ToolPorch / Score Converter
Standard Score ↔ Percentile Converter
Convert between standard scores, scaled scores, T-scores, z-scores, and percentile ranks — in either direction, with the descriptive range included. The third tool in the eval-day set, next to the age calculator and due-date calculator. Converting a single subtest score? The dedicated scaled score to percentile calculator defaults to that metric.
The conversion, shown
Every score metric is the same normal curve wearing a different scale. A standard score of 85 is one standard deviation below the mean: z = (85 − 100) ÷ 15 = −1.0. That same z is a scaled score of 7, a T-score of 40, and the 16th percentile — the area of the normal curve sitting below z = −1. This converter does that arithmetic exactly (normal-curve area computed to about six decimal places), which beats eyeballing the bell-curve poster on the file cabinet.
Common conversions at a glance
| Standard score | Scaled | T | z | Percentile | Common label |
|---|---|---|---|---|---|
| 130 | 16 | 70 | +2.0 | 98 | Very superior / well above |
| 115 | 13 | 60 | +1.0 | 84 | High average |
| 100 | 10 | 50 | 0 | 50 | Average |
| 85 | 7 | 40 | −1.0 | 16 | Low average |
| 78 | ~5.6 | ~35.3 | −1.47 | 7 | Below average |
| 70 | 4 | 30 | −2.0 | 2 | Below average |
| 69 | ~3.8 | ~29.3 | −2.07 | 2 | Well below average |
Frequently asked questions
How is the percentile calculated?
From the normal curve: your score is converted to a z-score using the metric’s mean and standard deviation (standard scores: M=100, SD=15; scaled: M=10, SD=3; T-scores: M=50, SD=10), and the percentile is the area of the curve below that z. The math is the standard normal distribution — public, publisher-independent, and shown on this page.
Why does my test manual show a slightly different percentile?
Manuals sometimes publish empirical percentiles from their norming sample rather than theoretical normal-curve values, and the two can differ by a point or two — especially at the extremes. For a report, the manual’s table is authoritative for that test; this converter gives you the normal-curve equivalent, which is what you need for cross-test comparison and for metrics the manual doesn’t list.
What do the descriptive labels mean?
They’re the conventional interpretation bands (e.g., 90–109 average, 80–89 low average). Publishers vary in their exact labels — one test’s "low average" is another’s "below average" — so match the label system of the test you’re reporting, and treat ours as the common default.
Can I convert a percentile back to a standard score?
Yes — switch the direction to "percentile → scores," enter the percentile rank, and you get the equivalent standard, scaled, T, and z-scores. Useful for translating parent-friendly percentiles back into score metrics for tables.
Is anything I enter stored?
No. All conversion happens in your browser; nothing is transmitted or saved.