Enter the first day of your last period, your average cycle length, and luteal phase length to see your fertile window and next few cycle predictions.
| Cycle | Period start | Fertile window | Ovulation |
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Ovulation typically occurs a fixed number of days — the luteal phase length, commonly 14 days — before your next period, not 14 days after your last one (that's only accurate for an exactly 28-day cycle). This calculator subtracts your luteal phase length from your cycle length to find ovulation day relative to your last period's start.
Sperm can survive up to about 5 days in the reproductive tract, while an egg is viable for roughly 24 hours after ovulation — so the fertile window spans about 5 days before ovulation through 1 day after, the period during which intercourse is most likely to result in conception.
Calendar methods are estimates based on average cycle behavior — actual ovulation timing varies cycle to cycle, especially with irregular cycles or luteal phases outside the typical 12–14 day range. Ovulation predictor kits (LH surge strips) or basal body temperature tracking give a more precise, cycle-specific read if you need higher confidence, particularly when trying to conceive.
Already expecting? See the Pregnancy Due Date Calculator.