How Is IQ Calculated? Formula, Norms & Modern

Modern IQ calculation uses deviation IQ: compare your performance to age norms (mean 100, SD 15). Old mental-age ÷ chronological-age × 100 is obsolete for adults.

Who This Article Is For

Students and curious adults searching the exact phrase 'how is IQ calculated'.

Key Takeaways

Classic formula (mental age / chronological age × 100) is historical—not how adult WAIS scores work.

Modern deviation IQ maps raw performance onto a normal curve with mean 100 and SD usually 15.

Full-scale IQ combines index scores (verbal, perceptual, working memory, processing speed).

Norms are age-banded; a raw score means different IQs at different ages.

Direct Answer: How Is IQ Calculated Today?

Today, IQ is almost always a deviation score: psychologists convert your raw subtest points into scaled scores using age-based norms, then combine those into index scores and a full-scale IQ centered at 100. About 68% of people fall between 85 and 115 on SD-15 scales.

The century-old ratio IQ (mental age ÷ chronological age × 100) fails for adults—mental age plateaus while chronological age rises. See APA Dictionary: intelligence quotient and APA Intelligence.

Step-by-Step: From Answers to an IQ Number

Though software does the math, the conceptual pipeline is stable across major batteries:

1. Administer subtests (e.g., vocabulary, matrix reasoning, digit span)
2. Sum raw points per subtest
3. Convert raw → scaled scores using age norms
4. Form index scores (VCI, PRI, WMI, PSI on WAIS-IV)
5. Derive Full-Scale IQ (FSIQ) from the composite
6. Report confidence intervals—scores have measurement error

Why Online Tests Differ From Clinical IQ

Many online tools estimate a single reasoning score from matrix items only. They may use proprietary norms. Clinical FSIQ samples more domains—see subtest profiles and WAIS vs online.

Sample item types: IQ test sample questions 2026.

Standard Deviation Choices (SD 15 vs 16)

Most modern Wechsler scales use SD 15. Some older Stanford-Binet editions used SD 16. Comparing scores across scales without noting SD misleads—read SD15 vs SD16.

Worked Intuition (Not a Real Score)

If you score at the 50th percentile among age peers, IQ ≈ 100. Around the 84th percentile ≈ 115; around the 98th percentile ≈ 130. Use the percentile chart rather than DIY z-score math unless trained.

Sources & further reading

External links open authoritative references used to fact-check this article. GuideIQ summarizes research; always read primary sources for clinical or legal decisions.

APA — Intelligence
Official psychology overview of intelligence and testing limits.
APA Dictionary — intelligence quotient
Definition covering ratio vs deviation IQ.
Pearson Assessments
Publisher of Wechsler clinical batteries and scoring guidance.

Common Interpretation Mistakes

Applying the childhood ratio formula to adults.

Averaging unrelated online quiz scores into 'true IQ'.

Ignoring confidence intervals around FSIQ.

90-Day Action Plan

Learn index scores before obsessing over FSIQ.

Prefer one well-normed test over five random quizzes.

Ask examiners for the norm year and SD used.

Frequently Asked Questions

What is the IQ formula?

Historically MA/CA×100; modern tests use norm tables for deviation IQ, not a single public equation.

Who calculates IQ scores?

Licensed examiners or validated scoring software after standardized administration.

Can I calculate IQ from SAT?

Only rough research conversions exist—not official IQ.

Why did my two tests disagree?

Different norms, domains, fatigue, and practice effects—see reliability guides.

Is 100 exactly average?

By design of the norm sample—yes, near the mean.

How Is IQ Calculated? Formula, Norms & Modern Scoring (2026)

IQ Test — 3 Minutes