Ovulation Calculator
Enter the first day of your last period and your usual cycle length. We'll show your predicted ovulation day, your 6-day fertile window, and peak fertility.
How we predict ovulation
Ovulation is set by the luteal phase, the second half of your cycle between ovulation and your next period. The luteal phase is stable at about 14 days across healthy cycles, so the variation in cycle length sits in the follicular phase before ovulation.[5] Ovulation day is therefore calculated as your last period plus (cycle length minus 14). Once you have a conception date, project forward with our Conception Calculator or our Due Date Calculator.
The 6-day fertile window ending on ovulation day comes from a 1995 study that tracked daily intercourse against hormonally confirmed ovulation across 625 cycles from 221 women.[4] Peak conception probability sits in the 2 days before ovulation and ovulation day itself.[4] We flag that narrower peak-fertility window in the results so you can focus timing when it matters most.
Read more: Due Date vs Conception Date.
Frequently asked questions
When do I ovulate in my cycle?
Ovulation typically happens about 14 days before the start of your next period. That means a 28-day cycle ovulates around day 14, and a 32-day cycle ovulates around day 18. The luteal phase from ovulation to your next period stays roughly constant at 14 days, so the variation across cycle lengths sits in the follicular phase before ovulation.[5]
How long is the fertile window?
The fertile window is a six-day span that ends on ovulation day. It covers the five days before ovulation plus ovulation itself.[4] Sperm can survive in the reproductive tract for up to five days, while the egg lives for about 24 hours, which is why fertility tapers off sharply the day after ovulation.[3] The two days before and including ovulation carry the highest conception probability.[4]
Can I ovulate more than once per cycle?
This calculator assumes one ovulation window per cycle. It cannot detect whether an ovulation event released one egg or more than one, so use it as timing context rather than proof of what happened biologically.
Is this calculator accurate if my cycles are irregular?
Calendar-based prediction works best for cycles between 21 and 35 days with a consistent length.[1] If your cycle varies by more than a few days each month, the predicted window is still a useful starting point, but confirming ovulation with basal body temperature, cervical mucus changes, or ovulation predictor kits will give you a more precise signal.[3] Irregular cycles longer than 35 days or under 21 days are worth discussing with a clinician.[1]
What are the physical signs of ovulation?
Trackable signs can include changes in cervical mucus and a small rise in basal body temperature after ovulation. These signs can help you understand a pattern, but they do not make the calendar estimate precise for every cycle.[3]
What if ovulation happens later than the calculator predicts?
Cycle timing can shift from month to month, especially when ovulation happens later than the calendar estimate. In that case, the next period may also arrive later than expected. If late or irregular cycles repeat, use the Period Calculator guidance or speak with a clinician.