Cycles are considered irregular if the gap between periods is shorter than 21 days or longer than 35 days, or if the length varies by more than about 7–9 days from one cycle to the next. Occasional variation is normal; a consistent pattern of irregularity is worth investigating.

Log the first day of every period for at least 3–6 months. Use your shortest and longest cycles to define a prediction window rather than a single date, and combine with ovulation signs (LH kits, basal body temperature) if you're trying to conceive.

Common causes include PCOS, thyroid disorders, significant weight change, excessive exercise, chronic stress, perimenopause, and certain medications. A gynaecologist can identify the cause with a few hormone and thyroid tests.

Enter the first day of your last period and your average cycle length (28 days is typical, but 21–35 days is also normal). The calculator counts forward to predict your next 6 period start dates, plus the ovulation day and fertile window within each cycle. The prediction assumes a roughly regular cycle.

Any cycle between 21 and 35 days is considered normal. The 28-day cycle is a popular average, not a universal standard. What matters more is consistency — if your cycle length varies by less than 7–9 days month to month, you're considered regular.

It gives a rough estimate, but predictions are less accurate when cycles vary. With PCOS, post-pregnancy, perimenopause, or chronic stress, cycles often shift by 7+ days. For irregular cycles, track 3–6 months of actual dates to find your personal average, and don't rely on calendar predictions for contraception.

No — it's a calendar tool. If you have heavy bleeding, severe pain, missed periods for 3+ months (when not pregnant), bleeding after menopause, or any new symptoms, see a gynaecologist. The calculator helps you track your cycle; it doesn't diagnose conditions.