Ovulation Calculator: Find Your Fertile Day

Ovulation is the day the egg is released — the center point of the fertile window. It usually happens about 14 days before your next period. Enter the date of your last period and your cycle length below to estimate your ovulation day.

How the ovulation day is calculated

The calculation counts backward, not forward, because the second half of the cycle is the steadier one. StatPearls describes the luteal phase as “relatively consistent within an individual, typically lasting 14 days,” while the first half stretches and shrinks. So ovulation ≈ next period − 14 days. In a 28-day cycle, that falls around day 14; in a 32-day cycle, around day 18.

How exact is “day 14”?

Not very, and it’s worth knowing before you plan around it. In a pooled analysis of 3,324 cycles from 581 U.S. women with no known fertility problems, ovulation landed on cycle day 14 in 8.7% of cycles, and the luteal phase came out at exactly 14 days in 9.7%. The Office on Women’s Health gives the outer bounds: the interval from ovulation to the next period “can be anywhere from one week (7 days) to more than 2 weeks (19 days).”

So read the result as the middle of a range. That’s also why the fertile window is drawn as several days rather than one: it absorbs the error.

Worked example

Last period started May 1, 30-day cycle:

• Next period: May 31
• Estimated ovulation: May 17 (31 − 14)
• Fertile window: May 12 to May 18

Signs that confirm ovulation

• Cervical mucus that’s clear, slippery, and stretchy, like raw egg white — it appears in the days before ovulation, which is what makes it useful.
• Basal body temperature slightly higher after ovulating: 0.4 to 0.8 °F (about 0.2 to 0.4 °C). It confirms after the fact and never predicts.
• A one-sided twinge in the lower abdomen (mid-cycle pain), in some women.
• The LH surge, which drugstore tests detect. It starts about 36 hours before ovulation and peaks about 12 hours before; StatPearls puts ovulation itself 36 to 44 hours after the surge begins. Even so, ASRM reports a maximum accuracy of 21% for pinning the exact day this way.

Since the highest chance of pregnancy is on the days around ovulation, also check the full fertile window with the fertile window calculator. And because this whole calculation counts backward from your next bleed, the next period calculator is the other half of it.

This calculator is informational and gives an estimate. Irregular cycles make the day of ovulation unpredictable; for fertility questions, see a gynecologist.