Cycle Length Checker

Your menstrual cycle length is measured from the first day of one period to the first day of the next. ACOG defines a normal cycle length as 21 to 35 days for adults, with the average being about 28 days. However, having a cycle that is consistently 25 days or 33 days is just as normal as 28, as long as it is regular for you. What matters most is consistency rather than hitting a specific number.

Cycle variation is the difference between your shortest and longest cycle over a given period. ACOG considers a variation of up to 7 to 9 days to be within the normal range. Greater variation may indicate irregular ovulation, which can be caused by conditions such as polycystic ovary syndrome (PCOS), thyroid dysfunction, hyperprolactinemia, or perimenopause. Lifestyle factors including stress, significant weight changes, excessive exercise, and shift work can also affect cycle regularity.

Tracking your cycle lengths over time is one of the most valuable pieces of health information you can bring to a medical appointment. It helps your healthcare provider identify patterns and decide whether further evaluation is needed. This tool performs a basic statistical analysis of your reported dates and provides general guidance. It is not a diagnostic tool and does not replace evaluation by your OB-GYN or healthcare provider. If your cycles are consistently shorter than 21 days, longer than 35 days, or if you experience significant irregularity, seek professional medical advice.