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Ovulation Calculator

Estimate your ovulation day and 6-day fertile window.

Short answer

Ovulation typically happens ~14 days before your next period. The fertile window covers the 5 days before ovulation and the day itself, since sperm survive up to 5 days.

Cycle

Fill in the fields above to see your result instantly.

Calculation method, coverage and version

Inputs used
First day of last period (YYYY-MM-DD), Average cycle length (days).
Calculation approach
The calculator applies the rules and assumptions described in “How it works” to the values you enter. Rounding can cause small differences from an official or provider calculation.
Coverage
Health & Wellbeing. UK nations, tax treatment, local rules and provider criteria can differ; check the official sources shown below before acting.
Content version
No page-specific review date is currently published. Treat the result as indicative and verify current rules independently.
Not included unless explicitly requested: personal circumstances not entered, provider discretion, future rule changes, professional fees and case-specific exceptions. Read the full calculator methodology.

How it works

Assumes a fixed 14-day luteal phase (most reliable phase). Ovulation = LMP + cycle length − 14. Fertile window = 5 days before ovulation through ovulation day.

Worked example

LMP 1 Apr, 28-day cycle: ovulation ~ 15 Apr; fertile window 10–15 Apr; next period ~ 29 Apr.

Who should use this

  • •People trying to conceive
  • •Anyone tracking cycles for awareness

Common mistakes

  • ×Using as contraception — too unreliable on its own
  • ×Assuming 28-day cycle when yours varies

Frequently asked questions

Is this safe to use as contraception?▾

No. Even careful fertility awareness methods have ~5-25% failure rates.

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