Ovulation usually happens about 14 days before your next period, not 14 days after your last one. That distinction matters, because the second half of the cycle is relatively fixed while the first half varies.
So a woman with 35-day cycles ovulates around day 21, not day 17. Calculators that assume day 14 for everyone get it wrong for most women.
This one works backwards from your expected period using your own cycle length.
Sperm survive up to five days in the right cervical mucus. An egg survives 12 to 24 hours.
That gives a fertile window of roughly six days: the five days before ovulation and the day itself. The two or three days immediately before ovulation carry the highest chance.
After the egg is gone, that cycle is over for conception.
Menstrual — bleeding, with both oestrogen and progesterone at their lowest.
Follicular — oestrogen rising as follicles develop. Many women report their best energy and mood here.
Ovulatory — a surge in LH releases the egg. Some women feel a one-sided twinge, and cervical mucus becomes clear and stretchy.
Luteal — progesterone rises then falls. Premenstrual symptoms belong to the end of this phase.
A calculator uses averages. Your body does not.
Ovulation shifts with stress, illness, travel, sleep disruption and training load. In perimenopause it becomes genuinely unpredictable, and some cycles have no ovulation at all despite a period arriving.
If your cycles vary by more than a few days, the prediction is a rough guide rather than a date.
This is not contraception. Calendar methods have high failure rates in typical use, because they cannot detect the shifts described above.
Two methods are more reliable than a calendar.
Cervical mucus. Around ovulation it becomes clear, slippery and stretchy, like raw egg white. This appears before ovulation, which makes it useful for timing rather than only confirming.
Basal body temperature. Temperature rises slightly after ovulation and stays up. This confirms ovulation happened but arrives too late to time intercourse, so it is for pattern-finding across cycles.
Ovulation predictor kits detect the LH surge and are the most practical option for most women.
Cycles consistently shorter than 21 or longer than 35 days, periods that stop for three months or more, or a year of trying without conception at under 35, six months at 35 and over.
Related reading: The menstrual cycle explained · Trying to conceive over 35 · Irregular periods in perimenopause · PCOS explained
Educational only, not medical advice, and not a contraceptive method.