About the Ovulation Calculator
Ovulation typically happens about 14 days before the next period starts, and the fertile window is the five days before it plus the day itself and the day after, because sperm can survive for several days. Enter the first day of your last period and your usual cycle length to see estimated dates. These are statistical estimates, not a measurement.
How to use the Ovulation Calculator
- Enter the first day of your most recent period.
- Enter your average cycle length, counted from the first day of one period to the first day of the next.
- Adjust the luteal phase if you know yours; 14 days is the typical default.
- Read your estimated ovulation date, fertile window and next period.
The formula behind it
Ovulation day = first day of last period + (cycle length − luteal phase). Fertile window = ovulation − 5 days to ovulation + 1 day.
Frequently asked questions
When do I ovulate in a 28-day cycle?
Around day 14, counting the first day of your period as day 1. The fertile window runs from about day 9 to day 15.
What if my cycle is irregular?
This estimate assumes regular cycles. If your length varies by more than a few days, the window is much less certain; tracking basal body temperature or using ovulation test strips is more reliable.
Does a longer cycle change when I ovulate?
Yes. Ovulation moves later — roughly cycle length minus 14 days — while the luteal phase stays fairly constant. In a 35-day cycle ovulation falls near day 21.
Can I use this to avoid pregnancy?
No. Calendar estimates are not reliable contraception. Use it for planning only, and speak to a clinician about contraception.
How long is the luteal phase?
It is usually 12–14 days and is the most consistent part of the cycle for most people.
This tool gives a general estimate based on averages and is not medical advice or a method of contraception. See a healthcare professional if you have irregular cycles or have been trying to conceive for a year (six months if over 35).