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Ovulation Calculator: Ovulation Day and Fertile Window

This free ovulation calculator estimates the day you are most likely to ovulate, the fertile window of six days around it and the date of your next period, based on the first day of your last period and your usual cycle length.

By Debmedia Technologies LLP · Kolkata, India · Updated

What Is It and When Do You Need It?

Many people want to know their likely fertile days when they plan a pregnancy or learn about their cycle.

Enter the first day of your last period and your usual cycle length.

How to Use This Tool

  1. Enter or paste your input in the box or boxes above.
  2. Choose the options, if there are any.
  3. Press the button to run the tool.
  4. Read the result and use the copy button where one is shown.

How the Calculation Works

The tool takes the first day of the last period as day 1. The next period is that date plus the cycle length. Ovulation is estimated at the cycle length minus the luteal phase. The fertile window runs from five days before to one day after ovulation. The same steps are repeated for the next three cycles.

Worked Example

With a last period starting 1 January 2025 and a 28 day cycle, ovulation is estimated on 15 January, the fertile window is 10 to 16 January and the next period starts on 29 January.

Common Uses

  • Planning a pregnancy.
  • Understanding the cycle.
  • Tracking dates.
  • Talking with a doctor.

Tips and Common Mistakes

  • Use your average cycle, not one cycle.
  • Irregular cycles make the estimate less reliable.
  • Do not use this tool to avoid pregnancy.
  • Check the first result against a case you already know, so you can trust the tool for the rest.
  • Nothing you type leaves your device. The work is done by your browser, so it also works with a slow connection once the page has loaded.

Next, you may need the pregnancy due date calculator, days until calculator, day of the week calculator, date difference calculator.

Everything runs in your browser. Nothing you type is uploaded, stored or sent to any outside service.

Frequently Asked Questions

Ovulation usually happens about 14 days before the next period. So the estimated day is the cycle length minus the luteal phase, counted from the first day of the last period.
The five days before ovulation and the day of ovulation itself, plus the day after, because sperm can live for several days and the egg for about a day.
It is an estimate. Cycles vary from month to month, so the real day can differ by several days.
No. It is not reliable for preventing pregnancy. Speak to a doctor or midwife for advice.

Disclaimer: results are calculated in your browser from the numbers you enter. Check important results before you rely on them for money, health, safety or legal decisions.