How ovulation is estimated
Ovulation usually happens about 14 days before your next period — so for a 28-day cycle that's around day 14, and for a 30-day cycle around day 16. An egg lives ~24 hours, but sperm survive up to 5 days, so the fertile window spans roughly the 5 days before ovulation plus ovulation day itself.
Trying to conceive?
The two or three days leading up to ovulation are the most fertile. Tracking signs like a small rise in basal body temperature or changes in cervical mucus, or using ovulation predictor kits, can pinpoint it more precisely than the calendar alone.
Important note
This is a calendar estimate, not a guarantee. Cycles vary month to month, especially if they're irregular. It is not a reliable form of contraception. For fertility concerns or to avoid pregnancy, speak with a healthcare professional.
Limitations
The "14 days before the next period" rule is a population average — the luteal phase (after ovulation) is fairly consistent for most people, but the follicular phase (before ovulation) varies more, which is why calendar-only predictions get less reliable the more irregular a cycle is.