Ovulation Calculator: Fertile Window and Cycle Dates

Ovulation Calculator

Estimate an ovulation date, a broader fertile-window range and upcoming period starts from the first day of the last menstrual period and your usual cycle pattern.

Calendar estimate only. Cycle timing varies, and an app or date calculation cannot confirm ovulation. Do not use this result as birth control, to diagnose a condition or to replace pregnancy testing or medical care.


Use day 1 of menstrual bleeding, not the last day.


Count from the first day of one period to the first day of the next.


Days from estimated ovulation to the next expected period.


Use 0 if cycles are very regular; larger variation widens the estimate.


Estimated cycle dates

Estimated ovulation—
Broader fertile window—
Next period estimate—

CyclePeriod startEstimated ovulationFertile windowNext period

How the ovulation estimate works

The calculator projects the next period from the average number of days between period starts. It then counts backward by the selected luteal-phase assumption.

Central ovulation estimate = projected next period start − luteal-phase days
Broader fertile window = earliest plausible ovulation − 5 days through latest plausible ovulation + 1 day

The five days before ovulation are included because sperm can remain capable of fertilization for several days; the extra day after the estimate reflects the short time an egg may remain viable. These are population-based timing assumptions, not confirmation that ovulation occurred.

Checked example

If the last period began January 1, the average cycle is 28 days, the luteal assumption is 14 days and variation is 0, the next period is projected for January 29. Ovulation is estimated for January 15, and the broader fertile window is January 10–16.

Limits and when the estimate may be unreliable

  • Stress, illness, travel, breastfeeding, perimenopause, recently stopping hormonal contraception and many health conditions can change cycle timing.
  • Cycles that vary substantially cannot be reduced to one dependable ovulation date. The variation input widens the displayed range but does not measure biological uncertainty.
  • This tool does not use basal body temperature, cervical mucus, hormone tests or ultrasound and cannot confirm ovulation.
  • Seek clinical advice for concerns about irregular or absent periods, fertility, pain, unusual bleeding or possible pregnancy. Use a pregnancy test according to its instructions when appropriate.

The input range is limited to 21–45 days for cycle length and 10–16 days for the luteal assumption. A warning appears when the average cycle falls outside the common adult range of 21–35 days.

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