Estimate a likely ovulation day and fertile window from the first day of a recent menstrual period and your usual cycle length. Calendar calculations are most useful when cycles are reasonably regular.
Ovulation & Fertile Window Calculator
Estimate your likely ovulation day and fertile window from a recent cycle.
Fertile-window estimates are approximate. ACOG describes ovulation as occurring about 14 days before the next period in an average cycle, with pregnancy possible from roughly 5 days before ovulation through about 1 day after. Irregular cycles reduce calendar accuracy.
What the fertile window means
ACOG explains that in an average cycle ovulation occurs about 14 days before the next menstrual period. Sperm can remain capable of fertilization for several days, while the egg has a much shorter lifespan after ovulation. That is why the fertile window spans several days rather than one exact moment.
Read ACOG’s fertility-awareness guidance.
Why calendar estimates can be wrong
Cycle length and ovulation timing can change with stress, illness, travel, postpartum changes, breastfeeding, perimenopause, polycystic ovary syndrome, some medications, and other factors. A cycle can also vary even when it is usually regular. This calculator therefore estimates a window; it does not confirm that ovulation occurred.
Other ways people track ovulation
Depending on the goal and clinical context, people may also observe cervical mucus, use urinary luteinizing-hormone tests, track basal body temperature, or receive ultrasound and laboratory monitoring through fertility care. Each method answers a somewhat different question and has limitations.
When to ask for fertility guidance
If cycles are very irregular, periods are absent, ovulation is difficult to identify, there are known reproductive-health conditions, or pregnancy has not occurred after an appropriate period of trying for your age and circumstances, discuss next steps with a qualified clinician.
