How this calculator works
This calculator uses elapsed calendar days, excluding the ovulation date and ending on the next period start. Different charts may use inclusive counting, so compare conventions before comparing results. Ovulation must be independently estimated or confirmed; an LH surge date is not necessarily the ovulation date. ASRM describes variation in luteal length and cautions that length alone does not establish a luteal-phase disorder. The 30-day form limit is an input guard, not a diagnostic boundary. The result is an estimate for information only and is not medical advice.
Formula
Length = next period date - ovulation date. Next period = ovulation + length. Ovulation = next period - length.
Worked example
Ovulation 2026-01-15 and the next period 2026-01-29 are 14 elapsed days apart.
Advertisement
Frequently asked questions
Does the calculator diagnose luteal phase deficiency?
No. A single date interval is insufficient for diagnosis.
Why might an app show one more day?
It may count ovulation as the first luteal day while this tool measures elapsed days.
Can an LH test provide the exact date?
It predicts an approaching ovulation but does not identify its exact time or confirm that ovulation occurred.