How do I calculate my cycle length?

Bottom lineCalculate your cycle length by counting from day 1 of one period (the first day of full flow) to day 1 of the next; a typical cycle is 21 to 35 days and normally varies by a few days, so track several cycles for a reliable average, which lets you predict your period and estimate ovulation.

Your cycle length is the number of days from the start of one period to the start of the next. Counting it is the foundation of predicting periods and your fertile window.

How to count

1. Mark day 1 as the first day of full flow (not spotting) of one period
2. Keep counting each day until your next period's day 1
3. The total number of days is your cycle length

For example, if your period started on the 3rd and the next on the 31st, your cycle is 28 days.

What's normal

• A typical cycle is 21 to 35 days in adults
• It's normal for length to vary by a few days month to month
• Track several cycles and take an average for a reliable figure

Why it matters

Your average cycle length lets you predict your next period and estimate ovulation (often around 12 to 16 days before your next period starts). Big swings in length can signal irregular cycles worth discussing with a clinician.

Femora calculates your average cycle length automatically from the dates you log and uses it to predict your period.

Try it: menstrual cycle calculator