Free tool
Ovulation & fertile window calculator
Enter the first day of your last period and your average cycle length to estimate your ovulation day, fertile window and next 3 period dates. Uses the standard cycle-length method. Nothing you type is stored or sent anywhere.
Estimated ovulation
This is a general estimate based on standard cycle-length calculations, not medical advice and not a diagnosis. Real cycles vary month to month. Your doctor or an ultrasound follicle scan is the authoritative way to confirm ovulation, especially if you are tracking for conception, avoiding pregnancy, or have an irregular cycle. Consult a doctor for anything beyond general planning.
Common questions
How is ovulation day calculated here?
From the first day of your last period (LMP) and your average cycle length. This tool estimates ovulation as LMP plus (cycle length minus 14 days), since the second half of a cycle (after ovulation, before the next period) is usually close to 14 days for most people. It is a standard estimation method, not a measurement of what actually happened in your body.
What is the fertile window and why does it start before ovulation day?
The fertile window shown here runs from 5 days before the estimated ovulation day through 1 day after it. Sperm can survive in the body for up to about 5 days, while the egg is typically viable for about 24 hours after release, so the days leading up to ovulation carry the highest chance of conception, not just the day itself.
How accurate is this for an irregular cycle?
Less accurate. This method assumes a fairly consistent cycle length and a roughly 14-day gap between ovulation and the next period. If your cycles vary by more than a few days month to month, if you have a diagnosed condition affecting ovulation, or if you are actively trying to conceive or avoid pregnancy, an ultrasound follicle scan or a doctor’s assessment is far more reliable than any date-based estimate.
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