However all tests and scales use 100 as the IQ of a person of average intelligence. Since the average IQ is said to be 100, hence anything above 100 is considered as above average and below 100 as below average.IQ below 70 is taken as that of mentally challenged person.IQ above 140 is generally considered as that of a genius.We already know that IQ = Mental Age/Chronological Age X 100. Hence, the normal or average IQ score is 100, which means that the mental and chronological ages coincide. very small percentage of the population reaches an IQ of over 140. From this chart, it can be thus concluded that a normal IQ score is generally considered to be around the 100 mark. IQ scores are thus calculated on a scale of 0 to 200.