Counting the length of your menstrual cycle is the foundation of monitoring women’s health. It helps predict the next period, estimate fertile days and notice any irregularities. In this guide we explain how to measure the cycle correctly and what its typical values mean.
From when to when do you count the cycle?
The cycle is counted from the first day of your period (the first day of bleeding) to the day before the start of the next period. The most common mistake is to count from the end of one period to the start of the next — this gives an incorrect result. The first day of bleeding is always day one of the cycle.
Worked example
Suppose the last period started on 3 May and the next one on 31 May:
- We count the days from 3 May to 30 May inclusive (the day before the next period).
- From 3 to 31 May is a difference of 28 days in the calendar.
- The cycle length is therefore 28 days.
More simply: we subtract the start date of one period from the start date of the next.
What cycle length is normal?
A cycle lasting from 21 to 35 days is considered normal, although the classic figure quoted is 28 days. The period itself, that is the bleeding, usually lasts from 3 to 7 days. It is worth remembering that not all women have a 28-day cycle and small differences are completely natural.
What does cycle regularity mean?
A cycle is considered regular if its length varies only within a small range from month to month, for example between 27 and 29 days. Variations of a few days are the norm. We speak of irregularity when the differences between cycles are large and unpredictable.
When should you consult a doctor?
- A cycle regularly shorter than 21 days or longer than 35 days.
- Very irregular, unpredictable cycles.
- Absence of a period for several months (other than pregnancy).
- Very heavy or very painful bleeding.
Why track your cycle?
Regularly noting the dates of your periods lets you predict future bleeding, estimate the fertile window and spot changes that may indicate hormonal problems. Today apps and calculators make this easier, but the principle of counting remains the same.
The information in this article is general in nature and does not replace medical consultation. If your cycle gives you cause for concern, consult a gynaecologist.
Count your cycle length and predict your period: Menstrual cycle calculator →