To calculate menstrual cycle length, you count the number of days from day one of your period (CD 1) up to, and including, the day before the next period starts. The count does not include the first day of the next menstrual period, because that's Cycle Day 1 of the next cycle.t is not the same as period length, which is the number of days your period lasts. The day your period starts is considered Cycle Day (CD) 1 -- the start of a new menstrual cycle. Have your average cycle length automatically calculated at MyMonthlyCycles.com whenever you update your free period calendar!