Find your fertile window, visualize your cycle on a calendar, track irregular cycles, and see your day-by-day conception odds, all science-backed, all free.
Most ovulation calculators assume a 28-day cycle with ovulation on day 14, but fewer than 15% of women have a textbook 28-day cycle. Ovulation can occur anywhere from day 11 to day 21, and cycle length varies month to month for most women. Ovulation predictor kits (OPKs) that detect the LH surge provide far more accurate timing than calendar-based calculation alone. The fertile window is approximately 5 days before ovulation plus the day of ovulation itself.
Ovulation is the release of a mature egg from an ovary. It happens once per cycle and is the only time pregnancy is possible. The key fact that surprises most people: ovulation does not happen 14 days after your last period, it happens approximately 14 days before your next period. For a 28-day cycle that math lines up at day 14, but for a 32-day cycle ovulation falls around day 18, and for a 25-day cycle around day 11. This single distinction makes a significant difference in prediction accuracy.
Once the egg is released it survives for only 12–24 hours. Sperm, by contrast, can survive inside the reproductive tract for up to 5 days in fertile cervical mucus. This asymmetry is why having intercourse in the days before ovulation is just as important as on ovulation day itself, sperm can wait for the egg, but the egg cannot wait for sperm.
The fertile window spans the 5 days before ovulation plus ovulation day itself, six days total. Research on conception probability by day relative to ovulation shows a clear peak pattern:
| Day (relative to ovulation) | Conception probability | Notes |
|---|---|---|
| 5 days before | ~10% | Sperm can survive but odds are lower |
| 4 days before | ~16% | Increasing probability |
| 3 days before | ~14% | Within the high-probability window |
| 2 days before | ~27% | High fertility |
| 1 day before | ~33% | Peak, highest single-day probability |
| Ovulation day | ~30% | Still very high, second best day |
| 1 day after | ~8% | Egg declining rapidly |
| 2+ days after | <1% | Egg no longer viable |
The most reliable physical sign. Discharge changes across your cycle: dry or minimal after your period, creamy and lotion-like in the week before ovulation, and then clear, slippery, and stretchy (often described as raw egg-white consistency) in the 1–2 days immediately before ovulation. This fertile-quality mucus protects sperm and helps them swim through the cervix. After ovulation, progesterone causes mucus to thicken and become hostile to sperm again.
Your resting body temperature rises by approximately 0.5–1°F (0.3–0.5°C) after ovulation due to the thermogenic effect of progesterone. This rise confirms that ovulation has already occurred, too late to act on for that cycle, but invaluable for predicting the pattern in future cycles. You must take your temperature immediately after waking, before getting out of bed or drinking anything, using a dedicated basal thermometer (accurate to 0.1°F).
Luteinizing hormone (LH) surges 24–36 hours before the egg is released. Over-the-counter ovulation predictor kits (OPKs) detect this surge in urine. A positive test means ovulation is coming within 12–48 hours, the signal to have intercourse over the next 2–3 days. Start testing on cycle day (cycle length − 17) to ensure you capture the surge, and test at the same time each afternoon (not first morning urine, as LH peaks in the afternoon).
About 20% of women feel a sharp or cramping pain on one side of the lower abdomen when the egg bursts through the ovarian wall. It lasts anywhere from a few minutes to several hours and alternates sides depending on which ovary releases that cycle. Useful as a confirmatory sign, but too inconsistent to rely on as a primary indicator.
A cycle shorter than 21 days or longer than 35 days, or one that varies by more than 5 days month to month, is considered irregular. The most common causes include polycystic ovary syndrome (PCOS), thyroid dysfunction, significant stress, extreme weight changes, intense exercise, and the adjustment period after stopping hormonal contraception. The irregular cycle tracker above uses your past cycle lengths to calculate an average and a range, giving you a realistic fertility window that accounts for the variability.
For women with highly irregular cycles, the calculator provides a useful estimate but ovulation predictor kits remain the most reliable real-world tool. Testing daily from the early end of the predicted fertile window through the late end ensures the LH surge is captured regardless of which day ovulation actually occurs.
The menstrual cycle has two distinct phases divided by ovulation. The follicular phase spans from the first day of your period to ovulation. Its length varies between women and between cycles, this is the variable half of the cycle. The luteal phase (ovulation to next period) is far more consistent, typically 12–14 days in most women. This is why the formula subtracts 14 from cycle length: the 14 days represents the luteal phase, working backwards from the expected start of the next period.
A luteal phase consistently shorter than 10 days (called luteal phase defect) can interfere with implantation and is worth discussing with a gynecologist if you are trying to conceive. BBT tracking across several cycles is the simplest way to estimate your own luteal phase length.
Detect your LH surge for accurate ovulation timing, more reliable than calendar calculation
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