Ovulation calculator
Estimate your ovulation day and fertile window from the first day of your last period and average cycle length. Educational calendar estimate only — not a substitute for clinical advice.
Defaults to your region. Switch to compare any other scale.
Assumes a typical ~14-day luteal phase (ovulation ≈ cycle length − 14). Luteal length can vary by a few days between people and cycles.
From the first day of one period to the day before the next. Typical range about 21–45 days.
Result
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Estimated ovulation
Enter the first day of your last period
Educational estimate
Educational calendar estimate only — not medical advice. Irregular cycles, PCOS, recent hormonal contraception, or fertility treatment need a clinician. Ovulation timing varies; this tool cannot confirm ovulation.
Formula
Ovulation day ≈ LMP + (cycle length − 14)Fertile window ≈ ovulation − 5 days through ovulation dayNext period ≈ LMP + cycle lengthEstimated due date (if conception on ovulation) ≈ ovulation + 266 daysAssumes a typical ~14-day luteal phase; individual luteal length varies
How it is calculated
- Enter the first day of your last menstrual period (LMP).
- Set your average cycle length (default 28; typical range about 21–45 days).
- Read the estimated ovulation date (primary result).
- Check the fertile window, next period, cycle day of ovulation, and optional due-date estimate.
- Treat results as a guide only — confirm with a clinician if cycles are irregular or you are in treatment.
Questions
How is ovulation estimated?
Calendar methods place ovulation about 14 days before the next period. With your average cycle length C, that is LMP + (C − 14). The fertile window is about the five days before ovulation through ovulation day, because sperm can live up to ~5 days and the egg about a day.
Is this medical advice or a fertility diagnosis?
No. This is an educational calendar estimate only. It cannot confirm ovulation, diagnose infertility, or replace LH strips, ultrasound, or a clinician. Seek medical care for irregular cycles, PCOS, endometriosis, or fertility treatment.
What if my cycle is irregular or I have PCOS?
Calendar estimates work best with fairly regular cycles. Large cycle-to-cycle swings, PCOS, recent hormonal contraception, breastfeeding, or perimenopause make the dates unreliable. Track symptoms or use ovulation tests and discuss planning with a clinician.
Why is the luteal phase fixed at 14 days?
After ovulation, the time until the next period is often the steadiest part of the cycle and averages about two weeks in many people. Teaching materials and ACOG-style fertility-awareness explanations therefore use ~14 days before the next period as a simple rule of thumb. Your own luteal length can be a few days shorter or longer.
How does this relate to the pregnancy due-date calculator?
If conception happens on the estimated ovulation day, adding 266 days gives an estimated due date (the same convention as dating from conception). For full pregnancy dating — including LMP + 280, IVF transfers, gestational age and leave dates — use the pregnancy due-date calculator.