Ovulation and fertile window calculator

The calendar method is the oldest and least precise way of locating ovulation: it assumes the phase after ovulation lasts fourteen days, which is true on average and wrong for many women. It gives a useful order of magnitude for knowing when to try — and a false sense of security for avoiding. Both uses are detailed below the tool.

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?

Frequently asked questions