Calculate your estimated ovulation date and 6-day fertile window from your last period date, cycle length and luteal phase. Cycle timeline included.
Ovulation does not happen on a fixed calendar date. It is anchored to the end of the cycle, not the beginning: subtract the luteal phase length from the cycle length and you get the day, counting from the first day of the last period, when the egg is released. A standard 28-day cycle with a 14-day luteal phase puts ovulation on day 14. A 32-day cycle with the same luteal phase puts it on day 18. The four-day difference matters when trying to conceive, and this calculator makes that arithmetic instant.
Three sliders control the result: the date of the first day of the last period, the average cycle length (20 to 45 days), and the luteal phase length (10 to 16 days). Adjust any of them and the ovulation date, fertile window and next expected period all update in real time. Nothing is sent anywhere — the calculation runs entirely in the browser.
Sperm can survive in the reproductive tract for up to five days. An egg survives roughly 12 to 24 hours after ovulation. Combining those two facts gives the six-day fertile window: the five days before ovulation plus ovulation day itself. The day after ovulation is included as a margin because the exact moment of ovulation can shift by several hours. Conception probability peaks on the two days before ovulation and ovulation day — declining sharply one day later. Ovulation day is computed as cycle length − luteal phase; for a 28-day cycle with a 14-day luteal phase that is day 14.
Cycle-based estimates carry an inherent uncertainty of ±2 days for regular cycles and wider for irregular ones. Actual ovulation can shift due to stress, illness, travel or hormonal variation. Entering a cycle length of 45 or 20 is allowed, but cycles outside 21 to 35 days are considered irregular and should be discussed with a clinician before making family-planning decisions from these figures alone. Entering a last period more than 90 days ago returns “Last period date is more than 90 days ago — please enter a recent cycle.” This calculator is a planning aid, not a clinical diagnostic tool.
Estimated Ovulation
Sunday, October 4, 2026
2026-10-04 · 15 days ago
Current phase
Luteal phase
Fertile Window
Sep 29 – Oct 5
6 days
Next Period
Oct 18
1 day ago
Ovulation Day
Day 14
of your cycle
Cycle Progress
100%
of 28-day cycle
Cycle timeline
Key dates
Fertile window opens
5 days before ovulation
Peak fertility — ovulation
Highest conception probability
Fertile window closes
1 day after ovulation
Next period expected
Day 29 — start of next cycle
Ovulation day is estimated as cycle length minus luteal phase (e.g. 28 − 14 = day 14 of your cycle). The fertile window is the 5 days before ovulation plus ovulation day itself and one day after. These are statistical estimates — actual ovulation can vary by several days due to stress, illness or cycle irregularity.