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Ovulation Calculator

Estimate ovulation timing and the fertile window from cycle assumptions.

Cycle-based ovulation estimates are less reliable with irregular cycles, recent hormonal contraception, postpartum cycles or PCOS.

Method

Ovulation is estimated as cycle length minus luteal phase length after the first day of the last menstrual period. The fertile window is shown as the five days before ovulation plus ovulation day.

Example

With a 28-day cycle and 14-day luteal phase, estimated ovulation is around day 14 of the cycle. The fertile window is shown as the five days before that plus ovulation day.

Common mistakes

Do not use this as contraception. Irregular cycles, recent hormonal contraception and postpartum cycles can make calendar estimates unreliable.

Limitations

This is not contraception or fertility treatment guidance. Ovulation tests, cervical mucus tracking and clinician advice may be more appropriate when timing matters.

References

Last reviewed: 2026-05-17

Before relying on this result

Use this calculator together with the formula, assumptions, limitations and examples on the page. If the topic involves health, tax, lending, investment, legal, safety or current-rate decisions, treat the number as an estimate and check the relevant primary source or professional guidance.

Calculator metadata last reviewed: 2026-05-14.