Free Ovulation Calculator
Enter your last period date and average cycle length to instantly calculate your ovulation day, fertile window, and next period. No sign-up needed.
Calculate Your Fertile Window
Predicted Ovulation Day
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How ovulation is calculated
Ovulation — the release of an egg from the ovary — occurs approximately 14 days before your next expected period, regardless of your cycle length. This is the luteal phase length, which is relatively consistent at 12–16 days for most people.
The formula this calculator uses is: Ovulation Day = First Day of Last Period + (Cycle Length − 14). For a 28-day cycle, that's day 14. For a 35-day cycle, it's day 21. For a 21-day cycle, it's day 7.
What is the fertile window?
The fertile window is the approximately 6-day window during which unprotected sex can result in pregnancy. Sperm can survive in the fallopian tubes for 3–5 days, and the released egg is viable for just 12–24 hours. Together, this creates a window from 5 days before ovulation through the ovulation day itself.
The day before ovulation and the ovulation day itself carry the highest probability of conception (approximately 30% per cycle in healthy, fertile couples under 35 according to ACOG guidelines).
Limitations of ovulation calculators
This calculator assumes that your cycle length is consistent and that ovulation always occurs 14 days before your next period. In reality, both assumptions can vary:
- Ovulation can occur earlier or later than expected due to stress, illness, travel, or hormonal fluctuations
- The luteal phase (post-ovulation) can range from 10–17 days across individuals
- People with polycystic ovary syndrome (PCOS) or very irregular cycles may ovulate unpredictably
For greater precision, combine this calculator with physical fertility signs: basal body temperature (BBT) charting, cervical mucus monitoring, or ovulation predictor kits (OPKs).
Ovulation Calculator FAQ
How do I calculate my ovulation date?
Ovulation typically occurs 14 days before your next expected period. The formula is: Last Period Date + (Cycle Length − 14) = Ovulation Day. For a 28-day cycle starting on 1 June, ovulation is predicted around 14 June.
What is the fertile window?
The fertile window is the 6-day period when pregnancy is possible: the 5 days before ovulation (when sperm can survive in the reproductive tract) plus the ovulation day itself (when the egg is viable for 12–24 hours).
Can I use the ovulation calculator to avoid pregnancy?
No. This calculator is not a reliable contraceptive method. Ovulation timing varies significantly between cycles. Speak with a healthcare provider about effective contraception options.
What if my cycle length is irregular?
Use your average cycle length for a rough estimate, but note that predictions will be less accurate. If your cycle varies by more than 7–9 days each month, consider tracking ovulation with an OPK kit or BBT thermometer for greater precision.