Ovulation Calculator
Estimate an ovulation day and fertile window from the first day of your last period and your usual cycle length.
Your details
Enter only the information needed for this estimate.
Complete the fields and select “Estimate my fertile window” to see your estimate.
Your estimated fertile window
This range marks days when conception may be more likely based on calendar timing.
Ovulation can shift from cycle to cycle, including in otherwise regular cycles.
Make it useful beyond today
Support your cycle beyond one estimate
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How the estimate works
The calculator counts backward from the next expected period using the luteal phase length you provide. It then places a broader fertile window around the estimated ovulation day.
Calendar estimates describe a likely range. They do not confirm that ovulation happened.
Ways to add context
Changes in cervical mucus, urine luteinizing hormone tests, and basal body temperature can add information that a calendar alone cannot provide.
- Track several cycles before treating an average as your usual pattern.
- Update the cycle length when your recent pattern changes.
- Consider clinician guidance if cycles are often shorter than 21 days, longer than 35 days, or absent.
Limits of this estimate
- Recent illness, travel, stress, breastfeeding, perimenopause, and stopping hormonal contraception can change ovulation timing.
- This estimate is not suitable for confirming ovulation or ruling pregnancy in or out.
- Do not rely on this calculator alone to prevent pregnancy.
Common questions
How should I use this estimate?
Use it as a planning starting point, then compare it with your own experience and any guidance from a qualified health professional.
Is this medical advice?
This calculator provides general educational estimates only. It is not medical advice, a diagnosis, or a substitute for care from a qualified health professional.
What happens to what I enter?
The calculator uses the values in your current browser session to produce the result. It does not save them in browser storage.