How the dates are estimated
Your next period is projected by adding your average cycle length to your last start date. Ovulation typically happens about 14 days before the next period begins (not 14 days after the last one — a common mix-up), and the fertile window spans roughly the 5 days before ovulation plus ovulation day itself, because sperm survive several days while the egg lives about 24 hours.
What counts as a normal cycle?
Anywhere from 21 to 35 days is normal for adults, and cycles can vary a few days month to month — stress, travel, illness and weight change all shift ovulation. Bleeding of 2–7 days is typical. Tracking 3–6 cycles gives a far better picture than any single month.
Irregular cycles and PCOS awareness
If your cycles are consistently longer than 35 days, shorter than 21, or unpredictable — especially alongside acne, excess facial/body hair growth, thinning scalp hair, or difficulty with weight — it's worth discussing PCOS (polycystic ovary syndrome) with a doctor. PCOS affects roughly 1 in 10 women and is diagnosed with an exam, blood tests and often an ultrasound — no online tool can do that. It is very manageable once identified, and irregular cycles are the single most common early clue.
Important honesty about accuracy
Calendar estimates are planning aids, not guarantees — ovulation moves. This tool must not be used as contraception; fertility-awareness methods that actually work require daily temperature and cervical-mucus tracking with proper training. For conception timing, the fertile-window estimate is a good starting point, refined by ovulation test kits.
Sources & references
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med. 1995;333:1517–1521.
- Bull JR, et al. Real-world menstrual cycle characteristics of more than 600,000 cycles. NPJ Digit Med. 2019;2:83.
- Teede HJ, et al. International evidence-based guideline for the assessment and management of PCOS. 2023.