How it works
Ovulation is estimated about 14 days before your next period, which is the first day of your last period plus your cycle length. The fertile window covers the five days before ovulation and the day after.
Ovulation ≈ last period + cycle − 14 days
The fertile window
- Most fertile: the five days before ovulation and the day after.
- Sperm can survive several days, so the days before ovulation matter.
- An ovulation test or basal body temperature is more precise.
Standards and sources
- 14-day luteal phase model
- Naegele-style cycle estimation
- For reference only; not medical advice
Last reviewed October 8, 2026
Frequently asked questions
How is ovulation calculated?
The next period is estimated from the first day of your last period plus your cycle length, and ovulation is placed about 14 days before it. The most fertile days are the five days before ovulation and the day after.
When am I most fertile?
Pregnancy is most likely in the fertile window: the five days leading up to ovulation plus the day after. Sperm can survive several days, so the days before ovulation matter.
How accurate is it?
It assumes a regular cycle and a 14-day luteal phase. Cycles vary, so treat it as a guide; ovulation tests and basal body temperature are more precise.