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. 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 comes from Wilcox et al. (NEJM 1995), which tracked daily intercourse against hormonally confirmed ovulation across 221 cycles. Peak conception probability sits in the 2 days before ovulation and ovulation day itself. 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.
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. 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. The two days before and including ovulation carry the highest conception probability.
Can I ovulate more than once per cycle?
A single cycle produces one ovulation event, though that event can release more than one egg within a 24-hour span and lead to fraternal twins. The next ovulation happens in the following cycle. Very occasional reports of mid-cycle breakthrough ovulation exist in the literature but are not the typical case and are not what fertility tracking methods are built around.
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. 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. Irregular cycles longer than 35 days or under 21 days are worth discussing with a clinician.
What are the physical signs of ovulation?
Common signs include clear, stretchy cervical mucus that resembles egg white, a mild one-sided pelvic twinge called mittelschmerz, and a small rise in basal body temperature of 0.3 to 0.5 degrees Celsius after ovulation. Some people notice breast tenderness or a brief uptick in libido. Not everyone experiences these signs, and their absence does not mean you are not ovulating.
How do stress, travel, or illness affect ovulation?
Acute stress, significant travel across time zones, poor sleep, or a febrile illness can delay ovulation by shifting the follicular phase. The luteal phase stays roughly the same length, so a late ovulation usually produces a late period rather than a missed one. A single delayed cycle is not a concern on its own; a pattern of irregular cycles or missed periods is worth a check-in with your clinician.