Estimate ovulation day and a five-day fertile start from cycle length and luteal-phase length.
| Ovulation day | – |
| Fertile from day | – |
Use this calculator for ovulation day = cycle length − luteal phase, and fertile start = ovulation day − 5 (not before day 1). A 28-day cycle with a 14-day luteal phase ovulates around day 14. Irregular cycles, late ovulation, and PCOS break the calendar model. This is not a contraceptive method and not an LH-strip reading.
Cycle (days): Typical length from first day of bleeding to the next first day.
Luteal (days): Luteal-phase length, usually about 14. Shorten it if you know yours is short.
Ovulation day: Estimated cycle day of ovulation.
Fertile from day: Five days before ovulation, floored at day 1.
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