When do I ovulate if my cycle is not 28 days?

The reason to count backwards is that the two halves of the cycle do not vary equally. The follicular phase - from the first day of bleeding to ovulation - is the part that stretches and shrinks: Bull and colleagues put its 95 percent interval at 10 to 30 days across 612,613 cycles. Anchoring to a fixed day 14 assumes the variation lives in the wrong half.

The luteal phase is steadier, but it is not a constant either. The same dataset found a mean of 12.4 days with a 95 percent interval of 7 to 17, and 18 percent of cycles with a luteal phase under 11 days. Mapping that interval onto your entered cycle length gives a band about ten days wide around the estimated ovulation date - for a 35-day cycle, roughly cycle day 18 to day 28.

That band is why this site shows a window rather than a date. It is also why a six-cycle forecast is less trustworthy than a one-cycle forecast: the same assumed cycle length is applied six times, so the error compounds six times too.

The fertile window is drawn around whichever ovulation date the arithmetic produced. ACOG's patient guidance puts it at five days before ovulation through one day after; Wilcox 1995 found conceptions came only from a six-day window ending on the day of ovulation itself. This site shows both, so the extra day is visible as an allowance for uncertainty rather than as a finding.