Ovulation Calculator
Enter the first day of your last period and your average cycle length to estimate your next ovulation date and fertile window.
How the Ovulation formula works
Based on the standard assumption that ovulation occurs about 14 days before the next period:
Next period = Last period start + Cycle length Ovulation date = Next period − 14 days Fertile window = Ovulation date − 5 days to Ovulation date + 1 day
Step-by-step calculation
- Add your average cycle length to the start date of your last period to estimate your next period's start.
- Subtract 14 days from that date to estimate ovulation, since the luteal phase (after ovulation) is fairly consistent at about 14 days regardless of total cycle length.
- The fertile window spans roughly five days before ovulation through one day after, reflecting sperm survival time and the egg's short viability window.
Worked example
Last period started June 1, with a 28-day cycle: Next period ≈ June 29. Ovulation ≈ June 15. Fertile window ≈ June 10–16.
Frequently asked questions
How accurate is this estimate?
It's based on average cycle patterns and works best for people with regular, predictable cycles. Cycle length naturally varies month to month for many people, so treat this as an estimate rather than a precise prediction — tracking methods like basal body temperature or ovulation test kits are more precise.
What if my cycle length varies?
Use your typical or average cycle length for the best estimate. If your cycles vary significantly, this estimate becomes less reliable, and tracking several cycles' actual lengths can improve accuracy over time.