Ovulation Calculator
Enter your last period's start date and average cycle length to estimate ovulation and your fertile window.
About this calculator
This ovulation calculator estimates ovulation as occurring 14 days before the next expected period — the standard luteal-phase assumption — then shows a five-day fertile window leading up to and including that day.
Cycle length varies naturally from month to month for most people, so this is a planning estimate. Tracking basal body temperature, cervical mucus, or using ovulation predictor kits will sharpen the estimate for an individual cycle.
How it works
Enter the first day of your last period
This anchors the calculation.
Enter your average cycle length
Defaults to 28 days — adjust if your cycles typically run longer or shorter.
Review the estimate
See the estimated ovulation day, fertile window, and next expected period.
Frequently asked questions
How is ovulation day estimated?
It's calculated as cycle length minus 14 days from the start of the last period, based on the typical 14-day luteal phase.
What is the fertile window?
Sperm can survive several days, so the fertile window is typically estimated as the five days before ovulation through the day of ovulation itself.
Is this a reliable form of contraception or conception planning?
No — it's a general estimate. For family planning, combine it with clinical guidance and, if needed, ovulation testing.