Estimate ovulation date and fertile window from your cycle length.
Ovulation = LMP + (cycle − 14) days. Fertile window covers the 5 days before ovulation plus the day after, since sperm survive ~5 days but the egg only ~24 h. Estimates are approximate; ovulation tests give cycle-specific accuracy.
Conception is far more time-bound than most people realise. Sperm survive in the female reproductive tract for about 5 days; the released egg lives only ~24 hours. The intersection of those two windows — the fertile window — is roughly 6 days per cycle, peaking the day before ovulation. Outside this window, the per-cycle conception probability drops near zero. People trying to conceive want to time intercourse to overlap the window; people avoiding pregnancy with fertility-awareness methods want to know when not to overlap. Either way, the underlying calculation is the same: from the first day of the last menstrual period (LMP) and the cycle length, estimate the ovulation date as cycle − 14 days, then bracket a 6-day window around it. This calculator returns the ovulation date, the fertile window, the next-period prediction, an estimated due date if conception happens this cycle, and the next two cycles for planning ahead.
Let LMP be day 1 of the cycle and L the cycle length in days. The corpus luteum phase (post-ovulation) is biologically fixed at about 14 days, so ovulation occurs at day L − 14. The fertile window covers 5 days before ovulation through 1 day after (5+1 = 6 days total). Next cycle starts at LMP + L; estimated due date if pregnancy this cycle = LMP + 280 days (Naegele's rule). Cycles 2 and 3 follow the same arithmetic against the projected next-LMP.
Enter the first day of your last period (LMP) and your average cycle length (21–40 days; the slider defaults to the textbook 28). The calc returns: estimated ovulation date, the fertile-window date range, the next-period prediction, the conception-cycle due date, plus the same set of dates for the next two cycles. Use the example presets to compare a textbook 28-day cycle against a short 24-day or long 32-day cycle.
LMP = 1 April 2026, cycle = 28 days. - Ovulation: 1 April + (28 − 14) = 15 April 2026. - Fertile window: 10 April – 16 April 2026 (5 days before through 1 day after). - Next period expected: 29 April 2026. - Due date if pregnant this cycle: 1 April + 280 = 5 January 2027. - Cycle 2 fertile window: 8 May – 14 May 2026; cycle 3: 5 June – 11 June 2026.
A 32-day cycle starting 10 April → ovulation 28 April; fertile window 23–29 April. A 24-day cycle starting 15 April → ovulation 25 April; fertile window 20–26 April. Short cycles ovulate sooner; long cycles later — the L − 14 rule is the constant.
The 14-day rule is a population mean. Individual luteal phases range from about 10 to 17 days. If yours is different (some women track this with basal body temperature charts), the LMP-based formula is off by that delta. The calc shows the population estimate; if you have a known luteal length, manually shift the result by the difference.
Irregular cycles. The calc assumes a stable cycle length. If yours varies by more than 3–4 days between cycles (PCOS, perimenopause, post-pill, breastfeeding), point estimates are unreliable. Use ovulation predictor kits (LH-surge tests) for cycle-specific accuracy, or symptothermal tracking for couples relying on fertility-awareness contraception.
Stress, illness, travel. Acute stressors can delay ovulation by days within a single cycle; the calc cannot account for that. If you suspect a delayed ovulation in the current cycle, recompute when the next period actually starts and you'll know the true cycle length retroactively.
Avoiding pregnancy with fertility-awareness alone. Calendar-based methods alone (this calc, the rhythm method) have a typical-use failure rate around 12–24 % per year — comparable to condoms with imperfect use. Symptothermal methods (cervical mucus + BBT + calendar) reach 1–5 % typical-use. Modern fertility apps based on LH-tests can reach single-digit percentages, comparable to oral contraceptives. Do not rely on this calc for contraception.
LMP date precision. "First day of last period" means the first day of flow, not spotting. If your last period started gradually, anchor to the first heavy-flow day.
Cycle length too short or too long. Cycles below 21 or above 40 days warrant a clinical consultation regardless of fertility goals. The calc clamps inputs into that window.