What is a normal menstrual cycle length?

Bottom lineA menstrual cycle is measured from the first day of one period to the first day of the next, and a normal length is 21 to 35 days (averaging about 28), with a few days' variation month to month being normal; see a doctor for cycles consistently under 21 or over 35 days, wide swings in length, or three or more missed periods when not pregnant.

A menstrual cycle is counted from the first day of one period to the first day of the next. A normal cycle length is 21 to 35 days, with the average around 28.

What's typical

• 21 to 35 days is the healthy adult range
• Teens often have longer, more variable cycles for the first couple of years
• Cycles can shift over time with age, stress, and life stage
• A small variation of a few days month to month is normal

What's worth checking

• Cycles consistently shorter than 21 days
• Cycles consistently longer than 35 days
• Wide swings in length from cycle to cycle
• Missed periods (3 or more) when not pregnant

These can reflect PCOS/PMOS, thyroid issues, perimenopause, stress, or other hormonal factors.

Why it matters

Your cycle length sets the timing of ovulation and your fertile window, so knowing it helps with both conceiving and avoiding pregnancy.

Find your average with the Menstrual Cycle Calculator, and see the four phases.

Femora calculates your average cycle length from your logs and sharpens its predictions over time.