A worked example: 28-day cycle from a January 1 period
A last period starting January 1, 2026, with a typical 28-day cycle, gives an estimated ovulation date of January 15 — 14 days in — with a fertile window from January 10 to January 16, and a next expected period on January 29.
The formula: ovulation date = LMP + (cycle length − 14)
Ovulation is counted backward from the next period, not forward from the last one: the luteal phase (from ovulation to the next period) is typically a fixed 14 days regardless of the total cycle length, so ovulation happens on cycle-length-minus-14 days after the last period started.
Why a longer cycle pushes ovulation later, not the 14-day count
The same January 1 period with a longer 35-day cycle shifts ovulation to January 22 (day 21, not day 14) — the fertile window moves to January 17-23, and the next period lands on February 5. The 14-day luteal-phase assumption stays fixed; it's the number of days before that 14-day countdown starts (governed by the cycle length) that changes.
What the fertile window represents
The 5 days before ovulation plus the day of ovulation itself reflect that sperm can survive several days awaiting ovulation, while the egg itself is viable for only about a day after release — so this window is not symmetric around the ovulation date.
Why this is an estimate, not a guarantee
This calculator uses your average cycle length, but actual ovulation timing can shift month to month, especially with irregular cycles — tracking basal body temperature or using ovulation predictor kits gives more precise, cycle-specific timing than a date-based estimate alone.