Menstrual cycle shortest and longest cycle lengths is considered very irregular. The average menstrual cycle lasts 28 days. The variability of menstrual cycle lengths is highest for women under 25 years of age and is lowest, that is, most regular, for ages 25 to 39. Subsequently, the variability increases slightly for women aged 40 to 44. The luteal phase of the menstrual cycle is about the same length in most individuals (mean 14.13 days, standard deviation 1.41 days) whereas the follicular phase tends to show much more variability (log-normally distributed with 95% of individuals having follicular phases between 10.3 and 16.3 days).
Menstrual cup find a larger cup uncomfortable. Length also needs to be considered: if a female's cervix sits particularly low, she may want to use a shorter cup. Capacity is important to women who have a heavier flow. The average menstrual cup holds around 20 ml. Some cups are designed to be larger and hold 37–42 ml. However, all cups currently available have a larger capacity than a regular tampon, which is 10–12 ml. A final consideration in selecting a menstrual cup is firmness or flexibility. Some companies offer a range of firmness levels in their cups. A firmer cup pops open