Ovulation Calculator

Estimate your most fertile days and ovulation date from your last period and cycle length.

Estimate your most fertile days and your likely ovulation date from your last period and your usual cycle length. This ovulation calculator maps out the fertile window when conception is most likely, whether you are trying to conceive or just tracking your cycle.

Read the guide: How to Find Your Fertile Window

How it works

  1. 1

    Enter your last period

    Add the first day of your most recent period.

  2. 2

    Set your cycle length

    Add your typical cycle length in days.

  3. 3

    See your window

    View your estimated ovulation day and fertile window.

Instant & 100% private — nothing is uploaded

Every calculation runs locally in your browser. The measurements, dates and details you enter stay on your own device and are never sent to a server — nothing is stored, logged or shared.

Frequently asked questions

When am I most fertile?
The fertile window spans roughly the five days before ovulation and the day of ovulation itself, when conception is most likely.
Can I rely on this for birth control?
No. Cycles vary and ovulation can shift, so calendar estimates are not a reliable method of contraception or a guarantee of conception.
How long is the fertile window?
It runs about six days: the five days before ovulation plus ovulation day itself. Sperm can survive several days, so intercourse in the days leading up to ovulation gives the best chance of conceiving.

Important

These calculators give general, informational estimates and are not medical advice, diagnosis or treatment. Everyone is different — for decisions about your health, talk to a qualified clinician.

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