How it works
Ovulation is estimated as 14 days before the next period: next period = last period start + cycle length, and ovulation = next period − 14 days. The fertile window shown is roughly 5 days before ovulation through 1 day after. This is an estimate only. Cycles vary, ovulation timing varies, and this tool must not be used as contraception or as a guarantee of conception. Confirm timing with a clinician if it matters for you.
Frequently asked questions
Why is it only an estimate?
Real cycles are rarely perfectly regular, and stress, illness and age all shift ovulation. A calendar estimate is a starting point, not a measurement.
Can I use this to avoid pregnancy?
No. Do not use this calculator as contraception — its fertile-window estimate is not reliable enough for that.