Ovulation Calculator
NewEstimate your ovulation day, fertile window, and next expected period from your last menstrual period date and average cycle length.
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estimated ovulation day
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fertile window
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next expected period
Estimates only — every body's cycle varies. Not a substitute for medical advice or dedicated fertility tracking methods.
Frequently Asked Questions
How is ovulation day estimated?
The calculator assumes ovulation typically occurs 14 days before your next period begins (the luteal phase — the time between ovulation and your next period — is fairly consistent at around 14 days for most people, even when the earlier follicular phase varies). So ovulation day = LMP date + (cycle length − 14 days).
Why is the fertile window 6 days, not just the ovulation day?
Sperm can survive in the reproductive tract for up to 5 days, while an egg is typically viable for about 24 hours after ovulation — so the fertile window spans roughly 5 days before ovulation through 1 day after, covering the full range of days intercourse could realistically lead to conception.
What if my cycle length varies month to month?
This calculator assumes a consistent cycle length, which many people don't have exactly — irregular cycles make ovulation prediction meaningfully less accurate. If your cycles vary significantly, consider tracking basal body temperature, cervical mucus changes, or using an ovulation predictor kit (OPK) alongside this estimate.
Is this a reliable method for trying to conceive or avoid pregnancy?
No — this is a general estimate based on average cycle math, not a substitute for clinically validated fertility awareness methods or medical guidance. If you're actively trying to conceive or avoid pregnancy, consult a healthcare provider about more precise tracking methods.