Ovulation Calculator

Estimate your next ovulation day, 6-day fertile window and next period — from just your last period date and cycle length. Nothing leaves your device.

How the estimate works

Ovulation usually happens about 14 days before the next period — not 14 days after the last one, which is why cycle length matters. The fertile window spans roughly six days: the five days before ovulation (sperm survive up to five days) plus ovulation day itself (the egg lives about 24 hours). Calendar estimates work best for regular cycles; if yours varies a lot, ovulation test kits or temperature tracking are more reliable.

Signs of ovulation

Beyond the calendar estimate, your body gives clues: cervical mucus becomes clear and stretchy (like egg white), basal body temperature rises slightly after ovulation, and some women feel a mild one-sided twinge. Ovulation predictor kits detect the hormone surge a day or two before. Combining these signs with the calendar gives a more reliable picture than dates alone.

Trying to conceive vs spacing pregnancies

To improve the chance of conceiving, the most fertile days are the two to three days leading up to ovulation. If you are instead trying to avoid pregnancy, remember that calendar estimates are not a reliable contraceptive — sperm can survive up to five days and ovulation timing varies, so speak to a healthcare provider about proper family-planning methods.

Why the fertile window is six days, not one

The egg itself survives only about 12–24 hours after release, which sounds like a very narrow target. The window is wider because sperm can survive up to five days in fertile cervical mucus — so intercourse several days before ovulation can still result in conception.

That gives a fertile window of roughly six days: the five days before ovulation plus the day itself. Chances are highest in the two days before ovulation rather than on the day, which is a common misunderstanding — waiting for a positive ovulation test can mean acting slightly late, since the LH surge precedes release by 24–36 hours.

The luteal phase is the fixed part

Cycle length varies mainly because the follicular phase (period to ovulation) varies. The luteal phase (ovulation to next period) is much more consistent, typically 12–14 days.

This gives a more reliable way to estimate: rather than assuming day 14, count back 14 days from your expected next period. On a 32-day cycle that means ovulation around day 18, not day 14 — a four-day error if you use the textbook figure. On a 24-day cycle it falls around day 10. Anyone with cycles that aren't 28 days should use this method instead.

A consistently short luteal phase, under about 10 days, is worth mentioning to a doctor, as it can affect implantation.

Signs you can track yourself

Three signals are commonly used together, since each has limitations alone. Cervical mucus becomes clear, stretchy and slippery — like raw egg white — in the days before ovulation; this is the most useful predictive sign because it appears before release. Basal body temperature rises by around 0.3°C after ovulation, taken immediately on waking, but it confirms ovulation retrospectively rather than predicting it. Ovulation predictor kits detect the LH surge and give 24–36 hours' notice.

Some also notice mittelschmerz — a one-sided twinge — or mild breast tenderness. Stress, illness, travel, disrupted sleep, significant weight change and conditions such as PCOS or thyroid disorders can all delay or suppress ovulation in a given cycle, so an irregular month is not necessarily meaningful on its own.

Frequently asked questions