Ovulation Calculator

How the fertile window works

An egg survives for about 12 to 24 hours after ovulation, but sperm can survive for up to five days. That creates a fertile window of about six days: the five days before ovulation and the day of ovulation itself.

The chance of conception is highest from intercourse in the two days before ovulation and on the day itself. By the time an ovulation test turns positive, the best window is already open, so it helps to know it is coming.

How the calculator estimates ovulation

The second half of the cycle, from ovulation to the next period, is fairly constant at about 14 days for most people. The first half is what varies. So ovulation is estimated by counting back 14 days from when your next period is due, rather than forward from your last one.

On a 28-day cycle that puts ovulation around day 14. On a 32-day cycle it moves to around day 18.

When a calendar is not enough

Calendar methods work best when cycles are regular. If your cycle length varies by more than a few days, combine them with ovulation predictor kits, which detect the rise in LH hormone 24 to 36 hours before ovulation, or with basal body temperature and cervical mucus tracking.

Very irregular or absent periods can be a sign of polycystic ovary syndrome, thyroid problems or other treatable causes. They are worth raising with a clinician rather than working around.

When to talk to a clinician

The usual advice is to seek help after 12 months of trying if you are under 35, or after six months if you are 35 or older. Go sooner if your periods are very irregular, very painful, or you have a known condition that affects fertility.