The method
How this ovulation calculator works
The calculator turns a period start date and typical cycle length into a planning estimate. It does not observe ovulation or replace medical care.
The dates we estimate
We estimate the next period by adding your cycle length to the first day of your last period. We then count backward by the luteal-phase setting, which defaults to 14 days, to estimate ovulation. The fertile window spans five days before estimated ovulation through the following day.
Why estimates vary
Calendar methods assume a pattern that may shift with stress, illness, travel, postpartum changes, breastfeeding, PCOS, perimenopause, or hormonal contraception. LH tests, cervical-mucus observations, and basal body temperature can add context.
Due-date estimate
For planning only, the calculator adds 280 days to each cycle's period start. A clinician may adjust a due date using ultrasound and other information.