Ovulation & Fertile Window Calculator

About the Ovulation & Fertile Window Calculator

This Ovulation Calculator estimates your likely ovulation day and the 6-day fertile window using your last period start date (LMP), average cycle length, and luteal phase. If you don’t know your luteal phase, a default of 14 days is common. The tool also predicts your next period dates so you can plan ahead.

Because real cycles vary, treat these as estimates. For higher accuracy, combine calendar predictions with LH ovulation tests (to detect the LH surge) and basal body temperature (BBT) tracking. If cycles are irregular (e.g., PCOS, postpartum, perimenopause), use a wider window or collect data across several cycles.

FAQ

• When does ovulation usually happen?
  Roughly one luteal-phase length before your next period. With a 28-day cycle and a 14-day luteal phase, ovulation is near day 14.
• What is the fertile window?
  The 5 days before ovulation plus ovulation day (about 6 days total) — the most likely time to conceive.
• What luteal phase should I use?
  If unknown, 14 days is a common default. Many people fall between 12–16 days.
• How accurate is an ovulation calculator?
  It provides estimates based on averages. Accuracy improves if you also track LH surge and/or BBT.
• My cycles are irregular — what then?
  Use longer/shorter averages, expect wider fertile windows, and consider additional tracking or clinical guidance.
• Can I get pregnant outside the fertile window?
  It’s unlikely but possible when cycles shift. Sperm can live up to ~5 days; ovulation timing may vary cycle-to-cycle.
• Does breastfeeding or stopping birth control affect timing?
  Yes — both can temporarily change cycle regularity and ovulation timing.

Disclaimer

The tools on this site are for educational purposes only and do not replace professional medical advice.

References

• American College of Obstetricians and Gynecologists (ACOG) – Ovulation and Fertility FAQ
• Mayo Clinic – Ovulation Calculator and Fertile Days
• National Health Service (NHS) – Fertility and the Menstrual Cycle
• Centers for Disease Control and Prevention (CDC) – Reproductive Health and Fertility
• World Health Organization (WHO) – Infertility and Reproductive Health Fact Sheet (2024)
• Wilcox AJ, Dunson D et al. (2000) – Timing of Sexual Intercourse in Relation to Ovulation and Fertility — New England Journal of Medicine
• Dunson DB, Baird DD et al. (2002) – Day-specific probabilities of conception during the menstrual cycle — Human Reproduction