My penis is small

i am doing hand practise since age 10 .I am 21 years old my penis is only 1.5 inch genarally and 4 inch when erect what is cause please suggest

If you are overweight, the layers of fat can deduct from the length for example, three inches of fat will make a 5 inch penis, 2 inches. Otherwise it's genetics.

4 inch u said..well depends on what rob is saying are u fat?

I've heard that 5 inches is average. So it's really not that bad.