How it works
Ovulation usually happens about 14 days before the next period starts. This luteal phase is fairly constant, so in a 28-day cycle ovulation falls about 14 days after your period starts, and in a 32-day cycle about 18 days after.
The fertile window covers the five days before ovulation plus the day of and after, because sperm can survive up to about five days and an egg about 12 to 24 hours. The two days before ovulation and the day itself have the highest chance of conception.
Cycles vary from month to month, so calendar methods are approximate and are not reliable as birth control. Results are estimates, not medical advice.
Formula
Ovulation = first day of last period + (cycle length − luteal phase); fertile window = ovulation − 5 days to ovulation + 1 day
Questions people ask
How long is the fertile window?
About six days: the five days before ovulation and the day of ovulation, sometimes extended a day after. Pregnancy is most likely from intercourse in the two to three days before ovulation.
Can I ovulate early or late?
Yes. Stress, illness, travel and changes in routine can shift ovulation by several days, which is why calendar predictions are only estimates.
How can I confirm ovulation?
Urine LH tests detect the hormone surge 24 to 36 hours before ovulation. A sustained rise in basal body temperature confirms ovulation has already happened, and cervical mucus changes can also help.