fertile window irregular cycle
This calculator estimates ovulation and the fertile window from two inputs: the LMP and the cycle length, which the tool accepts between 21 and 45 days. When the cycle is irregular, that length is not a fixed number, and each different value entered shifts the calculated window by whole days. This page shows the exact size of that shift and the limits the calculator applies to the input.
The 14-day luteal phase is subtracted from the total cycle, leaving only 7 days between the LMP and the estimated ovulation.
This is the value where the 14-day luteal phase and the 14-day follicular phase are equal, the same case Naegele's Rule also assumes.
24 days apart from the 21-day cycle for the same LMP, showing why a single cycle length does not serve every month of an irregular cycle.
Ovulation occurs approximately 14 days before the next expected period, regardless of total cycle length. In 28-day cycles, that is around day 14. In 35-day cycles, around day 21. This calculation is an estimate for regular cycles; irregular cycles require complementary methods.
Any length between 21 and 45 days. Values outside that range are rejected by the tool's validation, which treats this as a technical input limit, not as a clinical definition of what counts as a normal or irregular cycle.
Because the formula adds 5 days before the estimated ovulation date and 1 day after, a fixed 7-day total, whatever cycle length is entered. Only the window's position on the calendar changes, not its size.
The tool always uses the exact value entered, there is no range or average-cycle option: a different day count produces a different ovulation date. For a cycle that varies month to month, the displayed result is only valid for the length entered in that specific calculation.
Medical notice: this calculation is based on population-level estimates and does not replace medical care. Irregular cycles, hormonal conditions and ovarian disorders significantly alter results. Do not use this calculation as a contraceptive method.
The dates you enter stay in your browser only. We do not save or send this data.