How is IQ actually calculated? Is it still based on the old “mental age divided by chronological age” formula, or has the calculation method changed? When you take a modern IQ test, how do they convert your raw score into an IQ number?
The old mental age formula (IQ = mental age / chronological age × 100) hasn’t been used since the 1960s. Modern IQ tests use deviation scores based on how you compare to the normative sample for your age group.
Modern IQ calculation uses deviation IQ, not the outdated ratio method. Here’s how it works: (1) You complete the test and get a raw score (number of items correct); (2) Your raw score is compared to the normative sample of people your age; (3) This comparison is converted to a standard score with mean of 100 and standard deviation of 15; (4) Your position in the distribution determines your IQ—one standard deviation above average is 115, two is 130, etc. The deviation method solves problems with the old ratio formula, which didn’t work for adults and created weird artifacts. You can’t manually calculate your own IQ from raw scores because you need the detailed normative tables that come with professionally developed tests. This is why legitimate IQ testing requires standardized administration and scoring.