This calculator estimates your ovulation date by counting back 14 days from the start of your next expected period. That 14-day luteal phase (the time between ovulation and your next period) tends to stay fairly constant, so if your average cycle length is 28 days, ovulation is estimated around day 14.

Shorter or longer cycles shift this estimate accordingly: with a 32-day cycle, ovulation is estimated closer to day 18, since it’s the follicular phase (before ovulation) that varies most between people, not the luteal phase after it.

This is a statistical estimate based on average cycle behavior, not a direct measurement of when you actually ovulate. Ovulation predictor kits, basal body temperature tracking, or cervical mucus monitoring can confirm ovulation more precisely, especially if your cycles are irregular.

If you know your conception date instead, the calculator adds 266 days (38 weeks) from that date, since conception typically happens about two weeks after LMP.

For IVF pregnancies, dating is based on the embryo transfer date rather than LMP, since the embryo’s exact age is already known: 261 days from a Day 5 (blastocyst) transfer, or 263 days from a Day 3 transfer.

None of these methods predict an exact birth date only about 5% of babies arrive on their estimated due date. What they give you is a starting point your doctor will refine using ultrasound measurements, especially if your cycle is irregular or conception timing is uncertain.