How Is IQ Calculated? The Science Behind the Score

You've just finished an IQ test and you're staring at a number. But here's the question: where did that number actually come from?

Is it the percentage of questions you got right? Your raw score out of some maximum? The answer is neither — and understanding how IQ scores are actually calculated will change how you interpret your results.

IQ scores are not raw percentages. They are relative measures — your performance compared to everyone else in your age group, expressed on a standardized scale. Here's exactly how it works.

The Core Concept: Deviation IQ

The modern IQ score is called a Deviation IQ, introduced by David Wechsler in 1939. It answers one simple question: compared to other people your age, how far above or below average are you?

The scale is set so that:

A Brief History: How IQ Calculation Evolved

The formula for calculating IQ has changed significantly since intelligence testing began. Understanding this history explains why the modern method is far superior to earlier approaches.

Step-by-Step: How Your IQ Score Is Calculated

Here's the actual process that converts your answers into an IQ score — from raw responses to the final number:

IQ Score to Percentile: The Complete Table

Your percentile tells you what percentage of people your age scored lower than you. Here's how IQ scores map to percentiles:

The Flynn Effect: Why Tests Must Be Re-normed

One of the most fascinating discoveries in intelligence research is the Flynn Effect — named after philosopher James Flynn, who documented it in the 1980s. Average raw IQ scores have risen approximately 3 points per decade throughout the 20th century across many countries.

Think about what that means. If you took a test normed in 1950 today, you'd likely score significantly higher than 100 — not because you're smarter than an average 1950s person, but because education, nutrition, and exposure to abstract thinking have improved the entire population's performance on these specific tasks.

This is why tests must be regularly re-normed. Using outdated norms would systematically inflate IQ scores — a serious issue in legal and clinical settings, known as the obsolescence effect.

Confidence Intervals: Why Your Score Has a Range

No IQ test is perfectly precise. Every score comes with a standard error of measurement (SEM) — typically ±5 points for well-constructed tests. This means if your score is 115, your "true" IQ likely falls somewhere between 110 and 120.

This is why psychologists report IQ scores with a confidence interval rather than a single number. A score of "115 ± 5 (90% confidence)" is more honest than simply reporting "115."

The Bottom Line

IQ is not a raw score or a percentage — it's a standardized measure of where you stand relative to your age group. The Deviation IQ formula converts your raw performance into a number where 100 is always average, 115 is always one standard deviation above average, and so on.

Understanding this helps you interpret your results more accurately — and appreciate both what your score tells you and what it doesn't. A score of 112 doesn't mean you answered 112 questions correctly. It means you performed better than roughly 79% of people your age on a set of carefully normed cognitive tasks.