Ovulation calculator

An estimated fertile window and next period, worked out from your cycle length. Nothing you enter leaves this page.

Please read this first

Every date below is an estimate from two numbers. Cycles vary with illness, stress, travel, sleep, medication, breastfeeding, thyroid conditions, PCOS and the years approaching menopause — and even a regular cycle shifts by a few days month to month.

This is not contraception. It must not be used to avoid pregnancy: calendar methods are among the least reliable ways to do that, and this one knows nothing about your body beyond an average you typed in. It is also not a fertility treatment, not a pregnancy test and not a diagnosis. For any of those, speak to a doctor, nurse, midwife or pharmacist.

1 · Your cycle

First day of your last period

Usual cycle length (days)

First day of one period to the first day of the next.

Luteal phase (days)

Leave at 14 unless you have been told otherwise.

-Estimated ovulation

-Fertile window

-Next period

The two cycles after that

Projections that assume your cycle stays the same length. The further out they go, the less they are worth.

Projected cycles
Period starts	Estimated ovulation	Fertile window

The dates you enter are used to draw the result and nothing else. They are not sent to a server, not saved in your browser, not put in the address bar and not recorded in any analytics. Closing this tab is enough to be rid of them.

These are estimates

Every date this produces is an estimate from two numbers: when your last period started and how long your cycle usually is. It cannot know anything else about you, and cycles vary — with illness, stress, travel, sleep, medication, breastfeeding, thyroid conditions, PCOS and the years approaching menopause all shifting them. Even a regular cycle varies by a few days from month to month.

Treat the dates as a rough guide, not a schedule.

Why it counts backwards

The luteal phase — from ovulation to the next period — is the more consistent part of a cycle, usually twelve to sixteen days. The follicular phase before ovulation is what varies most. So this estimates the next period first and counts back from it. Counting forward from the last period assumes the variable part is fixed, which is why simple calculators are least accurate for exactly the people whose cycles are not twenty-eight days.

The fertile window

Sperm can survive up to about five days; an egg is viable for around a day. The window therefore opens roughly five days before ovulation and closes shortly after, and the tool marks the estimated ovulation day within it.

This is not contraception

It must not be used to avoid pregnancy. Calendar methods are among the least reliable approaches to that, and this one has no information about your actual cycle beyond an average you typed in. If you are trying to avoid pregnancy, speak to a doctor, nurse or pharmacist about a method that works.

It is also not a fertility treatment, not a pregnancy test and not a diagnosis. If you are trying to conceive without success, if your cycles are very irregular, or if anything has changed suddenly, that is a conversation for a clinician rather than a calculator.

Your information stays here

The dates you enter are used to draw the result and nothing else. They are not sent to a server, not stored in your browser, not put in the address bar and not recorded in analytics. Closing the tab is all it takes to be rid of them.