How do i calculate the number of days that my period lasts? it flows for like 6 days, but on the 7th day it drops just little like spotting. please

Cycle counting: Start counting from the first day of full flow. In other words, if you have very light spotting in the beginning, don't count those days. I would count all days with heavy, medium, or light flow. I would not count days at the end with very light spotting, although you might want to make separate note of how many days this typically continues if you are concerned about it.