How the ovulation date is estimated
Ovulation usually happens about 14 days before the next period, not 14 days after the last one. The time from ovulation to the next period, the luteal phase, is fairly stable, while the time before ovulation varies. So the calculator counts back from your next expected period:
Next period = first day of last period + cycle length
Ovulation = next period − luteal phase (14 days by default)
Fertile window = ovulation − 5 days to ovulation day
Worked example. Your last period started on March 3 and your cycles run 32 days. The next period is due on April 4 (March 3 + 32 days). Ovulation is estimated 14 days earlier, on March 21, and the fertile window runs from March 16 to March 21. With a 28-day cycle the same period would give ovulation on March 17.
Why the fertile window is six days
In a landmark study of 221 women trying to conceive, Wilcox and colleagues (NEJM, 1995) found that pregnancies came only from sex during a six-day span ending on the day of ovulation. The chance of conception rose from about 10% for sex five days before ovulation to about 33% on the day of ovulation itself. Sperm can survive several days, while the egg lives for about a day.
How reliable a calendar estimate is
Less than most people expect. A follow-up analysis by the same team (BMJ, 2000) found that only about 30% of women had their fertile window entirely within days 10 to 17 of the cycle, the days often given in guidelines, even with regular cycles. In data from more than 600,000 cycles logged in an app, Bull and colleagues (2019) found an average cycle of 29.3 days and an average luteal phase of 12.4 days, with wide variation. Ovulation predictor kits (which detect the LH surge 1 to 2 days before ovulation), cervical mucus changes and a rise in basal body temperature after ovulation all narrow the estimate.
Not a method of birth control
Fertility awareness methods can prevent pregnancy when they combine several signs and are learned properly, as ACOG explains, but a calendar calculator alone is not reliable enough to use as contraception.
When to see a doctor
ACOG advises an evaluation if you have been trying to conceive for 12 months and are under 35, or for 6 months if you are 35 or older. See a doctor sooner if your periods are very irregular or absent, very painful, or if you have a known condition such as PCOS or endometriosis.