Luteal Phase Calculator (luteal phase length from ovulation and next period)

Understanding the luteal phase calculator — concept and principle

This calculator works out the length of your luteal phase, the part of the menstrual cycle from just after ovulation to the day before the next period, using an ovulation sign and the dates of two periods. People tracking fertility use it to understand their cycle and to discuss it with a doctor.

Ovulation splits the cycle: with the default inputs the result is 12 days (luteal phase · typical).

How it is calculated

Estimated ovulation day = ovulation day itself, or the day after the first positive LH test, or the day before the first raised basal body temperature (BBT)

Luteal phase (days) = (next period start − 1 day) − ovulation day

Follicular phase (days) = ovulation day − period start + 1

Cycle length = next period start − period start

The principle

After ovulation the empty follicle becomes the corpus luteum, which produces progesterone to prepare the womb lining. Without pregnancy it lasts a fairly fixed time and then fades, bringing the next period. That is why the luteal phase is fairly constant for each person, while the follicular phase is what usually varies. An LH surge usually precedes ovulation by about a day, and the temperature rise follows it, which sets the offsets used here.

Worked example

Default inputs: period started 11 Oct 2026, first positive LH test 25 Oct, next period 8 Nov.

Ovulation = 25 Oct + 1 day = Mon 26 Oct 2026.
Luteal phase = (8 Nov − 1 day) − 26 Oct = 7 Nov − 26 Oct = 12 days → Typical length (11–17 days).
Follicular phase = 26 Oct − 11 Oct + 1 = 16 days.
Cycle length = 8 Nov − 11 Oct = 28 days.

How to read the result

Where does your result fall?

Most luteal phases last about 11–17 days. The ASRM Practice Committee (2021) describes luteal phase deficiency as 10 days or fewer, but notes it has not been proven to cause infertility on its own and that no test diagnoses it reliably. A luteal phase continuing well beyond 17 days without a period may mean pregnancy.

Common mistakes

Counting the first day of the next period as part of the luteal phase.
Using the last positive LH test instead of the first.
Drawing conclusions from a single cycle.

Keep in mind: This is not a diagnosis. Discuss cycle concerns or difficulty conceiving with a doctor.

Frequently asked questions

What is a normal luteal phase length?

Roughly 11–17 days, often around 12–14.

Can a short luteal phase stop pregnancy?

It has been suggested, but the ASRM notes it is not proven to be an independent cause of infertility.

Which ovulation sign is most accurate?

Ultrasound tracking is the most precise; LH tests are practical at home; temperature only confirms ovulation afterwards.

Sources: ASRM Practice Committee, "Diagnosis and treatment of luteal phase deficiency: a committee opinion", Fertil Steril 2021;115(6):1416–1423

Checked 9 Nov 2026. ASRM Practice Committee, “Diagnosis and treatment of luteal phase deficiency: a committee opinion”, Fertil Steril 2021;115(6):1416–1423 (luteal phase ≤ 10 days; not proven to be an independent cause of infertility); ovulation-marker offsets (day after LH surge onset, day before BBT rise) and the 11–17 day typical range are standard values.

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