We need to say that the first usable IQ test was invented by Binet and Simon, but the whole idea and definition was compiled by Stern.

So the first IQ test was invented – we define it nowadays as IQ test based on the Binet-Simon Scale. It is still used nowadays including the free IQ test provided here.

Soon after the invention of this scale US implemented it and was called the Stanford-Binet Intelligence Scale and soon became the standard intelligence test used in the U.S. – 1916. This score was calculated by dividing the test taker's mental age by their chronological age, and then multiplying this number by 100. For example, a child with a mental age of 12 and a chronological age of 10 would have an IQ of 120 (12 /10 x 100).

Stanford-Binet Intelligence Scale

If the mental age is the same as the chronological age, then the IQ will equal 100. An IQ of 100 thus indicates a child of average intellectual development. For a gifted child, the mental age is above the chronological age, and the IQ is higher than 140; for an intellectually challenged child, the mental age is below the chronological age, and the IQ is below 70.