Ovulation Calculator - Predict Your Fertile Window

How the ovulation calculator works

Ovulation is estimated using the standard formula: LMP + Cycle Length − Luteal Phase. For the most common 28-day cycle with a 14-day luteal phase, ovulation falls around day 14. The fertile window extends 5 days before ovulation (when sperm can survive in the reproductive tract) through 1 day after.

Understanding the luteal phase

The luteal phase is the second half of the cycle, from ovulation until the next period. It typically lasts 12–16 days and is more consistent than the follicular phase. If your cycles are irregular, the variation is usually in the follicular (pre-ovulation) phase. The default of 14 days is a good starting estimate.

Improving accuracy

To get more precise results, track basal body temperature (BBT) each morning or use over-the-counter LH (luteinizing hormone) test strips, which detect the hormone surge that triggers ovulation 24–36 hours before it occurs. Combining those methods with this calculator gives the best picture of your cycle.

Cycle phase overview

• Menstrual phase (days 1–5): the uterine lining sheds. FSH begins rising to stimulate follicle development. Estrogen and progesterone are at their lowest.
• Follicular phase (days 1–13): overlaps with menstruation. Rising estrogen thickens the uterine lining. One dominant follicle matures in the ovary.
• Ovulatory phase (day 14 in a 28-day cycle): the LH surge triggers ovulation - the egg is released and viable for 12–24 hours. The fertile window spans 5 days before through 1 day after ovulation.
• Luteal phase (days 15–28): the ruptured follicle becomes the corpus luteum, secreting progesterone. If no pregnancy occurs, the corpus luteum degenerates, progesterone drops, and menstruation begins.

BBT charting guide

Basal body temperature (BBT) is your temperature at complete rest, taken immediately upon waking before any movement. After ovulation, progesterone causes a sustained rise of approximately 0.2°C (0.4°F) that persists until menstruation - this is called the biphasic pattern.

To chart BBT effectively:

• Take your temperature at the same time each morning (within 30 minutes is acceptable).
• Use a dedicated BBT thermometer (reads to two decimal places: 36.65°C, not just 36.7°C).
• Measure before sitting up, eating, or drinking.
• Note disturbances (alcohol, illness, travel, poor sleep) as they can raise BBT abnormally.

BBT charting confirms that ovulation has already occurred - it cannot predict it in advance. Combine with LH testing for both prediction and confirmation.

LH test strips (OPK) guide

Ovulation predictor kits (OPKs) detect the luteinizing hormone (LH) surge in urine. The LH surge typically begins 24–36 hours before ovulation.

• When to start testing: for a 28-day cycle, begin on day 10–11. For shorter or longer cycles, start testing about 4–5 days before your estimated ovulation day.
• Reading the result: a positive OPK is when the test line (T) is as dark as or darker than the control line (C). A faint test line is negative - LH is always present at low levels.
• Timing intercourse: the most fertile window is the day of the positive OPK and the following day.