Ovulation calculator

Ovulation is estimated by counting backwards from the next period, not forwards from the last one — the second half of the cycle is the predictable part.

Why the window is six days long

An egg survives roughly 12 to 24 hours after ovulation. Sperm survive considerably longer — up to five days in fertile cervical mucus. The fertile window therefore opens about five days before ovulation and closes about a day after it, and intercourse in the two days immediately before ovulation carries the highest probability of conception.

This asymmetry surprises people: the days before ovulation matter more than the day itself, because sperm can wait but the egg cannot.

The luteal phase is the stable half

Cycle length varies mainly in the follicular phase — the stretch from the start of a period to ovulation. The luteal phase, from ovulation to the next period, is comparatively consistent within an individual, usually 12 to 14 days.

This is why counting backwards works better. With a 32-day cycle and a 14-day luteal phase, ovulation falls around day 18, not day 16 as a simple midpoint would suggest. Assuming ovulation at the midpoint of a long cycle is a common and consequential error.

Calendar prediction has real limits

Cycle length varies between cycles even in people who consider themselves regular. Studies tracking ovulation directly find that in a substantial minority of cycles it falls outside the predicted window, and stress, illness, travel, sleep disruption and weight change all shift timing.

Signs that track ovulation more directly:

Cervical mucus becomes clear, stretchy and slippery near ovulation. This is the most useful free signal, and it is predictive rather than retrospective.
Ovulation predictor kits detect the luteinising hormone surge that precedes ovulation by 24 to 36 hours.
Basal body temperature rises slightly after ovulation. It confirms that ovulation happened but cannot predict it in advance.

This is not a contraceptive method

Calendar-based prediction is not reliable enough to prevent pregnancy. Typical-use failure rates for calendar methods are high — considerably worse than hormonal contraception or barrier methods — because the fertile window moves and the calculation cannot see it move.

Structured fertility awareness methods, which combine daily temperature and mucus observation with formal rules and proper instruction, perform substantially better than calendar counting alone, but they require training and consistent daily tracking. If you are trying to avoid pregnancy, speak to a healthcare provider about options rather than relying on a predicted window.

Common questions

How long should we try before seeking advice?

General guidance is to consult a doctor after 12 months of trying if you are under 35, or after six months if you are 35 or over. Seek advice sooner if cycles are very irregular or absent, if there is known reproductive history, or if either partner has a relevant medical condition.

How often should we have sex during the fertile window?

Every one to two days through the window is the usual recommendation. Daily is fine and does not meaningfully reduce sperm quality in most men. Abstaining for long stretches to "save up" is counterproductive — it lowers motility.

Can I ovulate more than once in a cycle?

Multiple eggs can be released, but within a window of about 24 hours — which is how non-identical twins occur. A second, separate ovulation later in the same cycle does not happen, because hormonal changes after the first suppress it.

What if my cycles are very irregular?

Calendar prediction becomes unreliable. Ovulation predictor kits or mucus tracking are more useful, and persistently irregular cycles are worth discussing with a doctor, since conditions such as PCOS and thyroid disorders are common, treatable causes.