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Ovulation Calculator

Calculate ovulation date and fertile window from last period.

Typical range: 21–35 days

Usually 12–14 days

Understanding your menstrual cycle

A menstrual cycle has four phases: menstruation (days 1–5), the follicular phase (days 1–13), ovulation (day 14 in a 28-day cycle), and the luteal phase (days 15–28). Ovulation — the release of a mature egg from the ovary — is the only time conception is possible each cycle.

Signs of ovulation to watch for

  • Changes in cervical mucus (becomes clear and stretchy, like raw egg white)
  • A slight rise in basal body temperature (0.2–0.5°C) after ovulation
  • Mild pelvic pain or twinges on one side (mittelschmerz)
  • Increased libido
  • Positive LH surge on an ovulation predictor kit

Using this calculator alongside other methods

For the most accurate fertile window tracking, combine this calculator with BBT charting and OPK testing. The calculator gives you an estimated window to start testing. A positive OPK (LH surge) indicates ovulation is likely 24–48 hours away.

Frequently asked questions

When do I ovulate?
Ovulation typically occurs 14 days before your next expected period — not necessarily 14 days after your last period. For a 28-day cycle, this is day 14. For a 30-day cycle, it is day 16. For a 35-day cycle, it is day 21. This calculator uses your cycle length and luteal phase to find your personal ovulation day.
What is the fertile window?
The fertile window is the 6-day period during which conception is possible: the 5 days before ovulation plus the day of ovulation. Sperm can survive in the female reproductive tract for up to 5 days, and an egg is viable for 12–24 hours after release. The 2 days before ovulation and the day of ovulation are the most fertile.
What is the luteal phase?
The luteal phase is the second half of your menstrual cycle — from ovulation to the start of the next period. It is typically 12–14 days and is more consistent than the follicular phase (first half). If your luteal phase is shorter than 10 days, it may affect fertility and is worth discussing with a doctor.
Can I use this calculator to avoid pregnancy?
No. The calendar (rhythm) method of contraception has a high failure rate (up to 24% per year with typical use) because ovulation timing can vary due to stress, illness, travel, or hormonal fluctuations. For reliable contraception, consult a healthcare provider about evidence-based methods.
How accurate is this ovulation calculator?
This calculator is based on average cycle statistics. It is most accurate for women with very regular cycles. For irregular cycles (variation of more than 7 days), the predictions are less reliable. Ovulation predictor kits (OPKs) measuring LH surge, or basal body temperature (BBT) tracking, provide more personalised timing.

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