Ovulation Calculator: Fertile Window and Ovulation Date

Free ovulation calculator: enter your last period and cycle length to find your fertile window, ovulation day, next period and the best days to conceive. Works for any cycle length, irregular cycles and LH tests. Private, adds to your phone calendar.

🔒 Private by design: your dates are saved only in this browser (never uploaded, no account) and are shared by the three calculators below.

Day 1 of one period to day 1 of the next. 28 is the textbook average; most people are 24–35.
More options: period length, luteal phase, ovulation (LH) test
Ovulation to next period. 14 is typical (11–16 is normal).
Optional: from a temperature chart or an ultrasound. Replaces the estimate.
Optional. Ovulation usually follows 24–36 hours later, so this sharpens the estimate.

A longer or shorter cycle moves ovulation, so the due date shifts with it.

These are estimates from averages, not medical advice or contraception. Bodies vary and cycles change with stress, illness, breastfeeding and age. Talk to a doctor or midwife about your own situation, especially if your cycles are very irregular or you have been trying for 12 months (6 if you are over 35).

Ovulation calculator: find your fertile window

Enter the first day of your last period and your usual cycle length. The calculator estimates the day you ovulate, your 6-day fertile window (the five days before ovulation plus ovulation day), your next period and the earliest day a pregnancy test is reliable. It shows your chance of pregnancy day by day, a three-month calendar, and the next six cycles.

Why this one is easier to use

  • Works for any cycle length. Ovulation is counted back from your next period (the luteal phase, 14 days by default), not fixed at day 14, so a 24-day or 35-day cycle gets the right answer.
  • Irregular cycles: tick “My cycle length varies” and enter your shortest and longest cycle to see the widest window.
  • Add an ovulation (LH) test result and the estimate locks onto your real surge.
  • Add it to your phone calendar with one tap (an .ics file): fertile windows, ovulation days and expected periods for six cycles.
  • Private: nothing is uploaded and no account is needed. Your dates stay in your browser.
  • Linked with the period calculator (log your cycles to use your own average) and the due date calculator.

Questions

How is ovulation calculated?

Ovulation happens about 14 days before your next period starts. For a 28-day cycle that is day 14; for a 32-day cycle it is day 18. The calculator counts back from the expected next period by your luteal phase length.

When am I most fertile?

The two days before ovulation and ovulation day itself. Sperm can survive up to five days, while the egg lives only 12–24 hours, so sex in the days leading up to ovulation matters most. In the study the chart uses, the chance of pregnancy per day peaked at roughly 27–33% on those three days and was about 10% five days before.

How accurate is an ovulation calculator?

It is an estimate from averages. Even in regular cycles, ovulation can shift by a few days, which is why the whole window is shown. Ovulation (LH) test strips, cervical mucus and basal body temperature give more precise timing, and you can enter a positive LH test above.

Can I get pregnant outside the fertile window?

It is unlikely but possible, especially with irregular cycles, because ovulation can come earlier or later than expected. This tool is not a method of contraception.

When should I take a pregnancy test?

From the first day of a missed period. Testing earlier can give a false negative. If the test is positive, use the due date calculator.

How long should we try before seeing a doctor?

Doctors usually suggest after 12 months of trying, or 6 months if the woman is over 35, sooner if cycles are very irregular or absent.

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