Mental Age and Ratio IQ Calculator

Calculate the classic ratio IQ: mental age divided by chronological age, times 100. Enter both ages in years and months. Example: a child aged 8 years 0 months who performs like a typical 10-year-old has a ratio IQ of 10 / 8 x 100 = 125. The ratio method is historical and mainly meaningful for children, so the result is shown for learning, not for diagnosis.

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Mental age: years

Mental age: months

Chronological age: years

Chronological age: months

How to use the Mental Age and Ratio IQ Calculator

Enter the mental age (the age level at which the child performed) in years and months.
Enter the real chronological age in years and months.
Press Calculate to see the ratio IQ and how it compares with the SD 15 bands.
Read the note on limits: this method is historical and unreliable for adults.

Formula and method

Ratio IQ = (Mental age / Chronological age) x 100. William Stern proposed dividing mental age by chronological age in 1912, and Lewis Terman popularized the IQ number with the 1916 Stanford-Binet. Both ages are converted to months before dividing so partial years count.

Worked example: mental age 10 years 6 months = 126 months, chronological age 9 years 0 months = 108 months, so IQ = 126 / 108 x 100 = 116.7.

What is mental age?

Mental age is the age level at which a person performs on a test. If a 7-year-old answers questions as well as the average 9-year-old, the mental age is 9. It is a way to describe performance relative to age norms.

Why the ratio method is no longer used

The ratio method has several flaws. The spread of scores differs by age, so the same IQ means different things at different ages. Mental age levels off in adolescence, so ratios for adults make no sense. Since the 1930s, tests have used deviation IQ, where a score shows how far a person is from the average of their own age group in standard deviation units. See What Is IQ? for the history.

Ratio IQ examples

Mental age	Chronological age	Ratio IQ
6 y 0 m	6 y 0 m	100
9 y 0 m	6 y 0 m	150
5 y 0 m	6 y 0 m	83.3
12 y 6 m	10 y 0 m	125
7 y 3 m	8 y 0 m	90.6

Ratio IQ versus deviation IQ

A deviation IQ is 100 + 15 x z, where z compares the person with same-age peers. The ratio and deviation values agree roughly in childhood on well-designed tests but diverge at older ages and at the extremes. Use the IQ percentile calculator for deviation scores.

Limits and responsible use

Important: this is a statistics tool, not an IQ test. It converts a score you already have into a percentile and a descriptive band using a normal (bell-curve) model. Clinical IQ scores come only from a standardized test given and interpreted by a qualified psychologist, and online quizzes give estimates at best.

Mental age is a concept from tests given by trained examiners. This page cannot determine a mental age, and a result should never be used to diagnose a child or label a learning difference.

Frequently asked questions

How do you calculate ratio IQ?

Divide mental age by chronological age, both in the same unit, and multiply by 100.

What is the ratio IQ of a 5-year-old with a mental age of 6?

6 / 5 x 100 = 120.

Can adults have a mental age?

The idea is not meaningful for adults, because performance stops rising steadily with age. Early test makers capped age at 16.

What does a ratio IQ of 100 mean?

Mental age equals chronological age, which means average performance for that age.

Who invented the IQ?

William Stern proposed the intelligence quotient in 1912, and Lewis Terman popularized it in the United States.

Do modern IQ tests use mental age?

No. Modern tests report deviation IQ, based on comparison with same-age peers.

Sources

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Effortless Math Team

11 minutes ago