Six months of fertile windows.

Enter your last period and cycle length. We'll plot the next six months — periods, fertile windows, and ovulation days, all at a glance.

How ovulation timing is estimated from a calendar

This calendar works backward from your next expected period rather than forward from your last one. The luteal phase — the stretch between ovulation and the start of the next period — is consistently about 14 days for most people, even when the first half of the cycle (the follicular phase) varies quite a bit in length. So for a given cycle length, ovulation is placed 14 days before the next period is due: a 28-day cycle puts ovulation around cycle day 14, while a 30-day cycle puts it around cycle day 16. That's the math behind every shaded day on the grid above.

The fertile window itself is wider than ovulation day alone: it's the 5 days before ovulation plus ovulation day, a 6-day window in total. That range exists because sperm can survive in the reproductive tract for several days waiting for an egg, while the egg itself is only viable for a short window after release. Within those 6 days, conception probability isn't flat — it's highest in the 2–3 days immediately before ovulation, per Wilcox 1995 in the New England Journal of Medicine, the foundational study on timing intercourse relative to ovulation.

The biggest limitation is that this calendar math assumes your cycles are consistent — the same length, month after month. In reality, cycle length can vary by several days even for people who consider their cycles "regular," and the follicular phase (not the luteal phase) is where most of that variability shows up. The further your actual cycle strays from the length you entered, the further the predicted ovulation day drifts from the real one.

For that reason, treat this as a statistical estimate — roughly ±2 days of accuracy under typical conditions — rather than a precise readout of when you ovulated or will ovulate. If your cycles vary by more than about a week from month to month, layering this calendar with ovulation predictor (LH) tests or basal body temperature tracking will give you a more direct read on what's actually happening in a given cycle, rather than relying on averages alone.