How long is a normal menstrual cycle?
A normal menstrual cycle in adults lasts anywhere from 21 to 35 days, counted from the first day of one period to the day before the next. The often-quoted 28 days is just an average: in a study of over 600,000 cycles, the mean was 29.3 days, and only a minority were exactly 28. Teens can range from 21 to 45 days.
How to count your cycle length
Day 1 is the first day of real bleeding, not spotting. Your cycle ends the day before your next period starts. So if your period started on 1 March and the next one on 30 March, your cycle was 29 days. Our cycle length calculator averages a few cycles for you.
What's normal at different ages
| Life stage | Typical cycle length | Notes |
|---|---|---|
| First few years after your first period | 21 to 45 days | Cycles often vary a lot while ovulation settles |
| Adults, roughly 20 to 40 | 21 to 35 days | Average around 29 days; small monthly changes are normal |
| Late 30s and 40s | Often a little shorter | Average length drops slightly with age |
| Perimenopause | Shorter, longer or skipped | Cycles become less predictable before menopause |
In a large app-based study, average cycle length fell by about 0.18 days per year between ages 25 and 45, mostly because the first half of the cycle got shorter.
How much variation is normal?
Very few people have the same cycle length every month. A difference of up to about 7 days between your shortest and longest cycle is usually considered regular. A bigger spread, or cycles that keep falling outside 21 to 35 days, is worth a closer look.
Most of the variation comes from the first half of the cycle, the follicular phase, which can stretch when ovulation is delayed by stress, illness or travel. The luteal phase after ovulation is steadier, usually 10 to 17 days. Our guide to how long each phase lasts goes deeper.
Why your cycle length matters for mood
Cycle length decides when your harder days land. With a 35-day cycle, PMS arrives later than a 28-day calendar suggests, which can make low or irritable days feel random. Knowing your own length turns them into something you can plan around.
Lulu learns your real average and range from the cycles you log, then shows where your harder and easier days usually fall.
When to talk to a doctor
See a doctor if your cycles are regularly shorter than 21 days or longer than 35 (45 for teens), if the gap between your shortest and longest cycles is more than 7 to 9 days, if your periods stop for 3 months or more, or if you bleed between periods or after sex.
Frequently asked questions
Is a 21-day cycle normal?
Yes, 21 days is at the short end of normal for adults. Cycles regularly shorter than that are worth checking. See is a 21-day cycle normal.
Is a 35-day cycle normal?
Yes, 35 days is at the long end of normal. Longer cycles can mean you ovulate late or rarely. See is a 35-day cycle normal.
Why does my cycle length change every month?
Mostly because ovulation happens on a slightly different day each month. Stress, sleep, illness and travel can all move it.
Does cycle length include the days of bleeding?
Yes. Count from the first day of one period to the day before the next period, bleeding days included.
Sources
- NHS: Periods
- ACOG: Menstruation in Girls and Adolescents: Using the Menstrual Cycle as a Vital Sign
- Bull JR et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. npj Digital Medicine (2019)
- Cleveland Clinic: Menstrual cycle
This page is for information only and isn鈥檛 medical advice. If something worries you, please talk to a doctor.
Keep reading
Cycle length calculator: is my cycle normal?Free cycle length calculator: enter the start dates of your last few periods to get your average cycle length and see whether your cycle counts as regular.
How long is each phase of the menstrual cycle?How long each phase of the menstrual cycle lasts: period, follicular, ovulation and luteal phase lengths, what's normal, and why your cycle length changes.
How long does a period last?How long does a period last? Usually 2 to 7 days, about 5 on average. What changes it, how much blood is normal, and when a long period needs a doctor.
Why is my period irregular?Why is my period irregular? Stress, weight change, PCOS, thyroid issues, contraception, age and perimenopause. What counts as irregular and when to get help.Short menstrual cycle: what it means if your period comes every 2 to 3 weeksShort menstrual cycle under 21 days? Why periods can come every two to three weeks, from perimenopause to thyroid issues, what helps and when to check.Long menstrual cycle: what it means if your cycle is over 35 daysLong menstrual cycle over 35 days? Why ovulation comes late, common causes like PCOS, stress and thyroid, what a 40 or 45 day cycle means, and what helps.