Ovulation Calculator
Enter your LMP and cycle length to estimate your ovulation day and fertile window instantly.
How the ovulation window is estimated
Ovulation typically occurs about 14 days before the start of the next period. For any cycle length, the estimated ovulation day is calculated as LMP plus (cycle length minus 14) days. The fertile window spans the 5 days before ovulation through 1 day after, because sperm can survive several days in the reproductive tract and the egg is viable for roughly 24 hours after release.
The default cycle length is 28 days. If your cycle is shorter or longer, adjust the field to a value between 21 and 35 days. The next expected period is simply LMP plus the cycle length. All calculation happens locally in your browser.
FAQ
What is the LMP date? The first day of bleeding of your most recent menstrual period — the standard clinical reference point.
Why is the fertile window 6 days long? Sperm can survive up to 5 days, and the egg is viable for about 24 hours, so the combined window is roughly 6 days ending the day after ovulation.
My cycle varies — what should I enter? Use an average of your last 3 to 6 cycles. For highly irregular cycles, consult a healthcare provider.
Not medical advice. This tool provides estimates only. Always consult a qualified healthcare provider for clinical decisions.