to the 50th percentile between two assessments can be a smaller change in score than it sounds: about 8 points, inside the margin of error of many tests. The confidence interval calculator shows how wide that margin is.
Reading a report in four steps
- 01
Start with the composite, then read the parts
The Full Scale IQ or main composite is the summary. The index scores underneath show the pattern, and the subtest scaled scores show the detail. Our guides to WISC-V scores and WAIS index scores walk through both. - 02
Read the range, not just the number
Most reports print a 90% or 95% confidence interval. Two scores whose ranges overlap may not really differ. - 03
Translate each score once
Use the converter above to turn every metric into a percentile and a label, so a scaled score of 7 and a T-score of 40 are seen for what they are: the same position. - 04
Ask about gaps, not single scores
A big difference between two indexes says more about how someone thinks than any one number. Ask the psychologist whether the gap is unusual for people at that overall level, not only whether it is statistically significant.
Frequently asked questions
What percentile is a standard score of 85?
What does a scaled score of 7 mean?
What percentile is a T-score of 60?
Is a percentile the same as a percentage correct?
Why does the report give a different percentile from this calculator?
What is a stanine?
Next in IQ calculators
More in IQ calculators
- Mental age calculatorThe classic ratio formula
- Mental age testHow old does your mind act?
- IQ rarity calculatorHow rare a score is, as 1 in N
Related tools
- SAT, ACT and GRE to IQConvert a test score to IQ
- IQ score percentilesEvery score from 70 to 200
Curious where your own reasoning sits?
The LifeScore IQ test gives an adult estimate on the same standard score scale. It is an online estimate, not a clinical assessment, and never a substitute for one.
25 questions · about 12 minutes
Sources
- Pearson Assessments (2020). WISC-V interpretive considerations for sample report (scaled and composite score metrics, descriptive classifications).
- Wechsler, D. (2008). WAIS-IV Technical and Interpretive Manual. Pearson (qualitative descriptions of composite scores).
- GL Assessment. CAT4 quick data guide: standard age score, national percentile rank and stanine.
- Flanagan, D. P., & Alfonso, V. C. (2017). Essentials of WISC-V Assessment. Wiley.