Enter your last period and average cycle length to estimate your ovulation date and fertile window.

How to Use the Ovulation Calculator

Enter the first day of your last period and your average cycle length (28 days if you're not sure), and the calculator projects your likely ovulation date along with the six-day window around it.

Next period estimate = LMP + cycle length
Ovulation date        = Next period estimate − 14 days
Fertile window         = (Ovulation date − 5 days) to Ovulation date

Most cycle-tracking advice defaults to "ovulation happens on day 14," which is only true for a 28-day cycle and quietly wrong for anyone else. This calculator counts backward from the next period instead of forward from the last one, and that distinction is the entire reason it works better across different cycle lengths.

The Part of the Cycle That Barely Moves

Here's the counter-intuitive bit: the days before ovulation (the follicular phase) stretch or shrink depending on the person and the cycle, sometimes by a week or more. The days after ovulation (the luteal phase) don't — it holds steady at roughly 14 days almost regardless of total cycle length, because it's governed by a hormonal countdown that starts fresh at ovulation and ends the cycle. A person with a 35-day cycle isn't ovulating "late" in some vague sense; they're almost certainly ovulating around day 21, not day 14, with the same ~14-day luteal phase running out the clock afterward. That's why "14 days before the next period" beats "14 days after the last one" as a rule of thumb for anyone whose cycles don't land exactly on 28 days.

The fertile window itself isn't a guess about ovulation timing — it's built from separate, well-established biology: sperm can survive inside the body for up to about five days, and the egg is viable for roughly 24 hours after release. Stack those together and you get a six-day window ending on the estimated ovulation day, not a single-day target.

This is a statistical estimate based on average timing, not a measurement of this specific cycle. It cannot detect what actually happened inside any individual cycle — only basal body temperature charting, ovulation predictor kits, or cervical mucus observation do that. Do not use this calculator as a method of contraception or to avoid pregnancy. Cycle-based estimates carry meaningfully higher real-world failure rates than dedicated contraceptive methods, precisely because ovulation timing shifts from cycle to cycle in ways an average can't capture in advance. If you're managing fertility for any medical or family-planning purpose, talk to a doctor about a method suited to that goal.