Luteal Phase Length Calculator

The luteal phase is the second half of the menstrual cycle — the stretch from ovulation to the day before your next period begins. During this phase the empty follicle that released the egg transforms into the corpus luteum, which produces progesterone to prepare the uterine lining for a potential pregnancy. If fertilization does not occur, the corpus luteum breaks down, progesterone drops, and menstruation begins. A normal luteal phase lasts 11–17 days, with 12–14 days being most common. A short luteal phase (under 11 days) can impair implantation because progesterone levels fall before a fertilized egg has time to implant — a condition sometimes called luteal phase defect. A long luteal phase may suggest early pregnancy or, occasionally, a hormonal imbalance. Tracking your luteal phase length across multiple cycles helps identify patterns and can be valuable information to share with a reproductive endocrinologist or OB-GYN if you are experiencing difficulty conceiving. This calculator requires your total cycle length and the cycle day on which you ovulate — both of which can be determined through temperature charting or LH test strips.

The luteal phase begins the day after ovulation and ends the day before the next period. Its length equals: cycle length − ovulation day. For example, if your cycle is 28 days and you ovulate on day 14, your luteal phase is 28 − 14 = 14 days. The normal range is 11–17 days; under 11 days is considered short and may impair embryo implantation due to insufficient progesterone support; over 17 days is considered long and may indicate early pregnancy or hormonal irregularity. Ovulation day can be determined through a rise in basal body temperature (BBT), a positive LH surge test strip, or confirmation by ultrasound.