Cycle length
Days from the first day of one period to the day before the next; 21 to 35 days is the normal range (NHS).
What does cycle length mean?
Cycle length: Days from the first day of one period to the day before the next; 21 to 35 days is the normal range (NHS).
How it is used in the Splash library
Tracking three signs daily: Basal body temperature, cervical mucus and cycle length
From “Fertility awareness methods: the honest guide”That total is your cycle length.
From “What actually is a menstrual cycle?”In the app study, average cycle length got a little shorter with age in adulthood, then changes again in perimenopause.
From “What is a normal cycle length?”Natural variation: in a large real-world study, the same person's cycle length varied from month to month, and ovulation day varied a lot between people.
From “Why your predictions shift”In a study of over 600,000 cycles, average cycle length got slightly shorter with age (about 3 days shorter between the youngest and oldest groups), and the spread between one person's own cycles also got smaller as age rose through the late 20s and 30s.
From “Your cycle through life”
Cycle length explained
Cycle length is the number of days from the first day of one period to the day before the next one starts. Day 1 is the first day of proper bleeding, not spotting.
Tracking your cycle length over a few months shows your average and how much it varies, which is useful for predicting periods and noticing changes.
What's typical
The NHS describes 21 to 35 days as normal, with 28 as an average. FIGO uses 24 to 38 days for adults. A large study of over 600,000 cycles found an average of about 29 days (Bull, 2019).
Cycles that vary by up to 7 to 9 days are still considered regular in adults (FIGO). In the first few years after periods start, 21 to 45 days is common (ACOG).
Stress, illness, travel, weight change and some medicines can shift a cycle.
When to talk to a GP, nurse or pharmacist
- Your cycles are often shorter than 21 days or longer than 35 days.
- The length varies by more than about 20 days from month to month.
- Your periods stop for 3 months or more, or your usual pattern changes suddenly.
In the UK you can call NHS 111 for advice at any time. In an emergency call 999 (UK) or 911 (US). This is general information, not medical advice.
Where it comes up
Sources
Sources checked October 2026.
See it in your own cycle.
Log a few periods and Splash shows your patterns, privately, on your phone. Free, no account.