Ovulation & Fertile Window Calculator
Enter the first day of your last period and your average cycle length to see your predicted ovulation day, six-day fertile window, and the next three cycles ahead.
Your prediction
Next three cycles
| Cycle | Period starts | Fertile window | Ovulation |
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Important: Ovulation prediction is most useful when cycles are regular. For health concerns, consult a healthcare professional. This tool is not a contraceptive method.
Understanding the menstrual cycle
A menstrual cycle has two main phases. The follicular phase begins on the first day of bleeding and ends with ovulation. The luteal phase runs from ovulation to the start of the next period and is remarkably consistent — usually between 12 and 14 days long. Cycle length variability comes almost entirely from the follicular phase.
How ovulation is estimated
The calculator subtracts your luteal phase length from your cycle length to find the ovulation day. For a 28-day cycle with a 14-day luteal phase: 28 − 14 = day 14 (counted from the first day of your last period).
The fertile window
Sperm can survive up to five days inside the female reproductive tract, while the egg is viable for only 12 to 24 hours after ovulation. That gives a roughly six-day fertile window — the five days before ovulation and the day of ovulation itself. The two days before ovulation are statistically the most fertile.
Tips for better accuracy
- Track 3+ cycles to get a reliable cycle length average.
- Combine with ovulation predictor kits (LH surge tests) for confirmation.
- Watch for cervical mucus changes — clear, stretchy mucus signals fertility.
- Basal body temperature tracking confirms ovulation has already happened.
Medical disclaimer: This calculator is informational and not a contraceptive method. Talk to a healthcare provider for family-planning advice or fertility concerns.