Enter your ovulation date and the first day of your next period. The calculator returns your luteal phase length in days and classifies it as short, normal, or long.
How the result is calculated
- Inputs: the calendar date of ovulation and the first day of the next menstrual period.
- Length (days): the number of whole days from the ovulation date up to (but not including) the first day of the next period. This is the standard definition used in clinical and research settings.
- Validation: the tool checks that both dates are valid calendar dates and that the period date occurs after the ovulation date.
Classification bands
- Short: ≤ 9 days
- Normal: 10–14 days (most people fall here)
- Long: ≥ 15 days
These bands reflect common thresholds used in clinical reviews and epidemiologic studies of menstrual cycle phases.
Assumptions & notes
- Ovulation dating: accuracy depends on how ovulation was identified (e.g., ultrasound, serum LH/progesterone, urinary LH surge, cervical mucus, or basal temperature).
- Cycle variability: occasional short or long luteal phases can occur in healthy cycles. A single short phase does not diagnose a condition.
- No hormonal context: the calculator does not interpret progesterone levels or luteal phase adequacy; it only measures calendar length.
- Pregnancy cycles: if pregnancy occurs, a next period date may not exist; in that case this tool is not applicable.
- American Society for Reproductive Medicine (ASRM). Evaluation and treatment of luteal phase deficiency (Committee Opinion, 2021).
- StatPearls. Luteal Phase Deficiency (NCBI Bookshelf, regularly updated).
- Creinin MD, Keverline S, Meyn LA. “How regular is regular? An analysis of menstrual cycle regularity.” Obstet Gynecol. 2004;104(5 Pt 1):953–958. Journal page.
- Fehring RJ, Schneider M, Raviele K. “Variability in normal menstrual cycle length and luteal phase length.” J Obstet Gynecol Neonatal Nurs. 2006;35(3):376–384. PubMed.
How to use this calculator
- Enter the ovulation date (from OPK, BBT shift, or ultrasound).
- Enter the first day of your next period.
- Read your luteal phase length in days and the classification (short, normal, or long).
Spotting can make start dates tricky. Use the first day of full flow. If you’re unsure of ovulation, estimate it with OPK or BBT tools below.
Last updated: September 21, 2025
Frequently asked questions
What is the luteal phase?
The luteal phase is the time from ovulation to the start of your next period. It is usually stable for an individual compared with the follicular phase.
What counts as short, normal, or long?
Short is often defined as ≤10 days, normal about 11–16 days, and long as ≥17 days. Clinics may use slightly different cutoffs.
How do I know my ovulation date?
Use an LH surge from an OPK, a sustained BBT rise, or ultrasound monitoring. See OPK → Ovulation and BBT Ovulation to estimate timing.
I had spotting—what is day 1 of my period?
Use the first day of full flow, not light pre-period spotting. If flow started late at night, use the next morning as day 1 for consistency.
Does a short luteal phase affect implantation?
Some short luteal phases are normal. If your luteal length is consistently ≤10 days with difficulty conceiving, discuss it with your clinician. Tracking progesterone can add context; see the Progesterone Levels tool.
Can luteal length change cycle to cycle?
Yes, by a day or two. Larger swings often reflect uncertain ovulation timing or atypical cycles. Cross-check with OPK or BBT methods.
When should I take a pregnancy test?
Testing near your expected period reduces false negatives. For the best day to test by DPO, use the Pregnancy Test Accuracy calculator.