Menstrual Period Cycle Length Calculator: How to Track Your Cycle Time

Menstrual cycle length is the number of days from the start of one period to the start of the next. For example, if your period starts on June 1 and your next period starts on June 29, your cycle length is 28 days.

Many people think every cycle should be exactly 28 days, but that is not true. Cycle length can vary from person to person and even from month to month.

Use this simple formula: Cycle length = First day of next period − First day of last period.

Example: Last period June 1, next period June 29 = 28 days. July 2 to July 31 = 29 days. August 5 to September 1 = 27 days.

A normal cycle can vary. ACOG says a normal menstrual cycle is typically between 21 and 35 days, and a normal period generally lasts up to 7 days.

The Office on Women's Health gives a slightly broader range, saying periods are usually considered regular if they come every 24 to 38 days.

Tracking your period start date helps estimate your next period. Tracking your cycle length shows whether your cycle is regular. Tracking bleeding days helps you notice changes.

Tracking symptoms helps you connect cramps, mood, energy, or headaches to your cycle. Tracking missed or late periods helps you know when to test or contact a clinician.

Start by recording the first day of each period. Then count the days until your next period begins. After tracking for three to six months, you can estimate your average cycle length.

Example: January 28 days, February 30 days, March 29 days — average = 29 days.

Consider speaking with a healthcare professional if your cycles are suddenly much shorter or longer than usual, bleeding is very heavy, periods stop unexpectedly, or you have severe pain. Mayo Clinic notes that what is 'typical' can vary, but major changes are worth paying attention to.