Fertile Window Calculator

How the fertile window is estimated

Ovulation typically occurs about 14 days before the next period starts, not 14 days after the last one began. That distinction matters: the luteal phase after ovulation is relatively consistent at 12 to 16 days, while the follicular phase before it varies considerably. For a 32-day cycle, ovulation is around day 18, not day 14. Estimating forward from the last period is the most common source of error.

The fertile window spans roughly six days: the five days before ovulation plus ovulation day itself. Sperm can survive up to five days in fertile cervical mucus, which is why intercourse before ovulation can result in conception. The highest probability days are the two immediately before ovulation and the day of ovulation.

Calendar estimates are approximate

Even with regular cycles, ovulation day varies between cycles in most people. Research tracking ovulation directly has found that a minority of cycles ovulate on the day a calendar method predicts. Treat any calculated date as the centre of a range rather than a fixed point.

More reliable signals

Cervical mucus becomes clear, stretchy and slippery in the days approaching ovulation, which is the most accessible real-time indicator. Ovulation predictor kits detect the luteinising hormone surge 24 to 36 hours before ovulation, giving useful advance warning. Basal body temperature rises slightly after ovulation, which confirms it happened but comes too late to time intercourse for that cycle. Combining mucus observation with test strips is considerably more accurate than a calendar alone.

When to seek advice

General guidance is to consult a clinician after twelve months of trying without conception if under 35, or six months if 35 or over. Seek advice sooner where cycles are very irregular or absent, where there is known endometriosis, polycystic ovary syndrome or pelvic infection history, or where there is a known male factor. Fertility involves both partners and investigation should include both.

Processed locally

Everything is calculated in your browser. No dates or cycle information are stored, transmitted or logged. This is general information and not medical advice.