Ovulation and fertile days calculator

Ovulation happens about 14 days before the next period, not 14 days after the previous one. That distinction matters when your cycles are not 28 days long.

The fertile window covers the five days before ovulation plus the day itself.

Ovulation is found by subtracting 14 days from the expected next period. The five preceding days count as fertile because sperm can survive that long.

The fertile window lasts longer than ovulation

The egg lives twelve to twenty-four hours, but sperm can survive up to five days in favourable conditions. That is why the fertile window covers roughly the five days before ovulation and the day itself, not just the moment the egg is released.

The second phase of the cycle is the stable one

The phase from ovulation to the next period, called the luteal phase, lasts twelve to fourteen days in most women with fair regularity. What varies from cycle to cycle is the first phase. That is why the calculation works backwards from the end of the cycle rather than forwards from the start.

Signs from the body

Cervical mucus becomes clear and stretchy during fertile days. Basal temperature rises a few tenths after ovulation, which confirms it has happened but does not predict it. Ovulation tests detect the luteinising hormone surge one or two days ahead.

This is not contraception

Calendar-based methods have a high failure rate in real use, because cycles vary with stress, travel, illness and sleep. To avoid pregnancy, use a proven method.