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Ovulation Calculator

Enter your last period and cycle length to estimate your fertile window.

Estimated ovulation

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Fertile window

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Next period

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What this calculator does

This online calculator estimates your ovulation day, fertile window, and next expected period, based on the first day of your last period and your average cycle length. It's a planning estimate for tracking fertility or anticipating your next period.

Worked example

With a last period starting August 1, 2026 and a 28-day cycle, estimated ovulation falls around August 15, with a fertile window from August 10–15, and the next period expected around August 29.

Common mistakes

  • Assuming ovulation always happens on day 14. Day 14 only applies to a standard 28-day cycle — ovulation timing shifts with your actual average cycle length.
  • Treating estimates as exact. Stress, illness, and normal cycle variation can shift ovulation by several days even in someone with generally regular cycles.
  • Using one cycle's data only. Averaging your cycle length over several months gives a more reliable estimate than using a single recent cycle.

Frequently asked questions

How is ovulation day estimated?+

We assume ovulation typically occurs 14 days before the start of your next period, regardless of total cycle length. For a 28-day cycle, that's day 14; for a 32-day cycle, it's day 18.

What is the fertile window?+

The fertile window is usually estimated as the 5 days before ovulation plus the day of ovulation itself, since sperm can survive several days while the egg is viable for about 24 hours after release.

How accurate is this calculator?+

This is a statistical estimate based on average cycle patterns, not a measurement of your actual hormone levels. Ovulation predictor kits, basal body temperature tracking, or cervical mucus monitoring give more personalized accuracy than calendar-based estimates alone.

What if my cycles are irregular?+

Calendar-based estimates are less reliable with irregular cycles. Track several cycles to find your average length, or consult a healthcare provider for tools better suited to irregular cycles.

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