What is a normal cycle length?
The American College of Obstetricians and Gynecologists (ACOG) and the World Health Organization (WHO) define a normal menstrual cycle as 21–35 days, measured from the first day of one period to the first day of the next. Cycles outside this range for three or more consecutive months warrant evaluation.
A 2019 study analysing over 600,000 menstrual cycles from the Natural Cycles app dataset (Symul et al.) found that the median cycle length was 29 days, with most cycles falling between 25 and 30 days. Only about 13% of cycles were exactly 28 days.
Quick reference: Cycle 21–35 days = normal. Period 2–7 days = normal. Blood loss 30–80 ml = normal.
Is a 21-day cycle normal?
Yes. A 21-day cycle is at the lower end of the normal range. On a 21-day cycle, ovulation typically occurs around day 7 (21 − 14 = 7). Very short cycles — particularly those consistently under 21 days — may indicate a shortened luteal phase, which can affect fertility, and are worth discussing with a doctor.
Is a 35-day cycle normal?
Yes. A 35-day cycle is at the upper boundary of the normal range. On a 35-day cycle, ovulation typically occurs around day 21 (35 − 14 = 21). Cycles consistently longer than 35 days may indicate delayed or infrequent ovulation, which can be associated with PCOS. See our irregular periods guide.
What is a normal period duration?
A normal period lasts 2–7 days. Most people's periods last 4–5 days on average. Periods lasting fewer than 2 days (hypomenorrhoea) or more than 7 days (menorrhagia) may indicate a hormonal imbalance or uterine condition and are worth investigating.
What is a normal amount of bleeding?
Normal menstrual blood loss is 30–80 ml per cycle (approximately 2–6 tablespoons). Heavy menstrual bleeding (menorrhagia) is defined as loss exceeding 80 ml per cycle — practically, this often means soaking through a pad or tampon hourly for several consecutive hours, or needing to change protection at night.
Period flow intensity doesn't have to be measured precisely. In practice, if periods are heavy enough to significantly interfere with daily life, it's worth discussing with a doctor regardless of volume.
How much does cycle length vary from month to month?
Some month-to-month variation is normal. The Symul et al. dataset found that the median cycle-to-cycle variation is approximately 3–4 days. Variation of up to 7 days is considered within the normal range by ACOG. If your cycle varies by more than 7–9 days month to month, that meets the clinical definition of irregular cycles.