Ovulation Calculator
Predict ovulation day and your most fertile window.
Frequently Asked Questions
How is ovulation day estimated?
Ovulation usually happens about 14 days before the next period, so the calculator adds (cycle length - 14) days to the first day of your last period. For a 28-day cycle that is day 14.
What is the fertile window?
Sperm can survive up to five days and the egg about 24 hours, so the fertile window spans roughly five days before ovulation through the day after. The two days before ovulation have the highest conception chances.
How reliable is calendar prediction?
It is a useful estimate for regular cycles but cannot confirm ovulation. Ovulation predictor kits, basal body temperature and cervical mucus tracking give stronger evidence for any specific cycle.
Is the Ovulation Calculator free, and is my data private?
Yes. It is completely free and every calculation runs locally in your browser, so the numbers and dates you enter are never uploaded, stored, or shared with anyone.
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