Menstrual cycle calculator
Enter last period start and average length to see next period, a rough ovulation neighborhood, and a fertile-window sketch. Arithmetic — not another purple notification engine. Estimate only — not medical advice.
Enter a date to see an estimate.
What this page does
Built for people who want cycle math before a clinic visit, not a social tracker. Rough ovulation ≈ LMP + (cycleLength − luteal), default luteal 14.
Does not diagnose PCOS or store a cloud diary.
When to use
Establishing averages; figuring rough ovulation without OPKs.
When not to
Medical diagnosis. Emergency contraception timing advice.
Assumptions
Browser-only planning arithmetic. Defaults are disclosed on the page. Ultrasound and clinic policy override this tool.
Examples
LMP Jul 5, 2026 · cycle 28 → Rough ovulation Jul 19 · next Aug 2.
LMP Jul 5, 2026 · cycle 35 → Rough ovulation Jul 26 · next Aug 9.
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.
next = LMP + cycle; ovulation ≈ LMP + (cycle − luteal); fertile ≈ ovulation−5 … ovulation.
Related calculators
Next period focus: next period calculator. Luteal length: luteal phase calculator. Cycle-framed ovulation: ovulation-from-cycle page.
See also: period calculator, next period calculator, and ovulation from cycle calculator.
FAQ
- What is a normal cycle length?
- Often 21–35 days in broad teaching ranges; individuals vary.
- How do I calculate cycle day?
- Count from day 1 of the period.
- Ovulation from cycle length alone?
- Roughly — OPKs/temps are tighter.
- Does this store my cycles?
- Browser math; not a cloud diary.
- Birth control?
- Withdrawal patterns differ — not the intended use.
- Is this medical advice?
- Estimate only — not medical advice. Confirm dating, testing, and medical decisions with your clinic.