An average women's menstrual cycle is 28 days long, but it could range from 21 to 45 days. Factors that can change a menstrual cycle are age, stress, genetics and other underlying health issues. Calculating a women's next period is not a guarantee that it will start on that exact day, but it is a good estimate on when a women can expect it.
Record on your calendar the first day you start your menstrual cycle or the first day that bleeding occurs. This will allow you to have an accurate calculation.
Count the number of days from the first day of your period to the first day of your next menstrual cycle or the next onset of bleeding. An example would be if your first period started on March 1 and your second period started on March 29, your period would be 29 days long.
Repeat steps 1 and 2 for three to four consecutive menstrual cycles. Write down the average number of days for each cycle. For example, your first period in March was 29 days long, in April it was 27 and in May it was 30.
Add the numbers together, and divide by three. For example, 29+27+30=86, so 86/3=28.5 days. This example determined that the average menstrual cycle for this women is 28 days long based on three menstrual cycles. Calculating the length for several periods and finding an average can help determine a more accurate date for the start of each menstrual cycle.
If you find that your periods vary greatly in length, consult with your health-care provider.
Certain medications and life-changing events can also effect your menstrual cycle.