Ovulation Calculator – Fertile Window, 3-Month Forecast & Cycle History
Calculate your ovulation date and fertile window, with a 3-month cycle forecast and cycle history tracking for more accurate predictions. Enter past cycle lengths to get a personalised average and variability score.
Understanding Your Fertile Window
Ovulation typically occurs around 14 days before your next period starts — not 14 days after your last period. This distinction is important if your cycle isn't exactly 28 days. The fertile window spans the 5 days before ovulation plus the day of ovulation itself — six days in total. Sperm can survive in the reproductive tract for up to 5 days, while an egg is only viable for 12–24 hours after release.
The peak fertile days are the 2 days before and the day of ovulation. If you are trying to conceive, having sex on these days gives the highest probability of conception in any given cycle.
Why Cycle History Matters
A single cycle length gives a rough estimate, but tracking multiple cycles produces a more accurate average and shows your cycle variability. High variability (cycles varying by more than 7 days) makes calendar-based prediction less reliable — in this case, ovulation predictor kits (OPKs), basal body temperature (BBT) tracking, or cervical mucus monitoring will give you more accurate information about your specific ovulation timing each month.
Irregular cycles: If your cycle length varies significantly from month to month, or if you have cycles shorter than 21 days or longer than 35 days, speak to your GP. Irregular cycles can sometimes indicate conditions such as PCOS, thyroid disorders, or perimenopause that are worth investigating.
Signs of Ovulation
- Cervical mucus changes: Becomes clear, slippery, and stretchy (like egg white) around ovulation — the most reliable physical sign.
- Basal body temperature (BBT) rise: A slight rise of 0.2–0.5°C after ovulation, detectable with a BBT thermometer taken first thing in the morning.
- LH surge on OPK: Ovulation predictor kits detect the luteinising hormone surge 24–36 hours before ovulation — the most accurate timing method short of ultrasound.
- Mittelschmerz: Mild one-sided pelvic pain around ovulation — experienced by some but not all women.
When to seek help: If you have been trying to conceive for 12 months (or 6 months if over 35), speak to your GP about fertility investigations. Both partners should be assessed. NHS referral thresholds vary by region.