Predict your ovulation date and fertile window based on your last menstrual period and average cycle length. See your most fertile days for the next several cycles.
Ovulation is the release of an egg from the ovary. It typically occurs 14 days before the end of your menstrual cycle. For a 28-day cycle, this is around day 14. For a 30-day cycle, ovulation would fall around day 16.
An egg can only be fertilized for 12–24 hours after ovulation. However, sperm can survive in the reproductive tract for up to 5 days, creating a fertile window of about 6 days: the 5 days before ovulation and ovulation day itself.
Tip: For the best chance of conception, aim to have intercourse every 1–2 days during the fertile window, particularly in the 2–3 days leading up to ovulation.
Shorten a cycle from 28 days to 21 and the estimated ovulation date jumps from 14 days after your period starts to just 7 days after. With a five-day period, that puts the start of the fertile window on day three of bleeding, which surprises people who assume fertility only begins once a period is over. This calculator takes the first day of your last period, your average cycle length and your period length, then lists ovulation, a fertile window and the next period for up to 12 cycles ahead.
The page uses the calendar method. Ovulation date = first day of last period + (cycle length − 14) days. The 14 stands for the luteal phase, the stretch between ovulation and the next period, which tends to be steadier from month to month than the first half of the cycle. Later cycles repeat the arithmetic: cycle n starts at your last period date + (n − 1) × cycle length.
The fertile window is ovulation minus 5 days through ovulation plus 1 day. The five days before come from sperm survival, which ACOG and other reproductive-health bodies put at up to about five days in the reproductive tract, while an egg can be fertilized for only around 12 to 24 hours after release. A well-known 1995 study by Wilcox and colleagues in the New England Journal of Medicine found that conception was possible on roughly the six days ending on ovulation day. The dates the page prints cover seven calendar days, because the day after ovulation is included as a margin.
The offset shifts one-for-one with cycle length: a 21-day cycle gives 21 − 14 = 7 days after the period starts, a 28-day cycle gives 14 and a 35-day cycle gives 21. Period length only changes the period block that is displayed. It never feeds into the ovulation estimate.
Worked example. The last period began October 12, 2026 and the cycle length is 30 days. Offset = 30 − 14 = 16 days, so ovulation is estimated for Wednesday, October 28, 2026, and the fertile window runs October 23 through October 29. The next period is expected on October 12 + 30 = November 11. Cycle two repeats from that date: ovulation on Friday, November 27 and a window of November 22 to November 28. Everything is counted in whole days, so no rounding is involved.
Not to pinpoint it. The page assumes the same cycle length every month and a fixed 14-day luteal phase, so if your cycles swing by a week or more, ovulation can land well outside the highlighted days. A doctor or midwife can talk through ovulation test strips, tracking or ultrasound.
The explanatory text under the results describes five days before ovulation plus ovulation day, but the dates shown also include the day after ovulation as a margin. Chances of conception drop sharply once the egg is released, so the last day is the least important one.
No, 14 is an average rather than a rule. The luteal phase is usually somewhere around 12 to 14 days, but the first half of the cycle stretches or shrinks with stress, illness, travel and sleep, so real ovulation can be several days either side of the estimate.
No. This is a prediction for general education, not contraception and not medical advice. Calendar estimates miss ovulation often enough that ACOG treats fertility-awareness methods as less reliable than most other options, and sperm survival means sex before the window can still lead to pregnancy. Ask a doctor or nurse about contraception.
Probably less so. Cycles after stopping hormonal contraception, after pregnancy, while breastfeeding and during perimenopause are often irregular. Let a couple of natural cycles pass to learn your average length, or ask your clinician.