Ovulation & Fertility Calculator

Ovulation typically occurs 14 days BEFORE your next period starts, not 14 days after your last period. This is because the luteal phase (post-ovulation) is relatively constant at 12-14 days, while the follicular phase (pre-ovulation) varies. For a 28-day cycle, ovulation is around Day 14. For a 30-day cycle, it's around Day 16. For a 26-day cycle, it's around Day 12. Our calculator uses your cycle length and luteal phase to estimate your ovulation date.

The fertile window is approximately 6-7 days when pregnancy is possible: the 5 days before ovulation, ovulation day, and 1 day after. Sperm can survive in the female reproductive tract for up to 5 days, waiting for the egg. The egg is viable for only 12-24 hours after release. Conception is most likely when intercourse occurs 1-2 days BEFORE ovulation, allowing sperm to be present when the egg arrives.

Clinical studies show conception probability varies by day relative to ovulation: 5 days before: ~10%, 4 days before: ~16%, 3 days before: ~14%, 2 days before: ~27%, 1 day before: ~31% (peak), Ovulation day: ~33% (peak), 1 day after: ~0% (egg no longer viable). The 3 days from 2-days-before through ovulation day are your peak fertility window with combined ~30% daily chances.

The luteal phase is the time from ovulation to the start of your next period. It's typically 12-14 days and is remarkably consistent for each individual (unlike the follicular phase which varies). After ovulation, the empty follicle becomes the corpus luteum and produces progesterone. If pregnancy doesn't occur, hormone levels drop and menstruation begins. A 'luteal phase defect' (less than 10 days) can cause fertility problems because there's not enough time for implantation.

Implantation—when the fertilized egg attaches to the uterine lining—typically occurs 6-12 days after ovulation (6-12 DPO), with most occurring around 8-10 DPO. Some women experience 'implantation bleeding' (light spotting) and cramping. After implantation, the body begins producing hCG (the pregnancy hormone), which is what pregnancy tests detect. A positive test is usually possible 10-14 days after ovulation, or around the time of your expected period.

Due date is calculated as 266 days (38 weeks) from ovulation, or equivalently 280 days (40 weeks) from the first day of your last menstrual period (LMP). Since ovulation typically occurs 14 days after LMP in a 28-day cycle, both methods give similar results. However, for irregular cycles, calculating from ovulation date is more accurate. Note: Only about 5% of babies are born on their due date—it's an estimate, not a guarantee.

Calendar-based predictions are estimates with accuracy depending on cycle regularity. For regular cycles with consistent lengths, predictions can be accurate within 1-2 days. For irregular cycles, accuracy decreases significantly. To improve accuracy, combine calendar predictions with: (1) Ovulation Predictor Kits (OPKs) that detect LH surge, (2) Basal Body Temperature (BBT) tracking, (3) Cervical mucus monitoring. These methods detect actual physiological changes, while calendars only predict based on averages.

Common ovulation signs include: (1) Cervical mucus changes—becomes clear, stretchy, 'egg-white' consistency (EWCM). (2) LH surge—detected by ovulation test strips. (3) Basal body temperature rise—0.5-1°F increase after ovulation. (4) Mittelschmerz—mild pelvic pain or cramping on one side. (5) Breast tenderness. (6) Increased libido. (7) Light spotting (rare). Not everyone experiences all symptoms, and some have no noticeable signs.

Multiple ovulation (releasing more than one egg) can occur, but always within the same 24-hour window—not at different times in the cycle. This is how fraternal twins are conceived. Once ovulation occurs, hormone changes prevent further egg release that cycle. You cannot ovulate again later in the same cycle. Stories of 'late ovulation leading to twins' are misconceptions—the eggs were released together but may have been fertilized at slightly different times.

NO. This calculator should NOT be used for contraception or pregnancy prevention. Calendar-based fertility awareness methods have high failure rates (15-25% typical use) because: (1) Ovulation timing can shift unpredictably due to stress, illness, or hormonal changes. (2) Sperm can survive 5 days, outlasting miscalculations. (3) Cycle length varies. If you wish to avoid pregnancy, use proven contraception methods and consult a healthcare provider.

Ovulation Predictor Kits (OPKs) are home tests that detect the surge in Luteinizing Hormone (LH) that occurs 24-36 hours before ovulation. When you get a 'positive' OPK (test line as dark or darker than control), ovulation is likely within 24-36 hours. Start testing 2-3 days before expected ovulation based on calendar predictions. Test with afternoon urine (LH peaks in late morning). OPKs confirm ovulation is imminent, while calendar methods only estimate.

If cycles vary by more than 7-8 days, calendar predictions become less reliable. For irregular cycles: (1) Use OPKs to detect actual LH surge. (2) Track cervical mucus for fertile signs. (3) Monitor BBT to confirm ovulation occurred. (4) Consult a healthcare provider—irregular cycles may indicate PCOS, thyroid disorders, or other conditions affecting ovulation. You can still use this calculator with your average cycle length, but treat predictions as rough estimates.

The follicular phase is the first half of your menstrual cycle, from Day 1 (first day of period) until ovulation. During this phase, FSH (Follicle Stimulating Hormone) causes follicles in the ovary to develop, with one dominant follicle eventually releasing an egg. Estrogen rises, thickening the uterine lining. Unlike the luteal phase, follicular phase length varies—this is why cycle length varies. A 35-day cycle has a longer follicular phase than a 25-day cycle.

After ovulation, you must wait for: (1) Fertilization (within 24 hours of ovulation). (2) Travel to uterus (3-4 days). (3) Implantation (6-12 days post-ovulation). (4) hCG buildup to detectable levels (2-3 days after implantation). Earliest possible positive: 8-10 DPO (days past ovulation) with sensitive tests. More reliable: 12-14 DPO or the day of expected period. Testing too early yields false negatives. If negative at expected period but no bleeding, retest in 2-3 days.

No. This calculator runs entirely in your browser. Your period dates and cycle information are processed locally on your device and are never sent to our servers or stored anywhere. When you close or refresh the page, all data is cleared. We believe fertility data is deeply personal and have designed this tool with privacy as the priority. For persistent tracking, consider a dedicated fertility app with a clear privacy policy.