Ovulation calculator - fertile window and next period

Ovulation happens about 14 days before the next period, not on day 14 of every cycle. The second half of the cycle (the luteal phase) is fairly constant at 10-16 days, while the first half varies from person to person. That is why the calculator works from the cycle length: ovulation day = cycle length - luteal phase.

Example: a period starting on 1 October 2026 with a 28-day cycle gives ovulation around 15 October, a fertile window of 10-15 October and the next period on 29 October. With a 32-day cycle, ovulation moves to about 19 October.

The fertile window is 6 days: the 5 days before ovulation and the day of ovulation itself. Sperm can survive up to 5 days in the female reproductive tract, while the egg lives only 12-24 hours. The chance of conception is highest in the 2-3 days before ovulation and on the day itself, so couples trying to conceive are usually advised to have intercourse every 1-2 days during this window.

• Ovulation tests (LH strips) detect the hormone surge 24-36 hours before ovulation and are the easiest way to confirm the calendar estimate,
• basal body temperature rises by about 0.2-0.5 °C (0.4-0.9 °F) after ovulation, which confirms it afterwards,
• cervical mucus becomes clear, slippery and stretchy, like raw egg white, around ovulation,
• ultrasound follicle tracking is used in fertility clinics.

Combining methods gives much more reliable results than the calendar alone, especially with irregular cycles.

A cycle of 21-35 days is considered typical, and a difference of a few days from month to month is normal. If your cycles vary by more than 7-9 days, a calendar estimate is unreliable - track LH tests or temperature instead, and talk to a doctor if cycles are often shorter than 21 or longer than 35 days.

The calendar method is not reliable contraception: stress, illness, travel or hormonal changes can shift ovulation. If you are trying to conceive and it has not happened after 12 months (6 months if you are over 35), see a doctor. Once you are pregnant, the due date calculator estimates the birth date and pregnancy week.