For regular cycles it's usually accurate within 1–3 days. The more consistent your cycle length, the better the prediction. If your cycle varies a lot, the later cycles in the forecast are less reliable.

Add your average cycle length to the first day of your last period. For a 28-day cycle starting on the 1st, your next period is expected around the 29th. The calculator does this for the next six cycles automatically.

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.