What is the fertile window?

The days when pregnancy is most likely: the five days before ovulation and the day of ovulation, since sperm can survive up to five days.

How accurate is this?

It works best for regular cycles. Cycle length varies, so tracking signs or using ovulation tests can help.

Does the luteal phase vary?

Yes. It is often about 14 days, but ranges from person to person.

All tools

Ovulation Calculator

Enter the first day of your last period and your usual cycle length to estimate when you're most likely to ovulate.

Cycle Details

First day of last period

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Average cycle length (days)

Predicted ovulation day and fertile window from the last period start and average cycle length, using the standard luteal-phase method.

Formula

next period = last period + cycle length; ovulation = next period − 14 days; fertile window = ovulation − 5 to +1 days.

Reference Values

•Luteal phase ≈ 14 days
•Sperm survive up to 5 days
•Cycles 21–35 days are regular

FAQ

Common questions about Ovulation Calculator

How we calculate this

We use the standard published formula for this calculation, and we show the working so you can check it.

Spotted a mistake? Tell us and we'll check it within a few days. See how we check our calculators.

Medical disclaimer

This tool gives a general estimate for information only. It is not medical advice and cannot diagnose a condition. Talk to a doctor, midwife or other qualified professional about your own situation.