How the ovulation calculator works
It uses the calendar method: ovulation ≈ next period start − 14 days, fertile window = the five days before ovulation plus ovulation day. The six-day window comes from the Wilcox study, which followed 221 women trying to conceive and found no pregnancies from sex outside it.
The luteal phase (ovulation to period) is steadier than the follicular phase, but it still ranges from about 10 to 16 days between people. If your cycles vary by more than a week, or you often skip periods, use ovulation tests or talk to your doctor, who can check for common causes such as thyroid problems or polycystic ovary syndrome.
Questions people ask
When do I ovulate?
On average about 14 days before your next period starts. In a 28-day cycle that’s around day 14; in a 32-day cycle, around day 18. Timing varies from cycle to cycle, even in people with regular periods.
What is the fertile window?
The roughly six days ending on ovulation day. Sperm can survive up to five days, and the egg lives about 12–24 hours, so sex in the days before ovulation gives the best chance of pregnancy.
How accurate is an ovulation calculator?
Calendar predictions are a rough guide. Ovulation test kits (which detect the LH surge), cervical mucus changes and basal body temperature charts can pin it down better. Irregular cycles make calendar predictions much less reliable.
Can I use this to avoid pregnancy?
No. Calendar predictions alone aren’t reliable contraception. Speak to a clinician or sexual health service about effective methods.
When should I see a doctor about trying to conceive?
The usual advice is after 12 months of regular unprotected sex if you’re under 35, after 6 months if you’re 35 or older, or sooner if your periods are irregular or absent or you have known health concerns.
Sources
- Wilcox AJ et al. (1995). Timing of sexual intercourse in relation to ovulation. N Engl J Med 333:1517–1521.
- MedlinePlus (U.S. National Library of Medicine). Pregnancy: identifying fertile days.
- ACOG / ASRM. Guidance on when to seek fertility evaluation (12 months under 35; 6 months at 35+).
Every formula on VitalMetrics is listed with its original reference on Sources & methods.