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Estimate ovulation date and fertile window from LMP and cycle length — see next 3 cycles predicted.
Estimate ovulation date and fertile window from LMP and cycle length — see next 3 cycles predicted.
Enter values above and click Calculate — results will appear here with the formula explained.
Ovulation is estimated as 14 days before the next expected period, computed as LMP plus cycle length minus 14. In a 28-day cycle ovulation lands near day 14; in a 35-day cycle near day 21 — the 14-day luteal phase is relatively constant while the follicular phase varies. The fertile window shown (±3 days around ovulation plus ovulation day) reflects egg survival ~12–24 hours and sperm survival up to 5 days, commonly summarized as 6 fertile days: 5 days before plus ovulation day.
Accuracy depends on cycle regularity. The single-cycle estimate here uses one LMP and one average length; research shows luteal length varies 12–16 days across individuals and even within the same person, so day-14 is a population average, not a personal guarantee. Tracking the last 3 cycles and averaging smooths outliers: a 26, 28, 30 average of 28 is more reliable than any single month, while a 21–40 range signals anovulatory cycles worth clinical review.
This tool separates estimation from contraception on purpose. Fertility-awareness methods for contraception (FAM, symptothermal) require daily cervical-mucus scoring, basal-body-temperature charting and LH testing with strict abstinence or barrier rules — a calendar estimate alone has a typical-use failure rate near 24% per year (CDC contraceptive-effectiveness data). For planning pregnancy it is a timing aid; for preventing it, it is not sufficient.
Signs that refine the calendar prediction: clear stretchy cervical mucus and a positive LH surge 24–36 hours before ovulation, plus a sustained 0.3–0.5 °C basal-temperature rise after ovulation. Combining calendar + LH + mucus narrows the window far more than any single method — which is why fertility clinics combine all three for timed intercourse guidance.
Stress, illness, travel, weight change, thyroid issues, PCOS and postpartum status can shift ovulation by a week or suppress it entirely, rendering any calendar off. Medications and emergency contraception also reset timing. If cycles are consistently outside 21–35 days, highly irregular, or conception does not occur after 12 months of timed intercourse (6 months if over 35), seek medical evaluation rather than continuing calendar math — the underlying cause matters more than the date.
Privacy and YMYL note: reproductive data is sensitive. This calculator runs entirely in your browser — inputs are never sent to a server. Results are estimates for information only, not diagnosis or treatment, and do not replace professional medical advice. Confirm with a clinician and choose methods with known efficacy for contraceptive decisions.
Estimate ovulation date and fertile window from LMP and cycle length — see next 3 cycles predicted. Formula: Ovulation = LMP + cycleLength - 14 days.
Ovulation is estimated as 14 days before the next expected period, computed as LMP plus cycle length minus 14. In a 28-day cycle ovulation lands near day 14; in a 35-day cycle near day 21 — the 14-day luteal phase is relatively constant while the follicular phase varies. The fertile window shown (±3 days around ovulation plus ovulation day) reflects egg survival ~12–24 hours and sperm survival up to 5 days, commonly summarized as 6 fertile days: 5 days before plus ovulation day.
Accuracy depends on cycle regularity. The single-cycle estimate here uses one LMP and one average length; research shows luteal length varies 12–16 days across individuals and even within the same person, so day-14 is a population average, not a personal guarantee. Tracking the last 3 cycles and averaging smooths outliers: a 26, 28, 30 average of 28 is more reliable than any single month, while a 21–40 range signals anovulatory cycles worth clinical review.
This tool separates estimation from contraception on purpose. Fertility-awareness methods for contraception (FAM, symptothermal) require daily cervical-mucus scoring, basal-body-temperature charting and LH testing with strict abstinence or barrier rules — a calendar estimate alone has a typical-use failure rate near 24% per year (CDC contraceptive-effectiveness data). For planning pregnancy it is a timing aid; for preventing it, it is not sufficient.
Signs that refine the calendar prediction: clear stretchy cervical mucus and a positive LH surge 24–36 hours before ovulation, plus a sustained 0.3–0.5 °C basal-temperature rise after ovulation. Combining calendar + LH + mucus narrows the window far more than any single method — which is why fertility clinics combine all three for timed intercourse guidance.
Stress, illness, travel, weight change, thyroid issues, PCOS and postpartum status can shift ovulation by a week or suppress it entirely, rendering any calendar off. Medications and emergency contraception also reset timing. If cycles are consistently outside 21–35 days, highly irregular, or conception does not occur after 12 months of timed intercourse (6 months if over 35), seek medical evaluation rather than continuing calendar math — the underlying cause matters more than the date.
Privacy and YMYL note: reproductive data is sensitive. This calculator runs entirely in your browser — inputs are never sent to a server. Results are estimates for information only, not diagnosis or treatment, and do not replace professional medical advice. Confirm with a clinician and choose methods with known efficacy for contraceptive decisions.
With LMP 2026-01-01 and a 28-day cycle, ovulation is estimated around 2026-01-15 (Jan 1 + 14 days), with a fertile window roughly Jan 12–16 and next period Jan 29. With a 35-day cycle and the same LMP, ovulation moves to Jan 22 (Jan 1 + 21), fertile window Jan 19–23 and next period Feb 5 — showing why cycle length, not calendar date, drives the prediction. Track Jan, Feb and Mar LMPs averaging 30 days and the next estimate automatically recenters.
Last reviewed: September 2026 · Report an error