Ovulation Calculator

ovulation day = cycle length - luteal phase length, counted from the first day of the period. The luteal phase, from ovulation to the next period, is the steady part of the cycle at roughly 14 days for most people. The first half is what varies. So counting backward from the next period is more reliable than the usual "day 14" rule, which is only right if the cycle is exactly 28 days. The window runs from 5 days before ovulation to 1 day after, because sperm can survive up to five days in fertile cervical mucus while an egg lasts about 24 hours. That asymmetry is why the days before ovulation matter more than the day itself.

When am I most fertile?

The two days before ovulation, and then ovulation day itself. Studies of timed intercourse consistently put the peak there rather than on the day after, because sperm need to already be waiting when the egg is released.

My cycle is irregular. Is this still useful?

Less so, and honestly so. If your cycles vary by more than a few days, the ovulation day predicted here can be off by the same amount. Tracking basal body temperature or using ovulation predictor strips measures what actually happened rather than predicting it, and is the better tool for an irregular cycle.

Can I get pregnant outside the fertile window?

It is unlikely but not impossible, because ovulation can come earlier or later than predicted. Treat the window as where the chance concentrates, not as a fence. That cuts both ways, which is why this is a poor method of contraception.

Why count back from the next period instead of forward from the last one?

Because the second half of the cycle is the consistent half. Two people with 28 and 35 day cycles both ovulate about 14 days before their next period, but on day 14 and day 21 respectively. Counting forward gets the second one wrong by a week.

What if my luteal phase is short?

A luteal phase under about 10 days is worth raising with a doctor, since it can make it harder for a pregnancy to establish. You can only know your own by tracking ovulation and the next period across a few cycles, which is why it is an editable field here rather than a fixed 14.