Fertility · Cycle
Luteal phase length calculator.
Enter the day of your LH peak (or your ovulation day) and the first day of your next period to see how many days your luteal phase lasted.
Inputs
Result
Enter your numbers.
Luteal Phase Length Calculator: how it works
Your luteal phase is the number of days after your LH peak (or ovulation day) up to and including the day before your next period. This calculator counts those days. For example, an LH peak on 10 March and a period starting 24 March is 14 days apart, so the luteal phase is 13 days: 11 to 23 March.
The count follows the definition in Lenton, Landgren and Sexton (1984): the luteal phase begins the day after the LH peak and ends the day before menstruation. In their 327 luteal phases from apparently ovulatory cycles, the mean was 14.13 days with a standard deviation of 1.41 days. They estimated that every luteal phase of 9 days or less was abnormal, and that 74%, 22% and 2% of luteal phases of 10, 11 and 12 days were also abnormal. The calculator reports those same cut-offs: 9 or fewer days is short, 10 to 11 is borderline short, and 12 or more is not flagged as short.
The NHS describes ovulation as happening about 12 to 16 days before the next period, which is consistent with a luteal phase in the low-to-mid teens. If you use basal body temperature or a different test to mark the day, your count can be off by a day or so, and a short phase in one cycle does not mean you have a problem. Talk to a clinician if you are trying to conceive and your charts show short luteal phases cycle after cycle.
Worked example. LH peak on 10 March 2026, next period starts 24 March 2026. The two dates are 14 days apart, and the count excludes the marked day, so the luteal phase is 14 - 1 = 13 days. That is not flagged as short.
Frequently asked questions
How long should a luteal phase be?
In the Lenton 1984 study of 327 luteal phases the mean was 14.13 days with a standard deviation of 1.41 days. The authors judged every luteal phase of 9 days or less abnormal. This is one study, and your own cycles will vary by a day or two.
How is the luteal phase counted?
From the day after the LH peak up to and including the day before your next period starts. The marked day itself is not counted, so the count equals the gap between the two dates minus one.
Does this work with basal body temperature?
Only approximately. Temperature shows ovulation after it has happened, and the date you pick can be a day off. Use the last low-temperature day before the rise as the marked day and treat the answer as an estimate.
What if my luteal phase is short?
One short cycle is not a diagnosis. If short luteal phases keep recurring and you are trying to conceive, ask your clinician, who can look at the whole picture. This tool cannot diagnose a luteal phase defect.
Sources
Checked
- Lenton EA, Landgren BM, Sexton L: Normal variation in the length of the luteal phase of the menstrual cycle: identification of the short luteal phase (Br J Obstet Gynaecol, 1984)
- NHS: Periods
Runs entirely in your browser. Nothing you type is sent or stored.
Not medical advice
Not medical advice. This tool does not diagnose anything and cannot tell you whether a pregnancy is healthy or whether a cycle is normal for you. See your clinician for heavy or unusual bleeding, pain, dizziness, or any symptom that worries you, and get urgent care for severe pain or very heavy bleeding.