It estimates ovulation by subtracting the luteal phase from your average cycle length. The fertile window starts about five days before estimated ovulation and includes ovulation day plus the selected post-ovulation day.

Calendar estimates are less reliable with irregular cycles. Use several months of cycle data and consider LH tests, cervical mucus observations or clinician guidance for more precise timing.

A practical test date is around the expected period or about 14 days after estimated ovulation. Testing too early can produce false negatives.