Ovulation Calculator
How it works
Ovulation is estimated 14 days before your next expected period, since that second half of the cycle is far more consistent than the first: ovulation = last period start + (cycle length − 14) days. A 28-day cycle puts ovulation on day 14; a 35-day cycle moves it to day 21, still 14 days before the next period.
The fertile window spans the five days before the estimated ovulation date through the day after it, since sperm can survive for several days and the egg remains viable only briefly after release. Your next period is estimated as your cycle length after the first day of your last one.
This is an informational estimate, not medical advice — it does not replace guidance from a clinician or a certified fertility-tracking method, and it is less reliable the more irregular your cycle is.
Frequently asked questions
Why is ovulation estimated 14 days before the NEXT period, not 14 days after the last one?
Cycle length mostly varies in the first half of the cycle — the days leading up to ovulation. The second half, from ovulation to the next period, holds close to 14 days for most people. Counting forward 14 days from a longer or shorter cycle would miss that; counting back 14 days from the next expected period tracks it, which is why this calculator works from your cycle length rather than assuming day 14 always.
Why does the fertile window start several days before ovulation?
Sperm can survive in the reproductive tract for a few days, so intercourse in the days before ovulation can still lead to conception once the egg is released. The window here spans the five days before the estimated ovulation date plus the day after, when the egg is still potentially viable.
Does an irregular cycle make this less accurate?
Yes. This estimate assumes a fairly consistent cycle length; the more your cycle varies month to month, the less reliable a single date-based prediction is. A longer tracking history, basal body temperature, or ovulation predictor kits give a more personal picture than cycle length alone.
Is this medical advice?
No. This is an informational estimate built from average cycle timing, not a substitute for medical guidance or a certified fertility-tracking method. If you are trying to conceive or avoid pregnancy, talk to a clinician about an approach suited to your own cycle.