Which day starts a menstrual cycle? Use the first day, not the last
Cycle length is the interval between the starts of two periods. It is different from the number of days bleeding lasts.
Period calculator →Identify the correct starting points
If one period starts on 2026-04-03 and the next starts on 2026-05-01, the interval is 28 days. That is the recorded cycle length. Bleeding for five days during that interval does not make the cycle five days long. Use actual start dates rather than assuming that the end of bleeding is the start of a new cycle.
Estimate the next start
With 2026-05-01 as the most recent start and a cycle length of 28, the calculator returns 2026-05-29. The operation is simply start date plus cycle days. Do not count the first day again or add the cycle length to the last day of bleeding. The date-difference and date-addition tools can verify the two operations.
Respect the prediction limit
Cycles can vary, so the displayed date is a planning estimate rather than a guarantee. The tool does not predict ovulation, determine pregnancy or provide contraception. A noticeable change from your usual pattern, persistent irregularity or other concerning symptoms calls for appropriate medical advice. A calendar calculation alone cannot determine the cause of a change.
Try the calculators
- Period calculator
Estimate your next period start date from your last period and cycle length.
- Date difference
Count calendar days between two dates.
- Add days to date
Find the date before or after a number of days.