Luteal phase calculator
The luteal phase is the stretch from ovulation to the next period — often ~12–14 days when textbooks behave. Enter both dates to see length. This counts days; it does not diagnose “luteal phase defect.” Estimate only — not medical advice.
Enter a date to see an estimate.
What this page does
Charting people want a day count, not another purple notification. Enter ovulation and period start; read luteal days.
Pregnancy stops the “next period” input — do not use a missed period as luteal end.
When to use
Charting; comparing cycles; noticing a repeatedly short count before a clinic visit.
When not to
Interpreting progesterone labs; fertility treatment decisions.
Assumptions
Browser-only planning arithmetic. Defaults are disclosed on the page. Ultrasound and clinic policy override this tool.
Examples
Ovulation May 10 · period May 24 → 14 days.
Ovulation May 10 · period May 21 → 11 days (shorter than textbook 14 — still not a diagnosis).
Gotchas
Clinic dating after ultrasound can differ from browser math — the clinic wins for medical decisions.
Wrong input dates (retrieval vs transfer, spotting vs true LMP) slide every result.
Estimate only — not medical advice.
How this is calculated
The math stays in your browser. Enter the dates you actually know, read the estimate, and treat clinic ultrasound dating as the authority when it disagrees.
Nothing here is medical advice, a diagnosis, or a substitute for prenatal care.
- Enter the date(s) or numbers this page asks for.
- Check the assumptions under the result and on Methodology.
- Compare with your clinic portal if you have one.
- Use related tools if you only have a different kind of date.
- Confirm medical decisions with your clinician.
lutealDays = periodStart − ovulation.
Related calculators
Full cycle: menstrual cycle calculator. Next period: next period calculator. Rough ovulation from cycle length: ovulation-from-cycle page.
See also: menstrual cycle calculator, next period calculator, and ovulation from cycle calculator.
FAQ
- What is a normal luteal phase?
- Often cited around 11–17 days; your clinician interprets patterns.
- Can this diagnose a problem?
- No.
- Does a short luteal phase prevent pregnancy?
- Medical question — out of scope.
- How is this different from cycle length?
- Cycle length is period-to-period; luteal is ovulation-to-period.
- Mis-marked ovulation?
- Makes luteal length junk.
- Is this medical advice?
- Estimate only — not medical advice. Confirm dating, testing, and medical decisions with your clinic.