Trying to conceive: how to use it
Sex every two or three days during the fertile window is enough; sperm live up to five days, the egg only one. If your cycles are irregular, the estimate shifts by several days: urinary ovulation tests (which detect the LH surge one to two days before) and observing cervical mucus are more reliable than the calendar. With no pregnancy after a year of regular sex — six months after thirty-five —, the page on assisted reproduction describes the pathway. And a pregnancy, when it comes, is dated from the last period, not from ovulation: that is why doctors count in “weeks of amenorrhoea”.
Avoiding a pregnancy: why this is not contraception
Used alone, the calendar method lets a pregnancy happen in about one woman in four per year in typical use. The reason is simple: ovulation cannot be predicted with certainty, one cycle in ten departs from the average, and stress, a trip or an illness shift it. The natural methods that really work combine several signs — waking temperature, mucus, calendar — and require training: that is the symptothermal method, and the page on calendar methods explains the difference with this simple calculation. To choose a suitable contraception: which contraception for you?
What studies show about cycle length and the fertile window
Two large datasets explain why this calculation stays approximate. In an analysis of 612,613 ovulatory cycles from 124,648 users of a tracking app, the average cycle lasted 29.3 days, the phase before ovulation 16.9 days (95% interval: 10 to 30 days) and the phase after ovulation 12.4 days (7 to 17 days) (Bull et al., 2019). A fixed fourteen-day phase is therefore a convenient average, not a rule: from one woman to another, and from one cycle to the next, ovulation comes earlier or later than the calendar assumes. The authors, who are linked to the app’s maker, conclude that physiological signs such as basal body temperature should be tracked, not just cycle length; app users are not representative of the whole population.
On the fertile window, a US study followed 221 healthy women who wanted to conceive, over 625 cycles in which the day of ovulation could be estimated from urine hormone tests: conception occurred only when intercourse took place during the six days ending on the day of ovulation. The probability of conception ranged from 0.10 for intercourse five days before ovulation to 0.33 on the day itself (Wilcox et al., 1995). This is the six-day window the calculator shows, with one major caveat: the date of ovulation is still an estimate.
Irregular cycles: benchmarks and when to see a doctor
The UK’s National Health Service (NHS) puts the average cycle at around 28 days and treats regular cycles of 21 to 35 days as normal (NHS, 2023); periods are called irregular when the gap is under 21 days or over 35 days (NHS, 2026). Puberty, the approach of menopause, pregnancy, hormonal contraception, stress, large weight changes, very intense exercise, polycystic ovary syndrome and thyroid problems are among the common causes (NHS, 2026). The NHS advises seeing a GP if periods are irregular, last longer than seven days, come with bleeding between periods, or if pregnancy is taking a long time (NHS, 2026). In these situations the calculation on this page loses most of its value: a healthcare professional is the right person to ask.
Frequently asked questions
Can you ovulate during your period?
With a very short cycle, the fertile window can begin at the end of the period, because sperm survive several days: sex on day five of a twenty-one-day cycle can fertilise. Rare, but not impossible.
My cycles range from 26 to 32 days: what should I enter?
Make two estimates, with 26 and with 32: the real fertile window covers both, which shows precisely the limit of the calendar when cycles vary.
Are cycle-tracking apps more reliable?
Those that only do this calculation, no. Those that integrate temperature or hormone tests, more so — provided you understand what they measure. The page on cycle-tracking apps sorts them out.