All Tools

Ovulation Calculator

Enter your last period and cycle length to see your fertile window, ovulation day and next period.

  • No upload
  • Browser-based
  • Free
  • No signup
Estimated ovulation day
—
enter your last period to start
Fertile window—most likely to conceive
Next period—if no conception
Due date if you conceive—40 weeks
  • Most fertile days—

Dates are estimated in your browser from your cycle — nothing is uploaded or stored. An estimate, not medical advice.

Calculated in your browser.

How to use Ovulation Calculator

  1. Enter the first day of your last menstrual period.
  2. Set your average cycle length (the default is 28 days).
  3. Leave the luteal phase at 14 days unless you know yours differs.
  4. Read your estimated ovulation day, fertile window, next period and the due date if you conceive.

Common use cases

  • Trying to conceive.
  • Natural cycle awareness.
  • Planning ahead.

Tips

  • The two days before ovulation are typically the most fertile — aim for those.
  • Cycle length is measured from the first day of one period to the first day of the next.
  • For irregular cycles, combine this estimate with ovulation test strips or temperature tracking.

Troubleshooting

Frequently asked questions

How does the ovulation calculator work?
Ovulation usually happens about one luteal phase (14 days by default) before your next period. The tool counts forward from the first day of your last period using your cycle length, so ovulation day is roughly cycle length minus luteal length after your period started.
What is the fertile window?
The days you are most likely to conceive — about five days before ovulation through the day after, because sperm can survive up to five days and the egg about a day. The two days before ovulation are usually the most fertile.
How accurate is it if my cycle is irregular?
It is an estimate based on an average cycle. If your cycles vary a lot, the fertile window shifts too. Tracking basal body temperature or using ovulation test strips gives a more precise signal; enter your average cycle length for the best estimate here.
Does it store my cycle information?
No. All dates are calculated entirely in your browser — nothing you enter is uploaded, saved or shared, and no signup is needed.