Find your expected ovulation date and fertile window based on your last period and cycle length.
A free, in-browser ovulation calculator. Enter the first day of your last menstrual period (LMP) and your typical cycle length to see your estimated ovulation day, your 6-day fertile window, and when to expect your next period. All calculations run locally in your browser — your cycle information is never uploaded.
Enter your LMP
Pick the first day of your last menstrual period. That is day 1 of your current cycle.
Set your cycle length
Default is 28 days. If your cycles are reliably longer or shorter, set the actual average between 20 and 40 days.
Read your fertile window
The ovulation date, the 6-day fertile window, and your next expected period update immediately.
The luteal phase — the time between ovulation and the next period — is roughly constant at about 14 days across most people and cycles. So ovulation is estimated as: LMP + (cycle length − 14). For a 28-day cycle that is day 14; for a 32-day cycle it is day 18.
Wilcox et al. (NEJM, 1995) showed that almost all pregnancies occur when intercourse happens in the 6-day window ending on the day of ovulation — the 5 days before plus the ovulation day itself. Sperm can survive up to 5 days in the reproductive tract, while the egg only lives about 12–24 hours after ovulation.
No. Calendar-based methods are explicitly not reliable contraception on their own. Ovulation day varies between and within cycles — especially with stress, illness, travel, or irregular cycles. Combine calendar tracking with other signs (basal body temperature, cervical mucus, LH ovulation tests) or use medical contraception.
No. Every calculation runs in your browser. Your LMP and cycle length stay on your device.
This tool uses the luteal-phase-constant model first described by Hermann Knaus (1929), independently of Kyusaku Ogino. The luteal phase from ovulation to menses averages ~14 days with a standard deviation of only 1–2 days across healthy cycles, so ovulation is estimated as LMP + (cycle length − 14). The fertile-window definition (5 days before ovulation through the ovulation day, 6 days total) comes from Wilcox, Weinberg, and Baird's landmark NEJM 1995 study, which used urinary hormone metabolites to time ovulation precisely in 221 healthy women and found essentially no conceptions outside that window. Limitations: the model assumes a regular cycle and that ovulation timing is stable, which is often not the case — especially under stress, while breastfeeding, near menarche/menopause, or with hormonal conditions like PCOS. Calendar-based predictions are a starting point, not a diagnostic.