Estimated period dates
Each row is a forecast using one fixed average cycle length. The final bleeding day is inclusive: a five-day period beginning October 29 ends November 2. Actual starts and durations can differ.
For a separate calendar estimate of ovulation and the fertile window, open the Ovulation Calculator .
The formula
Cycle number 1 represents the next period. Date arithmetic uses whole Gregorian calendar days at UTC midnight; leap years and month lengths are handled automatically. Bleeding duration does not change the gap between period starts.
A 28-day cycle beginning October 1
With a last start of October 1, 2026, a 28-day average cycle, and five bleeding days, the next estimated period is October 29 through November 2. The following period begins November 26 and ends November 30.
How to use this calculator
- Enter the first day of your most recent period.
- Use your average start-to-start cycle length and usual bleeding duration.
- Choose how many future cycles to show, then calculate and open the forecast table.
Measure the interval between starts
Write down the first bleeding date for consecutive periods. The elapsed days between those starts give one cycle length; the number of bleeding days is a separate input. If you have several recorded intervals, calculate their average and round to a whole day for this calculator. Entering a rounded average creates a planning estimate, rather than a record of what happened.
Read dates that cross a month boundary
The forecast continues across months and years without restarting the cycle on the first day of a month. It also counts February 29 in leap years. A range can begin in one month and finish in another, so the table prints complete ISO dates for both ends. Duration counts the starting day as day one. The displayed dates refer to the calendar dates entered and do not shift with a timezone change.
Update the forecast when a new period begins
Once the next period actually starts, replace the previous start date with that observed date. A fixed average repeated over many cycles can drift when a cycle is longer or shorter than expected. This tool stores no cycle history and does not calculate uncertainty bands. Use the table for tentative scheduling; it does not establish ovulation, confirm pregnancy, or identify the reason for a late period.
Assumptions & limitations
What this calculation assumes
- The same whole-day average cycle and bleeding duration apply to every forecast.
What to keep in mind
- Accepts cycle lengths of 21–90 days and bleeding durations of 1–14 days; these are input limits, not medical classifications.
- Input and forecast dates must stay within 1900–2100. Calendar estimates cannot be used as contraception.
Common questions
Is cycle length the same as bleeding duration?
No. Cycle length is the interval between two period starts. Bleeding duration counts days within one period.
Why does a five-day period end four days after its start?
The first day is already included. October 29, 30, 31, November 1, and November 2 are five dates.
Will this work with irregular cycles?
It can repeat an average, but a changing cycle makes future dates less dependable. It cannot predict the variation or explain its cause.