As the length of the menstrual cycle varies from one woman to another, it is important to know the personal length to calculate the next period, which can be ascertained by counting and recording the number of days for a few consecutive menstrual cycles before taking an average to determine the next period.
The average length of the menstrual cycle is 28 days.
Start recording the periods
Record the first day of a menstrual cycle on a calendar. This is the first day that bleeding occurs. Use an online cycle length calculator to record the data for an easier approach.
Count the days between the first period and the next one
At the next onset of bleeding, count the number of days from the first day of the first period to the day before the second period begins. For example, if the first period started on May 1 and the second on May 30, the menstrual cycle would be 29 days long.
Record the numbers for the next couple of periods
Repeat the steps above for at least three more consecutive menstrual cycles and write down the number of days for each month.
Calculate the average length of the menstrual cycle
Add the numbers recorded for each month and divide them by the number of months taken into account. The result determines the average length of the menstrual cycle. This can help determine the start of the next period more accurately.
Bear in mind, however, that factors such as stress, diet and health issues may affect the menstrual cycle.