As your child begins to use a wider range of sound, he will better perceive the differences in the words spoken to him. This contributes to an rapid growth in his vocabulary. Below is an illustration of a child's rate of vocabulary growth. At all stages, the vocabulary that a child comprehends exceeds the vocabulary that he uses in speech - at a ratio of (+ -) 5 to 1. If a child could speak 10 words, he can more or less understand 50 words.

Age	Words Spoken	Words Understood
15 months	10	50
18 months	50	250
2 years	200	1,000
3 years	1,000	5,000
4 years	1,600	8,000
5 years	2,200	11,000
6 years	2,600	13,000

Let me just clarify that this is an estimation set at the highest bar possible. Do not despair if your child is not at par. As I've said, children differ in their phases of development. There is, however, a certain point that if a milestone is not met, a trip to the developmental pediatrician is a must.