Ovulation Calculator – Find Your Fertile Window

How to use the ovulation calculator

1. Enter the first day of your last period.
2. Enter your average cycle length — the number of days from the first day of one period to the first day of the next.
3. If you know it, enter your luteal phase length; otherwise leave it at 14 days.

You’ll see your most fertile days, your estimated ovulation date, when your next period is due, and a table covering your next six cycles.

How it works

Worked example

Your period started on 1 January 2026, your cycle is 28 days and your luteal phase is 14 days:

• Ovulation ≈ 1 January + 28 − 14 = 15 January
• Fertile window ≈ 10 to 16 January
• Next period ≈ 29 January

With a 35-day cycle, ovulation moves to about 22 January and the fertile window to 17–23 January.

Tips for trying to conceive

• Have sex every one to two days during the fertile window rather than trying to hit one exact day.
• Track your cycles for a few months to learn your real average length.
• Use ovulation tests in the days before your predicted ovulation.
• Start a daily folic acid supplement — many health services recommend it before and during early pregnancy.
• See a doctor if you have been trying for 12 months (or 6 months if you are 35 or over), or if your periods are very irregular.

Irregular cycles

If your cycle length changes by more than about a week from month to month, a calendar estimate becomes less reliable. Ovulation tests, temperature tracking or advice from a doctor are better guides. Conditions such as polycystic ovary syndrome (PCOS) and thyroid problems can affect ovulation.

Your data stays private

The dates you enter never leave your device. Nothing is stored or uploaded, so you can use the calculator as often as you like. Once you are pregnant, the due date calculator estimates when your baby will arrive. This tool gives general information, not medical advice — see our disclaimer.