Ovulation Calculator
Estimate your ovulation date and fertile window based on your menstrual cycle. Plan for conception or track your cycle.
Enter the first day of your last period and cycle length to estimate your ovulation date and fertile window.
Understanding Ovulation: The Key to Fertility
Ovulation is the release of a mature egg from the ovary. It happens once per cycle, typically 14 days before your next period — not 14 days after your last one. This distinction matters enormously if your cycle isn't exactly 28 days.
In a 28-day cycle, ovulation occurs around day 14. In a 35-day cycle, it's around day 21. In a 24-day cycle, it's around day 10. The time from ovulation to your next period (the luteal phase) is remarkably consistent at 12–14 days. It's the first half of the cycle (the follicular phase) that varies.
The egg survives only 12–24 hours after release. But sperm can live up to 5 days in the reproductive tract. This creates a fertile window of approximately 6 days — 5 days before ovulation plus ovulation day itself. Timing intercourse within this window is the single most important factor for natural conception.
The Menstrual Cycle Phases
| Phase | Days (28-day cycle) | What Happens | Fertility |
|---|---|---|---|
| Menstruation | Days 1–5 | Uterine lining sheds — your period | Very low |
| Follicular phase | Days 1–13 | Follicles develop, one becomes dominant, oestrogen rises | Increasing after day 8 |
| Ovulation | Day ~14 | LH surge triggers egg release, egg viable 12–24 hours | Peak fertility |
| Luteal phase | Days 15–28 | Corpus luteum produces progesterone, prepares uterus | Drops rapidly after day 15 |
What this means for you: Your most fertile days are the 2–3 days immediately before ovulation. By ovulation day itself, the window is closing. If you're trying to conceive, start timing intercourse from about 5 days before your expected ovulation date. Use our Conception Calculator if you're already pregnant and want to know when it happened.
Signs of Ovulation
Cervical Mucus Changes
As ovulation approaches, cervical mucus becomes clear, stretchy, and slippery — resembling raw egg white. This "fertile mucus" is designed to help sperm swim and survive. After ovulation, mucus becomes thicker and less abundant.
Basal Body Temperature
After ovulation, progesterone causes a 0.2–0.5°C rise in your resting temperature. Take your temperature at the same time every morning before getting up. The rise confirms ovulation occurred — useful for tracking but tells you after the fact, not before.
LH Surge (OPK Tests)
Ovulation predictor kits detect the LH surge that triggers egg release 24–36 hours before ovulation. A positive OPK means ovulation is imminent — the most time-sensitive signal available without medical monitoring.
Mittelschmerz (Ovulation Pain)
About 20% of women feel a twinge or mild cramp on one side of the lower abdomen during ovulation. It can last minutes to hours. While not reliable enough to use alone, combined with other signs it helps confirm your fertile window.
Conception Probability by Timing
| Days Relative to Ovulation | Probability of Conception |
|---|---|
| 5 days before | ~4% |
| 4 days before | ~8% |
| 3 days before | ~15% |
| 2 days before | ~25% |
| 1 day before | ~30% (peak) |
| Ovulation day | ~12% |
| 1 day after | ~0% (egg has typically expired) |
Source: Wilcox et al., New England Journal of Medicine, 1995. These are per-cycle probabilities for healthy couples. Peak conception probability is actually the day before ovulation, not ovulation day itself — because it takes sperm several hours to reach the egg after ejaculation.
Related Fertility Tools
How to use this tool
Enter the first day of your last menstrual period
Select your average cycle length
Click Calculate to see your ovulation date and fertile window
Common uses
- Planning conception timing
- Tracking your menstrual cycle
- Identifying your most fertile days
- Predicting your next period
- Understanding your reproductive cycle
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