Ovulation calculator

Estimated ovulation day and fertile window for your next cycle, from last period and cycle length.

Estimate ovulation day and the fertile window from your last period start and cycle length.

Ovulation ≈ next period −14 days; fertile window = ovulation −5…+1 days. A rough estimate, not contraception or medical advice. Runs in this browser.

How to use

  1. Enter the first day of your last period.
  2. Enter your typical cycle length.
  3. Read ovulation day and the fertile window.

What it supports

Ovulation ≈ cycle −14 days from the next expected period; fertile window is the 5 days before plus the day after. Cycle 20–45 days; the estimate covers the next cycle only. An estimate — not contraception or medical advice.

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