Ovulation Calculator

The calculator above gives you a calendar estimate of when you ovulate and when your fertile window opens, for this cycle and the next two. Below is exactly how it works, how far to trust it, and what to do when your cycles do not follow the average.

How the ovulation calculator works

It uses the calendar method that ASRM describes for counseling: assume the second half of the cycle, the luteal phase, is about 14 days, and count back from your next expected period.¹ The calculator runs these steps:

• Estimated ovulation = cycle day (cycle length − luteal phase length), using 14 days unless you change it. If your period started on the 1st and your cycle is 28 days, that is the 14th.
• Fertile window = the five days before estimated ovulation, ovulation day, and the day after.
• Next period = first day of your last period + cycle length.
• Start LH tests = about three days before estimated ovulation.

The biological fertile window is the six days ending on ovulation day, because sperm survive several days and the egg does not.² We add the day after ovulation as a buffer, since a calendar estimate can easily be a day off.

Why 14 days, and when to change it

The luteal phase is steadier than the first half of the cycle. ASRM's 2026 committee opinion puts a typical luteal phase at 12 to 14 days, with a range of 11 to 17.³ In a study of 612,613 cycles, cycle length differences came mostly from the follicular phase, not the luteal phase.⁴ That is why 14 days is a reasonable default. If you have measured your own luteal phase, enter it; see what is the luteal phase? for how.

Ovulation day by cycle length: 28, 30, 32 and 35-day cycles

Day 1 is the first day of your period. With the default 14-day luteal phase, the calculator places ovulation on cycle day (cycle length − 14), the same count ASRM uses: day 14 of a 28-day cycle and day 16 of a 30-day cycle.¹

The most fertile days are the two before ovulation and ovulation day itself.² Real cycles often run a little later than the calendar: in large hormone datasets, the most common ovulation day in 28-day cycles was day 15 or 16.⁵⁶ That is one reason the window shown includes the day after estimated ovulation.

How accurate is it?

It is a starting point, not a measurement. Calendars assume everyone with the same cycle length ovulates on the same day, and they do not:

• In 949 volunteers who tested urine daily, apps predicted the actual ovulation day no better than 21% of the time. Only 15% of the women had the 28-day cycle that 34% believed they had.⁵
• In nearly 76,000 cycles logged with a connected ovulation test, ovulation in 28-day cycles spread over 10 days, most often on day 15.⁶
• In a study of 221 women, only about 30% had their fertile window entirely within cycle days 10 to 17.⁷

ASRM concludes that calendar apps can help you understand your own patterns, but should come with a candid look at their limits.¹

How to use your result

1. Start having sex every one to two days from the first day of the fertile window shown. ASRM says that frequency gives the highest pregnancy rates.¹
2. Start LH tests on the date shown, or a few days earlier if your cycles vary.
3. When you get a positive LH test, have sex that day and the next one or two.
4. After ovulation, switch to the DPO calculator to see when a pregnancy test can give a reliable answer.

For a fuller plan, read the fertile window guide and the best time to have sex to conceive.

When a calculator is not enough

A calendar cannot see what your hormones are doing. That matters most if:

• Your cycles are irregular. Most people's cycles fall between 23 and 35 days, and in the connected-test dataset 52% of users had cycles that varied by five days or more.⁶ Run the calculator with your shortest and longest cycles and test across the whole range, or use a tracker that confirms ovulation rather than only predicting it.
• You have PCOS. PCOS can stop the ovaries releasing an egg every month.⁸ A tracker that confirms ovulation with progesterone or temperature tells you more than a date estimate.
• You have tried for a while. ASRM suggests seeing a specialist after six months of trying at 35 or older.¹ Our Try hub lists the other signs it is time to get checked.