Part of the topic: 👶 Family & children
Ovulation calculator: fertile days & next period
Enter the first day of your last period and your usual cycle length. The calculator estimates ovulation, the fertile window and your next periods.
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🔒 Runs entirely in your browser – nothing you enter is uploaded or stored.
How to use it
- Enter the first day of your last period.
- Enter your average cycle length (from the first day of one period to the first day of the next).
- Read the estimated ovulation date, the fertile window and the next periods.
Last checked: October 2026 · Based on: Calendar method; ~14-day luteal phase. Found a mistake? Use the form below – we fix confirmed errors and update the date.
Frequently asked questions
How is ovulation calculated?
The phase after ovulation (luteal phase) usually lasts about 14 days. So ovulation is estimated at cycle length minus 14 days after the first day of your period – day 14 in a 28-day cycle.
What are the fertile days?
Sperm can survive up to about 5 days, the egg about 12–24 hours. The fertile window is therefore the 5 days before ovulation plus the day of ovulation.
Can I use this for contraception?
No. Cycles vary from month to month, so a calendar estimate is not reliable contraception.