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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.

How irregular cycles affect predictions

A regular cycle is defined as one where cycle length varies by less than 7 days from month to month. Women with polycystic ovary syndrome (PCOS), thyroid disorders, or high stress levels often experience irregular cycles where ovulation timing is unpredictable. For cycle lengths that vary by more than 7 days, calendar-based prediction has error margins of 5–10 days. In these cases, ovulation predictor kits (OPKs) that detect the LH surge are significantly more reliable — start testing from day 10 of your cycle and test daily or twice daily as you approach the predicted window.

Who uses an ovulation calculator

Couples actively trying to conceive use it to identify the 2–3 peak fertile days in each cycle when conception probability is highest. Women tracking their cycle for health awareness use it alongside period tracking apps to build a multi-month picture of their natural cycle variation. Healthcare providers recommend it as a first step before fertility investigations, as it helps couples confirm they are timing intercourse correctly before more invasive assessments. Women recovering from hormonal contraception who want to understand when their natural cycle has re-established use it to monitor cycle length as it normalizes over 3–6 months.

Privacy and sensitivity of this calculator

Reproductive health data is among the most sensitive personal information. This calculator runs entirely in your browser — no dates, cycle lengths, or results are stored, logged, or transmitted to any server. There are no account requirements and no usage tracking tied to this page. Verify this yourself by opening the Network tab in browser DevTools while using the calculator — you will see only static asset requests (CSS, fonts), with zero data sent to any external endpoint.

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 h 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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