Luteal Phase Calculator

Calculate luteal phase length from cycle data, with ovulation date, fertile window, next period estimate, and short-phase guidance.

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First day of last period

Cycle length (days)

Ovulation day (optional)

Day of cycle (day 1 = period start)

Enter your last period date to calculate your luteal phase.

Health — Pregnancy

Luteal Phase Calculator

The luteal phase is the second half of the menstrual cycle — the period between ovulation and the start of the next period. Its length affects the likelihood of successful implantation and early pregnancy. This calculator estimates your luteal phase from your cycle data.

About this calculator

Reviewed March 2026

Methodology

Luteal phase = cycleLengthDays − ovulationDayOfCycle. Default ovulationDayOfCycle = cycleLength − 14. Fertile window: ovulationDate −5 to +1. Short < 10 days, normal 10–16 days, long > 16 days, consistent with standard reproductive medicine references.

Limitations

Ovulation day estimation assumes a fixed relationship between LMP and ovulation (luteal phase length ~14 days), which varies across individuals.
Short luteal phase diagnosis requires clinical assessment, not cycle calculation alone.
Cycle irregularities reduce the accuracy of all estimates.

Disclaimer

This calculator provides estimates for educational use. Fertility concerns should be discussed with a qualified healthcare professional or fertility specialist.

See our calculation methodology and editorial standards for how this estimate is produced.

What is the luteal phase?

The luteal phase begins at ovulation and ends when the next period starts. During this time, the corpus luteum produces progesterone, which thickens the uterine lining in preparation for potential implantation. Luteal phase length equals cycle length minus ovulation day. For a 28-day cycle with ovulation on day 14, the luteal phase is 14 days.

Luteal phase = Cycle length − Ovulation day

Ovulation day is 1-indexed from the first day of the period. Default assumes ovulation on day (cycle length − 14).

Short and long luteal phase

A normal luteal phase is 10–16 days. A phase shorter than 10 days (sometimes called luteal phase defect) may indicate insufficient progesterone support, which can contribute to difficulty conceiving or early pregnancy loss. A phase longer than 16 days is less common and may warrant investigation. The luteal phase is typically the more consistent half of the cycle — the follicular phase (before ovulation) varies more between cycles and individuals.

Fertile window and cycle tracking

The fertile window spans approximately 5 days before ovulation through 1 day after, reflecting the combined lifespan of sperm (3–5 days) and the egg (12–24 hours). Tracking luteal phase length over multiple cycles using basal body temperature (BBT) or ovulation predictor kits (OPKs) provides more accurate ovulation timing than the standard formula.

Frequently asked questions

Why does luteal phase length matter for fertility?

The luteal phase must be long enough to allow implantation and early embryo development. A phase shorter than 10 days may not provide sufficient progesterone support, though the evidence on whether this alone causes infertility is still debated.

Can I lengthen my luteal phase?

In clinical settings, progesterone supplementation can be used. Some people find lifestyle improvements (reducing intense exercise, managing stress, ensuring adequate nutrition) help, but evidence is limited. Speak with your GP or fertility specialist if you have concerns.

How do I find my ovulation day?

Ovulation is estimated to occur approximately 14 days before the end of your cycle. For a 28-day cycle this is day 14; for a 35-day cycle, day 21. BBT charts and ovulation predictor kits provide more accurate timing.

Luteal Phase Calculator

Luteal Phase Calculator

What is the luteal phase?

Short and long luteal phase

Fertile window and cycle tracking

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