FAQ
Why do different IQ tests give different numbers?
Different tests give different numbers because they use different standard deviations around the same mean of 100. A score is just how far you are from average measured in SD units, so a wider SD produces a higher-looking number for the same ability. The underlying percentile is what really compares across tests.
Which IQ test uses which standard deviation?
The Wechsler scales use an SD of 15, the Stanford-Binet uses 16, and the Cattell uses 24. That is why the same top-2% standing reads as 130 on Wechsler, 132 on Stanford-Binet, and 148 on Cattell. Knowing the SD is essential before comparing two scores.
What stays the same when you convert between scales?
The percentile stays the same when you convert an IQ between SD scales. Converting changes only the label on the same point in the distribution, never your actual rank among other people. So always convert through the percentile, not by adding or subtracting points directly.
This is an educational estimate for self-understanding, not a clinical diagnosis.