Ovulation Calculator (Fertile Window)
Estimate your ovulation day and most fertile days from the first day of your last period and your cycle length — for the next three cycles.
For general information only — not medical advice. Results are estimates; talk to a doctor or qualified health professional about your own health.
How to use
- 1Enter the first day of your last period and your average cycle length.
- 2The fertile window is the 5 days before ovulation plus the day of ovulation.
How it's calculated
Ovulation ≈ next period start − 14 days (the luteal phase). Fertile window = ovulation − 5 days to ovulation + 1 day.
Frequently asked questions
How accurate is an ovulation calculator?
It's an estimate based on averages. Cycle length and ovulation vary; ovulation tests, basal temperature or cervical mucus tracking are more precise.
Why is the fertile window six days?
Sperm can survive up to 5 days, and the egg about 12–24 hours after ovulation.
Can I use this as birth control?
No — calendar methods alone are not a reliable form of contraception.