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LMP Sep 1 on a 28-day cycle → ovulation around Sep 15, fertile window Sep 10–16, due date if conceived: June 8.
Conception math runs on one reliable anchor: the luteal phase — the time between ovulation and the next period — is about 14 days in almost everyone, while the follicular phase before it varies. That is why ovulation is estimated backward from cycle length, not forward from the period. This calculator runs that estimate and shows the full 6-day fertile window, because sperm survive up to 5 days while the egg lasts about 24 hours — the days before ovulation matter as much as the day itself.
Couples use it the week it matters: LMP Sep 1 on a 28-day cycle → ovulation ~Sep 15, fertile window Sep 10–16, and the due date if it works.
It is an estimate with a typical error of ±2 days, because the underlying assumption — a fixed 14-day luteal phase — is an average, not a law. In studies of confirmed cycles, ovulation day varies even in women with clockwork 28-day cycles. That is exactly why the fertile window matters more than the predicted day: sperm survive up to 5 days in fertile cervical mucus, so intercourse on the 3–4 days before ovulation is statistically as effective as the day itself. For confirmation, ovulation predictor kits detect the LH surge 24–36 hours before ovulation, and basal-body temperature (a ~0.5°F sustained rise) confirms it happened. If cycles are irregular, the calculator becomes less reliable and the OPK route becomes the primary tool.
Count backward from the next period, not forward from the last: ovulation ≈ cycle length minus 14 days. A 33-day cycle ovulates around day 19; a 24-day cycle around day 10. With truly irregular cycles (variance over 7 days), calendar math breaks down — track the shortest cycle in the last 6 months and treat its ovulation estimate as the window's start, the longest cycle's estimate as the end. Persistent irregularity is also diagnostic information: cycles routinely over 35 days or under 21 days are worth discussing with a clinician, since PCOS and thyroid issues commonly announce themselves through cycle length before any other symptom.
The 1–2 days before ovulation carry the highest per-day probability — around 25–30% for couples under 35 — because sperm need to already be present when the egg releases; the egg survives only 12–24 hours while sperm wait up to 5 days. Intercourse on ovulation day itself is slightly worse than the day before, and the day after drops sharply. Practical cadence from fertility clinics: every 1–2 days across the fertile window beats daily (which can reduce sperm concentration) and beats once (which risks missing the window entirely). The due-date bonus: if conception happens this cycle, pregnancy dating starts from the LMP — ovulation + 266 days — which is why the calculator shows it alongside.