Pregnancy calculator
Luteal Phase Calculator
Measure the second half of your cycle. Enter when you ovulated — and your next period, if it's started — to find your luteal phase length, see whether it's typical, and track your days past ovulation.
Last reviewed 20 May 2026
Optional — enables cycle-day detail
From OPK, BBT, or tracking
Optional — gives an exact luteal phase
Luteal phase length
Add next period
Enter your next period's first day for an exact figure
Days past ovulation (DPO)
1 DPO
Counting from your ovulation date
Estimated next period
9 June 2026
If luteal phase ≈ 14 days
Luteal phase length varies between cycles. One measurement is a data point, not a diagnosis. Charting several cycles gives a far more reliable picture. Medical disclaimer.
How to use this calculator
- Enter the date you ovulated (from a positive OPK or your BBT shift).
- Optionally enter the first day of your last period — this adds your follicular-phase length and ovulation cycle day.
- If your next period has already started, enter its first day for an exact luteal phase length. If not, you'll still see your current DPO and an estimated next period.
Background: the science
Two phases, two behaviours
The menstrual cycle has a follicular phase (period start to ovulation) and a luteal phase (ovulation to the next period). The follicular phase is the variable one — it lengthens or shortens cycle to cycle and person to person. The luteal phase is the steady one, usually holding within a 12-16 day band for a given individual.
What drives the luteal phase
After ovulation, the collapsed follicle transforms into the corpus luteum, which secretes progesterone. Progesterone matures the uterine lining into a state ready to receive an embryo. With no pregnancy, the corpus luteum has a fixed lifespan of roughly two weeks — it regresses, progesterone drops, and menstruation begins. That fixed corpus-luteum lifespan is exactly why the luteal phase is so consistent.
Why luteal length matters for conception
Implantation typically occurs 6-12 days past ovulation. If the luteal phase is very short (under ~10 days), the lining may begin to shed before or just as an embryo is trying to establish itself. That's the mechanism behind concern over a 'short luteal phase'.
How to interpret your result
- 12-16 days: typical — ample time for implantation.
- 10-11 days: borderline — note it, and watch the pattern over future cycles.
- Under 10 days: short luteal phase — one cycle is not a diagnosis, but a repeated pattern is worth raising with your provider.
- Over 16 days: unusual — if your period is overdue, consider a pregnancy test.
Always interpret luteal length across several charted cycles, not from one measurement.
What this calculator does NOT do
- It does not diagnose a luteal phase defect or any fertility condition.
- It does not measure progesterone — only a blood test can.
- It does not replace evaluation by a provider or fertility specialist.
Sources
- American Society for Reproductive Medicine — Practice Committee opinion on the clinical relevance of luteal phase deficiency.
- Lenton EA, et al. Normal variation in the length of the luteal phase of the menstrual cycle. British Journal of Obstetrics & Gynaecology. 1984.
- ACOG / ASRM — guidance on optimizing natural fertility.
See our methodology. Not a substitute for medical advice — read the medical disclaimer.