Formula
Ovulation ≈ last period + (cycle length − 14) daysFertile window = ovulation − 5 days … ovulation + 1 dayNext period = last period + cycle length How it works
Ovulation is the release of an egg from the ovary. The second half of the cycle — from ovulation to the next period, called the luteal phase — is fairly constant at about 14 days for most people, while the first half varies. So the best calendar estimate of ovulation is your cycle length minus 14 days, counted from the first day of your last period.
Sperm can survive in the reproductive tract for up to five days, while the egg lives for only about a day. That makes the fertile window roughly six days long: the five days before ovulation and the day of ovulation itself (plus a day of margin here). The two or three days just before ovulation carry the highest chance of conception.
Cycles vary from month to month, and illness, stress or travel can shift ovulation. If your cycles are irregular, ovulation predictor kits (which detect the LH surge), basal body temperature charts or cervical-mucus tracking are more reliable than any calendar. This calculator is for information only, is not medical advice and should not be used for birth control.
Frequently asked questions
What if my cycle length varies?
Use your average, but treat the dates as approximate. For a wider safety margin when trying to conceive, run the calculator with your shortest and longest cycles and combine the windows.
Can I get pregnant outside the fertile window?
It is unlikely but possible, because ovulation can come earlier or later than predicted. Calendar methods alone are not dependable contraception.