How is ovulation estimated?

It assumes ovulation happens about 14 days before your next period, so ovulation day is your cycle length minus 14. The fertile window is roughly the five days before ovulation plus the day itself.

It is a guide, not a guarantee. The luteal phase varies between people, and cycles shift month to month, so combine it with signs such as cervical mucus or ovulation tests if you are trying to conceive.

Does it predict twins?

No calculator can predict twins. Whether you conceive twins depends on factors like family history, age and, for some, fertility treatment, not the timing of ovulation.

Ovulation calculator: find your fertile window

Ovulation calculator

First day of your last period

Cycle length (days)

An estimate, worked out in your browser. Nothing you enter is sent anywhere.

How the window is worked out

The calculator assumes ovulation happens roughly 14 days before your next period, so it counts back from your expected next period based on your cycle length. The most fertile days are the few days leading up to ovulation and the day itself.

Use it as a guide

Cycles are not clockwork. If you are trying to conceive, pair this estimate with signs your body gives, such as changes in cervical mucus, or with ovulation tests, and speak to your GP if you have questions.

Quick answer

This ovulation calculator estimates your ovulation day and most fertile window from the first day of your last period and your usual cycle length, assuming ovulation about 14 days before your next period. It runs in your browser. It is an estimate: cycles vary, so use it as a guide alongside other fertility signs.

How to use it

Enter your last period. Set the first day of your most recent period.
Enter your cycle length. Pick your usual number of days from the first day of one period to the next.
Read your fertile window. You get an estimated ovulation day and the days around it when you are most fertile.

Frequently asked questions

How is ovulation estimated?: It assumes ovulation happens about 14 days before your next period, so ovulation day is your cycle length minus 14. The fertile window is roughly the five days before ovulation plus the day itself.
How accurate is it?: It is a guide, not a guarantee. The luteal phase varies between people, and cycles shift month to month, so combine it with signs such as cervical mucus or ovulation tests if you are trying to conceive.
Does it predict twins?: No calculator can predict twins. Whether you conceive twins depends on factors like family history, age and, for some, fertility treatment, not the timing of ovulation.