Fertile window from last period.
Ovulation Day = LMP + (CL − 14) Fertile Window Start = Ovulation Day − 5 Fertile Window End = Ovulation Day EDD (Naegele's Rule) = LMP + 280 days
Ovulation typically occurs approximately 14 days before the start of your next expected period. Because cycle length varies among women, the ovulation day is calculated by subtracting 14 from your average cycle length, then adding that number of days to the first day of your last menstrual period (LMP).
The fertile window spans the 5 days before ovulation plus the day of ovulation itself (6 days total), because sperm can survive up to 5 days in the reproductive tract.
If conception occurs on the estimated ovulation day, the estimated due date (EDD) is calculated using Naegele's Rule: add 280 days (40 weeks) to the LMP date.
Ovulation Day = June 1 + (30 − 14) = June 1 + 16 = **June 17, 2025** Fertile Window Start = June 17 − 5 = **June 12, 2025** Fertile Window End = **June 17, 2025** EDD = June 1 + 280 days = **March 8, 2026**
Result: Ovulation Day: June 17, 2025 | Fertile Window: June 12–17, 2025 | Estimated Due Date (if conceived): March 8, 2026
In this example, a woman with a 30-day cycle whose last period began on June 1, 2025 would be most fertile between June 12 and June 17, with ovulation most likely on June 17. If she conceives during this window, her estimated due date would be around March 8, 2026. Keep in mind that ovulation timing can shift by several days due to stress, illness, or hormonal fluctuations, so these are estimates, not guarantees. Confirm ovulation with an LH surge test (OPK) or basal body temperature (BBT) tracking for greater accuracy.
The average menstrual cycle lasts 28 days but healthy cycles range from 21 to 35 days. It is divided into four phases:
The luteal phase (from ovulation to next period) is relatively fixed at approximately 14 days in most women, which is why the formula subtracts 14 from the cycle length. Women with shorter cycles (e.g., 24 days) ovulate around Day 10; women with longer cycles (e.g., 35 days) ovulate around Day 21.
An egg survives only 12–24 hours after ovulation. However, sperm can live inside the fallopian tubes for up to 5 days. This creates a 6-day window of fertility: the 5 days before ovulation plus the day of ovulation itself. Studies show the highest probability of conception occurs when intercourse takes place 1–2 days before ovulation.
Naegele's Rule, published in the 19th century, adds 280 days (40 weeks) to the LMP to estimate the due date. It assumes ovulation on Day 14 and a 28-day cycle; adjustments may be needed for longer or shorter cycles. Ultrasound dating (especially an 8–12 week scan) typically provides a more precise EDD.
⚕️ Disclaimer: This calculator is provided for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Cycle irregularities, polycystic ovary syndrome (PCOS), thyroid disorders, and other factors can significantly affect ovulation timing. Always consult a qualified healthcare provider or OB-GYN for personalized guidance.
Start testing with an OPK about 2 days before the estimated ovulation day shown by the calculator. For a predicted ovulation on Day 17 of your cycle, begin testing on Day 15. Test at the same time each day (mid-morning is generally recommended), avoiding first morning urine.
Implantation typically occurs 6–12 days after fertilization. A home pregnancy test is most reliable from the first day of your missed period, which is approximately 14 days after the estimated ovulation day. Testing too early can produce a false negative.
Polycystic ovary syndrome (PCOS) often causes anovulatory cycles (cycles where no egg is released) or highly irregular cycles, making calendar-based predictions unreliable. Women with PCOS should work with a gynecologist and may benefit from medically supervised cycle tracking or fertility treatment.
Breastfeeding can suppress ovulation through elevated prolactin levels (lactational amenorrhea), so the calculator's formula — which relies on regular menstrual cycles — is not applicable during exclusive breastfeeding. Cycles can resume unpredictably once breastfeeding frequency decreases.
For a typical 28-day cycle, ovulation occurs around Day 14, which is 14 days after the first day of your period. For other cycle lengths, subtract 14 from your cycle length to get the approximate day of ovulation (e.g., for a 32-day cycle, ovulation is around Day 18).
An ovulation calculator is a reasonable first estimate for women with regular cycles, but it can be off by 2–4 days. Actual ovulation can shift due to stress, illness, travel, or hormonal changes. For greater accuracy, pair calculator results with OPK testing or BBT charting.
You can enter your best estimated average cycle length, but results will be less reliable. Women with cycles that vary by more than 7 days month to month are advised to use OPKs, which directly detect the LH surge regardless of cycle length.
Enter the average number of days between the first day of two consecutive periods. For best accuracy, average 3–6 recent cycles. If your cycles have been 27, 29, and 28 days, enter 28.
It uses the same Naegele's Rule formula your OB-GYN typically uses at the first visit. However, doctors commonly adjust the EDD after an early ultrasound (8–12 weeks), which is more precise. Treat the calculator's EDD as a preliminary estimate.
The chance is very low but not zero. Research published in the New England Journal of Medicine found that pregnancy from intercourse outside the fertile window (Days -5 to 0 relative to ovulation) was rare. Practically, the fertile window defined by this calculator covers the vast majority of conception risk.
Not always. Even women with regular cycles can experience occasional early or late ovulation due to factors like stress, travel across time zones, illness, or dietary changes. That's why real-time tracking methods like OPKs add valuable confirmation.
Only sources that have been reviewed are shown. Unverified citations are never published.