Ovulation Calculator — Fertile Window & Next Period

This calculator uses the calendar (rhythm) method recommended by the American College of Obstetricians and Gynecologists (ACOG) and the NHS. It needs only two things you already know: the first day of your last menstrual period (LMP) and your average cycle length — the number of days from one period's first day to the next. An optional luteal-phase setting lets people with a doctor-measured luteal phase fine-tune the result.

The key insight is that ovulation is tied to your next period, not your last one. The luteal phase — the gap between ovulation and the next period — stays close to 14 days regardless of how long your overall cycle is. So the math counts backwards:

1. Next period = last period + cycle length. The period after that adds another cycle length, and so on.
2. Ovulation day= next period − luteal phase = last period + (cycle length − luteal phase). For a 28-day cycle this is about day 14; for a 32-day cycle it is about day 18. This is where the popular “day 14” rule goes wrong for anyone whose cycle is not 28 days.
3. Fertile window = the 5 days before ovulation through about 1 day after — six days in total. The lead time comes from sperm surviving up to five days in fertile cervical mucus (NHS); the short tail reflects the egg living only 12–24 hours after release (Mayo Clinic).
4. Most-fertile days = the two days before ovulation plus ovulation day, when conception is most likely (Mayo Clinic).
5. Implantation window = 6–12 days after ovulation, when a fertilised egg typically embeds in the uterine lining (American Pregnancy Association).
6. Earliest reliable test = 14 days after ovulation, around the day the next period is due. Testing earlier risks a false negative because hCG has not yet risen enough to detect.
7. Estimated due date if you conceive this cycle = ovulation + 266days (38 weeks from conception). For a 28-day cycle this equals Naegele's clinical rule of LMP + 280 days, which the tool cross-checks automatically.

Every date is plain calendar arithmetic — no time zones, no rounding — so the results are exact and reproducible. Because cycles shift with stress, illness, and age, treat the output as a well-grounded estimate, not a guarantee.

• ACOG — Fertility Awareness-Based Methods of Family Planning (luteal phase, fertile window)
• NHS — How can I tell when I'm ovulating? (ovulation ~14 days before next period; sperm survival)
• Mayo Clinic — Getting pregnant: How to get pregnant (egg viability; most-fertile days)
• American Pregnancy Association — Implantation (6–12 days after ovulation)

The rules and formulas on this page were last cross-checked against these sources on 2026-06-09. This tool gives estimates for planning and education; it is not a contraceptive method and does not replace medical advice. For fertility or family-planning concerns, consult a qualified doctor.