Ovulation Calculator

How to use it

Enter the first day of your last period.
Enter your average cycle length.
Read your estimated ovulation day and fertile window.
Check the table for the next three cycles.

How ovulation is estimated

Ovulation usually happens about 14 days before your next period, not 14 days after your last one. The phase after ovulation (the luteal phase) is fairly constant, while the phase before it varies. So the calculator:

predicts your next period: first day of last period + cycle length,
subtracts 14 days to get the most likely ovulation day,
marks the fertile window as the ovulation day and the 5 days before it.

It also shows a wider possible ovulation range, 10 to 16 days before the next period, because the luteal phase isn't exactly 14 days for everyone.

Worked example

Your last period started on June 1 and your cycle is 30 days.

Next period: June 1 + 30 days = July 1
Estimated ovulation: July 1 − 14 days = June 17
Fertile window: June 12 – June 17
Possible ovulation range: June 15 – June 21

With a 28-day cycle, ovulation would fall on day 14; with a 32-day cycle, around day 18.

Why the fertile window is six days

An egg survives for about a day after ovulation, but sperm can live in the reproductive tract for up to five days. That means sex in the five days before ovulation, as well as on the day itself, can lead to pregnancy. The two or three days just before ovulation are usually the most fertile.

Making the estimate more accurate

A calendar estimate works best with regular cycles. To pinpoint ovulation more precisely, you can combine it with:

Ovulation predictor kits (LH tests), which detect the hormone surge 24–36 hours before ovulation.
Basal body temperature tracking, which shows a small rise after ovulation has happened.
Cervical mucus changes, which become clear and stretchy around ovulation.

If your cycles are irregular, the calculator's dates are less reliable. Track your periods with the period calculator for a few months to find your average.

Important limits

This calculator is meant to help you understand your cycle and plan if you're trying to conceive. It isn't a method of birth control and can't guarantee when you ovulate. If you've been trying to get pregnant for 12 months without success (or 6 months if you're 35 or older), talk to a doctor. The calculator is informational and isn't medical advice. Your dates are calculated in your browser and never uploaded.

Ovulation day by cycle length

Assuming a 14-day luteal phase, here's the typical ovulation day counted from the first day of your period:

Cycle length	Likely ovulation	Fertile window
24 days	day 10	days 5–10
26 days	day 12	days 7–12
28 days	day 14	days 9–14
30 days	day 16	days 11–16
32 days	day 18	days 13–18
35 days	day 21	days 16–21

Your own luteal phase may be a few days shorter or longer, which is why the calculator also shows the wider 10–16 day range.

Frequently asked questions

How many days after my period do I ovulate?

It depends on your cycle length. Ovulation is usually about 14 days before your next period, so around day 14 of a 28-day cycle and around day 21 of a 35-day cycle.

What are my most fertile days?

The day of ovulation and the five days before it, with the two to three days before ovulation usually being the most fertile.

Can I get pregnant right after my period?

It's less likely but possible, especially with short cycles, because sperm can survive up to five days.

How accurate are ovulation calculators?

They give a reasonable estimate for regular cycles but can be off by several days. Ovulation tests or temperature tracking are more precise.

Does this work with irregular periods?

It can give a rough idea, but with irregular cycles the predicted dates are much less reliable.

Sources

Last reviewed: November 14, 2026 · Kitpebble editorial team

Change log

November 14, 2026: Sources expanded; FAQ and content reviewed.

For general information only; not medical advice or a diagnosis. How we write and review our pages

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