Estimate your fertile window based on cycle length per standard reproductive physiology. Combine with OPKs for more precise tracking per ACOG.
According to ACOG, ovulation typically occurs 14 days before your next period — not 14 days after your last. The fertile window is approximately 6 days (5 days before ovulation + ovulation day) per published reproductive research. Enter your cycle length above for a personalized estimate.
Written by Ash K · Last updated: June 2026 · Sources cited below
Ovulation typically occurs 14 days before your next period — not 14 days after the last one. This distinction matters because cycle length varies. In a 28-day cycle, ovulation is around day 14. In a 32-day cycle, it's around day 18. In a 26-day cycle, it's around day 12.
Enter your last period start date and average cycle length. The calculator estimates your ovulation date and 6-day fertile window.
Key Takeaway: Ovulation = cycle length minus 14 days. The fertile window spans 5 days before ovulation through 1 day after — 6 days total. Sperm survive up to 5 days in the reproductive tract; the egg survives 12–24 hours. This overlap creates the fertile window. The calculator identifies these days based on your cycle length.
| Cycle Length | Estimated Ovulation Day | Fertile Window |
|---|---|---|
| 26 days | Day 12 | Days 7–13 |
| 28 days | Day 14 | Days 9–15 |
| 30 days | Day 16 | Days 11–17 |
| 32 days | Day 18 | Days 13–19 |
| 35 days | Day 21 | Days 16–22 |
The fertile window is approximately 6 days: 5 days before ovulation + the day of ovulation itself. Research by Wilcox et al. (1995) in the New England Journal of Medicine established this window through a prospective study of 221 women.
Peak fertility is the day before ovulation and ovulation day itself. Conception probability is highest during these 2 days (approximately 25–30% chance per cycle).
Note: This calculator estimates ovulation based on average cycle length. Actual ovulation timing varies by ±1–2 days even in regular cycles. For more precise tracking, combine this estimate with ovulation predictor kits (OPKs) that detect the LH surge 24–36 hours before ovulation, or basal body temperature tracking.
The Fertile Window — Per Published Reproductive Research
Cervical mucus changes — becomes clear, stretchy, and egg-white consistency around ovulation. This is the most reliable observable sign.
Basal body temperature rise — BBT increases 0.2–0.6°F after ovulation due to progesterone. This confirms ovulation occurred but doesn't predict it in advance.
Ovulation predictor kits (OPKs) — detect the LH (luteinizing hormone) surge that triggers ovulation 24–36 hours later. The most actionable timing tool for conception.
Mittelschmerz — mild pelvic pain on one side during ovulation. Not everyone feels it, and it's not reliable enough to use as a sole indicator.
Tip: For the best conception probability, have intercourse every 1–2 days during the fertile window — not just on the estimated ovulation day. Since ovulation timing can shift by 1–2 days, covering the full window maximizes your chances.
For early pregnancy tracking after conception, see our HCG doubling time calculator and due date calculator.
Bottom Line: Ovulation occurs approximately 14 days before your next period. The fertile window is 6 days (5 before + ovulation day). Enter your cycle length to estimate your dates. For maximum accuracy, combine with OPKs or cervical mucus tracking.
When do I ovulate?
Approximately 14 days before your next expected period. For a 28-day cycle, that's day 14. For a 30-day cycle, day 16. This calculator estimates based on your average cycle length.
How many days after my period do I ovulate?
It depends on cycle length. For a 28-day cycle: approximately day 14 (14 days after period starts). For a 35-day cycle: approximately day 21. Count from the first day of your period.
Can I get pregnant outside my fertile window?
Pregnancy is extremely unlikely outside the 6-day fertile window. Sperm can survive up to 5 days, and the egg survives 12–24 hours. Outside this overlap, conception is not expected.
How accurate is an ovulation calculator?
Calculators estimate based on averages — actual ovulation can shift ±1–2 days even in regular cycles. They're most accurate for women with consistent cycle lengths. OPKs provide same-cycle precision.
This tool is for informational and educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your healthcare provider with questions about your health.