Ovulation Calculator
Estimate your ovulation date and fertile window from your last period and cycle length.
What is the Ovulation Calculator?
This tool estimates ovulation using the standard calendar method: working backward 14 days from your next expected period, since the time between ovulation and the next period (the luteal phase) is typically the most consistent part of the cycle.
The formula
Next Period = Last Period Start + Cycle Length, then Ovulation = Next Period − 14 days. The fertile window is estimated as the 5 days before ovulation through the day of ovulation itself, since sperm can survive in the body for several days while an egg survives roughly 24 hours after release.
How to use it
- Enter the first day of your last period.
- Enter your average cycle length (count from the first day of one period to the first day of the next).
- Review the estimated ovulation date, fertile window, and next period date.
An important accuracy note
The 14-day luteal phase is an average, not a fixed biological constant — real luteal phase length varies meaningfully between individuals, and even cycle-to-cycle within the same person, so this estimate can be off by several days either way. This tool is for general awareness, not a method of contraception or a fertility diagnosis; anyone trying to conceive or avoid pregnancy with precision should talk to a doctor about more reliable tracking methods (like basal body temperature or ovulation predictor kits).
Want the "why" behind this tool? Read Why Ovulation Is Calculated Backward From Your Next Period, Not Forward.