Ovulation Calculator

Ovulation is the release of a mature egg from an ovary, typically occurring about 14 days before the start of the next menstrual period. The fertile window covers the 5 days before ovulation and the day of ovulation itself, since sperm survive several days while the egg is viable for roughly 24 hours.

Enter the first day of your last period and average cycle length to estimate ovulation date and the fertile window. Accuracy depends on cycle regularity — the calculation assumes the luteal phase (ovulation to next period) is roughly fixed at 14 days while the follicular phase varies.

The biological cycle has two named phases. The follicular phase starts at menstruation and lasts until ovulation; its length varies between cycles and individuals (10 to 22 days is common). The luteal phase starts at ovulation and lasts until the next period; it's relatively fixed at 12 to 16 days because it's determined by the lifespan of the corpus luteum. The calendar method assumes a 14-day luteal phase: ovulation date = next expected period - 14 days. With a 28-day cycle, that's day 14. With a 32-day cycle, day 18. With a 24-day cycle, day 10. The fertile window opens 5 days before ovulation (sperm survival in cervical mucus) and closes the day after.

A worked example. Last period started March 1, average cycle length 30 days. Next period expected April 1. Ovulation: April 1 - 14 = March 18. Fertile window: March 13 through March 19 (5 days before through the day after ovulation). For a person actively trying to conceive, intercourse 2 to 3 days before predicted ovulation is statistically the most likely to result in pregnancy because viable sperm are already present when the egg arrives. For a person trying to avoid pregnancy, this window is roughly when intercourse is least safe — but the predictive accuracy is too low to use as contraception alone.

Limitations and what disrupts the prediction. The 14-day luteal assumption fails for some individuals (luteal phase defects can shorten it to 10 days). Stress, illness, travel across time zones, weight changes, intense exercise, thyroid disorders, and PCOS can delay or skip ovulation entirely. Postpartum, perimenopause, and the months after stopping hormonal contraception produce highly irregular cycles where calendar prediction may be off by a week. Combining calendar predictions with basal body temperature tracking (BBT rises 0.5°F after ovulation) and ovulation predictor kits (LH surge 24 to 36 hours before ovulation) tightens accuracy substantially. For pregnancy avoidance, fertility awareness methods that combine multiple signals have a typical-use failure rate around 12 to 24 percent — not reliable enough for primary contraception. Use a barrier or hormonal method if pregnancy is to be avoided.