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Find your ovulation day and fertile window from last period date and cycle length.
Ovulation is the release of a mature egg from the ovary, typically occurring around day 14 of a 28-day menstrual cycle. The fertile window spans approximately 5 days before and on the day of ovulation.
Estimated Ovulation Day
September 5, 2026
Fertile Window
Aug 31 – Sep 5, 2026
Next Expected Period
September 19, 2026
Cycle Length Classification
Normal Cycle (28 days)
| Cycle Length | Ovulation Day |
|---|---|
| 21 days | Day 7 |
| 24 days | Day 10 |
| 26 days | Day 12 |
| 28 days | Day 14 |
| 30 days | Day 16 |
| 32 days | Day 18 |
| 35 days | Day 21 |
Formula
Ovulation day = LMP + (cycle length − 14) | Fertile window: ovulation day − 5 to + 1LMP = Last Menstrual Period — the first day of the most recent period
cycle length − 14 = The luteal phase is consistently ~14 days; ovulation precedes the next period by 14 days
Fertile window = Sperm survive up to 5 days; egg survives 12–24 hours after ovulation
Worked Example
Ovulation day for a 30-day cycle starting March 1
Did you know? An egg is only viable for 12–24 hours after ovulation, but sperm can survive in the fallopian tubes for up to 5 days. This is why the fertile window extends several days before ovulation — having intercourse before ovulation is often more effective than after.
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