Ovulation Calculator

Ovulation is the moment an egg is released from the ovary. It happens roughly 14 days before your next period — which is why we count back from your cycle length, not forward from your period. This "count back" approach is reliable because the luteal phase (from ovulation to the next period) stays fairly constant within a person, while the follicular phase (the part before ovulation) is what varies with cycle length.

The calculation

Ovulation = period start + (cycle length − luteal phase)

With a 28-day cycle and a 14-day luteal phase, ovulation falls on day 14. With a 30-day cycle it falls on day 16.

The fertility window — why six days

The timing matters because of how long sperm and the egg can survive:

• Sperm can live up to 5 days inside the body.
• The egg survives only about 24 hours after release.

That asymmetry is the secret to the "fertile window": because sperm can wait, pregnancy is possible from roughly five days before ovulation through the day after. In practice this gives a conception window of around six days per cycle. It also explains why trying to time intercourse purely to the day of ovulation can miss the boat — the days before are often the most important.

Example

Last period started 1 August 2026, 28-day cycle, 14-day luteal phase:

Ovulation = 1 Aug + 14 days = 15 August 2026
Fertile window = 10–16 August 2026
Next period = 29 August 2026

Why tracking beats a calendar

The luteal phase is usually close to 14 days, but not always identical, and cycle length can shift with stress, illness and lifestyle. For that reason a calendar estimate is a good starting point but not a guarantee. More precise methods include tracking basal body temperature (it rises slightly after ovulation), observing cervical mucus changes, or using ovulation test strips that detect the hormone surge preceding release. Combining these with a cycle-tracking app brings the prediction much closer to reality.

Accuracy note

Cycle length varies. Tracking basal body temperature, cervical mucus or using ovulation test strips pinpoints ovulation much more precisely than a calendar estimate. This tool is for information, not medical advice or contraception — calendar methods are not a reliable form of birth control, and if you are trying to conceive for a year or more without success, it is worth speaking to a healthcare professional.