How is the next period date calculated?
A menstrual cycle runs from the first day of one period to the first day of the next. This calculator starts with the first day of your last period, repeats your average cycle length forward and projects the start and end dates of the next six cycles.
For example, if the last period started on August 1 and the average cycle is 28 days, the next start is projected around August 29. With a typical five-day period, the projected end is September 2. This is a calendar forecast, not a guarantee that bleeding will begin on that exact date.
What does cycle length mean?
ACOG defines the menstrual cycle as the number of days from the first day of one period to the first day of the next. Adult cycles commonly fall in the 21–35 day range. Using several recent cycle lengths can be more representative than relying on a single month.
Why are irregular cycles harder to forecast?
If cycle length changes substantially from month to month, one average value becomes less predictive. Stress, illness, pregnancy, breastfeeding, perimenopause, medications and other health factors can alter timing. The calculator cannot diagnose the reason for an early, late, missed or unusual period.
Can this be used for contraception?
No. Projecting period dates does not identify ovulation with certainty and should not be relied on as birth control. The Ovulation Calculator also provides only an approximate calendar estimate.
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Frequently asked questions
How is the next period date predicted?
The calculator adds your average cycle length repeatedly to the first day of your last known period until it reaches the next predicted start date.
What counts as cycle day 1?
Cycle day 1 is the first day of menstrual bleeding.
Why can the prediction be wrong?
Cycle timing naturally varies and can be affected by stress, illness, pregnancy, medications, breastfeeding, perimenopause and other factors.
What if my cycles are irregular?
A single average becomes less representative when cycle lengths vary widely, so treat the forecast as a rough planning estimate.
Does this calculator diagnose a missed or abnormal period?
No. It only projects dates from the pattern you enter and cannot determine why bleeding is early, late, heavy or absent.