How to use the ovulation calculator
- Enter the First day of your last period. Count from the first day of full bleeding, not spotting.
- Set Cycle length (days), from the first day of one period to the first day of the next. Use 28 if you are unsure.
- Leave Luteal phase (days) at 14 unless ovulation tests or temperature charting showed a different number for you.
- Read the ovulation day, fertile window, peak days, next period and due date, then the next three cycles.
How the dates are calculated
The tool uses the calendar method described by the American Society for Reproductive Medicine (ASRM). The luteal phase is presumed to be about 14 days, so ovulation falls on cycle day (cycle length minus luteal phase), counting the first day of your period as day 1. The fertile window is the 6 days ending on ovulation day. A 28-day cycle gives ovulation on day 14 and a window of days 9 to 14. A 30-day cycle gives day 16 and days 11 to 16.
Your next period is the first day plus your cycle length. ASRM notes that the chance of pregnancy peaks in the two days before ovulation, and Wilcox and colleagues found it climbs from about 10% five days before ovulation to about 33% on ovulation day. The due date is 280 days from the first day of this period, shifted by the gap between your ovulation day and day 14, as in ACOG's dating guidance.
Worked example
The starting values are a period that began 10 days ago, a 28-day cycle and a 14-day luteal phase. Today is day 11, ovulation is on day 14 (in 3 days), the fertile window is days 9 to 14 (it began 2 days ago), peak fertility is days 12 to 14, and your next period is in 18 days. For a period starting September 20, 2026, that is ovulation on Sat, Oct 3, 2026, a fertile window of Mon, Sep 28 to Sat, Oct 3, a next period on Sun, Oct 18, and a due date of Sun, Jun 27, 2027 if you conceive that cycle.
Limits and edge cases
A calendar estimate is a guess. ASRM cites a study of 949 volunteers in which the best calendar app predicted the exact ovulation day only 21% of the time. Ovulation test strips, which detect the LH surge that comes shortly before ovulation (Cleveland Clinic), are more precise, and our period tracker helps you learn your real cycle length.
It accepts cycles of 21 to 45 days and luteal phases of 10 to 16 days, and handles irregular cycles, hormonal birth control, breastfeeding and perimenopause poorly. It is not contraception. Once you conceive, use the pregnancy due date calculator.
Frequently asked questions
When do you ovulate in a 28-day cycle?
Around day 14, counting the first day of your period as day 1. The fertile window runs from day 9 to day 14, because sperm can survive for up to 5 days while the egg lives only 12 to 24 hours after ovulation.
How do I calculate ovulation if my cycle is not 28 days?
Subtract 14 from your cycle length to get the ovulation day. A 32-day cycle gives day 18 and a fertile window of days 13 to 18. Change the luteal phase field if your own number is not 14.
What are the best days to get pregnant?
The two days before ovulation and ovulation day. ASRM defines the fertile window as the 6 days ending on ovulation day and says sex every 1 to 2 days in that window gives the best chance.
When should I see a doctor about trying to conceive?
ASRM defines infertility as no pregnancy after 12 months of regular unprotected sex, and says to be evaluated after 6 months if you are 35 or older. See a clinician sooner if your periods are very irregular or absent.
Sources: ASRM, Optimizing natural fertility (2022); Wilcox et al., NEJM 1995; ACOG, Methods for Estimating the Due Date.