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Ovulation Calculator

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Estimate your fertile window based on your last period's start date and your average cycle length. This uses the common assumption that ovulation happens about 14 days before your next period starts, regardless of overall cycle length.

Ovulation Calculator

Live
Estimated ovulation day
Enter a date above
Fertile window
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Next period est.
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An estimate based on averages — actual ovulation timing varies and can shift cycle to cycle.

How this is estimated

Ovulation day ≈ Next period date − 14 days Next period date = Last period start + cycle length Fertile window ≈ 5 days before ovulation through ovulation day
Example

Last period started June 1, 2026, with a 28-day cycle: next period ≈ June 29, so ovulation ≈ June 15, with a fertile window of roughly June 10–15.

Step-by-step guide

  1. Enter the first day of your last period.
  2. Enter your average cycle length — count from the first day of one period to the first day of the next.
  3. Read your estimated ovulation day and fertile window.

Common mistakes

Assuming ovulation always happens on day 14 of the cycle — it's actually about 14 days before the next period, which is a different reference point if your cycle isn't exactly 28 days.
Using an average from an irregular cycle history — if your cycle length varies a lot month to month, this estimate becomes less reliable, and tracking methods like basal body temperature or ovulation test strips can add more precision.

Frequently asked questions

How accurate is this estimate?

It's a reasonable starting estimate based on average cycle patterns, but actual ovulation timing can shift from cycle to cycle. For more precision, many people combine this with ovulation predictor kits or basal body temperature tracking.

What if my cycle length varies a lot?

This calculator works best with a fairly consistent cycle. If yours varies significantly month to month, consider using your shortest recent cycle for a more conservative fertile-window estimate, or speak with a healthcare provider.

Can this be used as birth control?

This is an estimate for planning purposes and isn't reliable enough to be used as a sole method of contraception. Speak with a healthcare provider about proven contraceptive methods if that's your goal.

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