Ovulation Calculator

Estimate ovulation date and fertile window from your last period and cycle length.

First day of last period
Select date

Typical range: 21–35 days

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How it works

Enter the first day of your last menstrual period and your average cycle length (typically 28–35 days). The tool estimates ovulation around day 14 before the next period (i.e. cycle length − 14 days after your period starts) and marks a fertile window of roughly 5 days before to 1 day after ovulation. Next period start, due-if-conceived date, and the following 3 cycles are also shown. All computation runs locally — no data is stored or transmitted.

Common use cases

  • Identifying the most likely fertile days when trying to conceive.
  • Tracking cycle regularity and anticipating the next period.
  • Getting a rough estimate of conception timing for early pregnancy planning.

Frequently asked questions

How accurate is this estimate?

This tool uses the standard calendar method (ovulation at cycle length − 14 days). Actual ovulation varies by person and cycle. For precise tracking, combine with basal body temperature or LH test strips.

My cycle is irregular — is this useful?

Less so. The calendar method assumes a consistent cycle. For irregular cycles, track several months of period start dates and use the average cycle length as a rough guide.

Is this data stored anywhere?

No. All calculation happens in your browser and nothing is sent to any server.