Luteal Phase Calculator

The luteal phase is the second part of the menstrual cycle. It starts the day after you ovulate and ends the day before your next period begins. To measure it, you need two real dates from your own tracking: the day you ovulated and the day your next period started.

What Luteal Phase Length Measures

After ovulation, the follicle that released the egg makes progesterone. If you do not get pregnant, progesterone falls and your period starts. Luteal phase length tells you how long that stage lasted. It does not measure hormone levels or prove you ovulated; it relies on the ovulation date you enter. This page looks back at past cycles. To predict future dates, use the Ovulation Calculator or the Period Calculator.

The Formula

Luteal phase (days) = next period start date minus ovulation date, minus 1

Follicular phase (days) = ovulation date minus period start date, plus 1

Cycle length = follicular phase + luteal phase

This rule counts ovulation day as the last day of the follicular phase, the same way large cycle studies count it³ ⁴.

How This Calculator Computes Your Result

You can enter calendar dates or cycle day numbers, where day 1 is the first day of your period. You also choose how you found ovulation: a known ovulation date, your first positive LH test, or your first high temperature on a basal body temperature (BBT) chart. The calculator adds one day to a positive LH test date, because in an ultrasound study ovulation was best predicted within 24 hours after the first positive test⁶. It steps back one day from the first high temperature, because the rise comes after ovulation⁴.

Results are in whole days. Enter up to 6 cycles to get your average (to one decimal place), your shortest and longest luteal phase, and how many were 10 days or fewer. Add the date your period started to see your follicular phase and cycle length. Tools that count ovulation day in the luteal phase show one day more for the same dates.

What Your Result Means

The American Society for Reproductive Medicine (ASRM) describes a typical luteal phase as 12 to 14 days, with a range of 11 to 17 days¹. It describes luteal phase deficiency as a clinical diagnosis linked to a luteal phase of 10 days or fewer¹. Large studies agree on the middle of the range. In 1,060 cycles tracked with LH monitors, the average was 12.4 days³. In 612,613 cycles from app users, the average was also 12.4 days, with most between 7 and 17 days⁴. An older study that counted from the day after the LH peak found an average of 14.13 days².

An average of 11.5 days counts as the low end of normal. A result over 17 days most often means the ovulation date was set too early. Your follicular phase varies much more: in the app data most follicular phases fell between 10 and 30 days⁴, which is why a long or short cycle usually reflects the time before ovulation³.

How to Find Your Ovulation Date

LH tests. Urine tests detect the surge of luteinizing hormone (LH) that triggers ovulation. In a study that checked ovulation by ultrasound, ovulation was best predicted within 24 hours after the first positive test⁶. The limit: in 31% of cycles, a day with a positive reading followed by negative days showed up before ovulation⁶. Using that early positive makes your luteal phase look too long.

BBT charting. Take your temperature every morning before you get up. After ovulation it rises by about 0.2 to 0.3 °C⁴. The limit: BBT confirms ovulation only after the fact, and the estimated ovulation day is the last low reading before the rise⁴. Missed readings make the shift harder to place⁴.

Apps. A date an app predicts from past cycle lengths alone, without LH tests or temperatures, is a guess rather than a measurement. Count the first day of full flow as your period start, not light spotting.

Why One Cycle Is Not Enough

Your luteal phase can shift by a few days from one cycle to the next. In the LH monitor study, the same woman's luteal phase varied by 3.6 days on average across cycles, compared with 5.8 days for the follicular phase³. Short readings are also common in healthy cycles: 18% of cycles in the app data had a luteal phase under 11 days⁴. An average of 3 or more cycles gives a fairer picture than one result.

When a Short Luteal Phase Matters

ASRM states that "No diagnostic test for LPD has been proven to be reliable in the clinical setting," and that luteal phase deficiency has not been proven to cause infertility on its own¹. In a study of women trying to conceive, those with a short luteal phase (11 days or fewer counting ovulation day, which is 10 or fewer on this calculator's count) had lower fertility in the first 6 months, but their chance of pregnancy by 12 months was not significantly different⁵. ASRM advises checking and treating related conditions first, such as thyroid or prolactin problems¹.

It is worth talking to a doctor if several cycles in a row come out at 10 days or fewer, if you are trying to conceive and it is taking longer than expected, or if your cycles are very irregular. Your tracked dates help a doctor see the pattern.