Estimate the fertile window
Cycle tracking puts ovulation ~14 days before the next period — the fertile window around it is what planning needs.
Tools used in this guide
The job
For timing conception — or simply understanding the cycle — the useful dates are ovulation day plus the five days before it (sperm survive ~5 days; the egg ~1). A regular 28-day cycle makes the math simple enough to do by hand; the tool does it for the coming cycle.
Last period and cycle length
Enter the first day of the last period and the typical cycle length (20–45 days). Ovulation is estimated at next-period minus 14 days; the window is the five days before through the day after.
Worked example
Last period started Sep 1, cycle 28 days: next period ≈ Sep 29, ovulation ≈ Sep 15, fertile window ≈ Sep 10–16. A 32-day cycle from the same start gives next period ≈ Oct 3, ovulation ≈ Sep 19, window ≈ Sep 14–20.
Where it stops
The −14-day luteal estimate assumes a textbook cycle — real luteal phases vary 10–16 days, and irregular or PCOS-affected cycles make calendar math unreliable. It is not contraception and not a diagnosis; LH tests or BBT confirm what the calendar only estimates.