Chronological age
Mental age = (10.00 × 115) ÷ 100 = 11.50 years
Mental age11 years 6 months
In tithes11.5
About 1.5 years above chronological age. The ratio formula is historical only — modern IQ uses deviation scoring instead.
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Mental Age Calculator
Enter chronological age and IQ to see the mental age the score implies. Formula: mental age = (chronological age × IQ) ÷ 100.
The formula
Mental age = (chronological age × IQ) ÷ 100
Enter chronological age as years and months. The calculator also shows the equivalent in tithes (tenths of a year): one tithe equals 1.2 months (10 tithes = 12 months).
Mental age was introduced by Alfred Binet and Théodore Simon in their 1908 revision of the Binet-Simon Scale. The IQ score is, historically, the ratio of mental age to chronological age multiplied by 100.
Modern IQ testing no longer uses the ratio formula — the Stanford-Binet switched to a deviation-IQ in 1960 — but the concept of mental age remains useful as an intuitive translation of a score, especially for children.
A note on history
Binet himself did not invent the IQ. He invented the mental-age concept and the scale that produces it. The IQ — the ratio of mental age to chronological age, times 100 — was William Stern’s 1912 idea, popularised by Lewis Terman at Stanford in 1916.
The ratio IQ has been replaced in modern testing by deviation IQ, which expresses a score as standard deviations from the mean of an age-matched population. Read more about Binet here.
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