Ovulation Calculator

How ovulation is estimated

The luteal phase — from ovulation to the next period — is the stable part of the cycle, about 14 days for most people. The follicular phase before it is what varies. So ovulation is estimated as the cycle length minus the luteal phase: day 14 of a 28-day cycle, day 18 of a 32-day cycle, day 10 of a 24-day cycle. The fertile window is the five days before ovulation plus the ovulation day (some add the day after), because sperm live up to five days and the egg about one.

The table shows this cycle's dates for a period starting March 2, 2026 at different cycle lengths.

Which days matter most

Not all fertile days are equal. Studies timing intercourse against confirmed ovulation find the highest chance of conception on the two days before ovulation and the day itself, roughly a one-in-three chance per act for a couple with no fertility problems. Five days before, the chance is a tenth of that; two days after, it is essentially zero. Intercourse every day or two through the window covers the uncertainty in the ovulation estimate.

Confirming ovulation

A calendar estimate assumes a regular cycle and a 14-day luteal phase. Real cycles vary by a few days even when 'regular', and irregular cycles can ovulate anywhere. Home methods narrow it down: an LH ovulation predictor kit turns positive a day or so before ovulation; a rise in basal body temperature confirms it afterwards; cervical mucus changes in the days before. Using this calculator to know when to start testing, then a kit to pin the day, is the practical combination.

Planning ahead: the next six cycles

The calculator lists six cycles so you can plan around travel and work. Each row gives the period start, fertile window, ovulation day and the due date a conception that cycle would produce. If your cycles vary, run it with your shortest and longest cycle length and treat the union of the two windows as fertile.

Luteal phase length

Most luteal phases are 12–14 days; anything from 10 to 16 is considered normal. If you have tracked ovulation and know yours, enter it: someone with a 12-day luteal phase and a 28-day cycle ovulates on day 16, not 14, and the default would put the window two days early. A consistently short luteal phase (under 10 days) is worth mentioning to a clinician.

Avoiding pregnancy

The same arithmetic is the basis of calendar-based contraception, but it is unreliable for that purpose: the fertile window estimated here has an error of several days in either direction, and cycles change. Fertility-awareness methods that work combine daily temperature and mucus observations over many cycles, and even then have a typical-use failure rate far above most contraceptives. Use this page to plan for conception, not against it.