Is my cycle length normal?
How to measure your cycle, why health bodies draw the line in slightly different places, and when a long or short cycle is worth raising with a doctor.
The short answer
A cycle runs from the first day of one period to the first day of the next. The U.S. Office on Women’s Health calls 24 to 38 days normal; other bodies use 21 to 35. A 28-day cycle is an average, not a rule. A single long cycle is common, but if your cycles are regularly longer than 38 days or shorter than 24, it is worth talking to a doctor.
If your cycle is 24 days, or 40, or different every month, you have probably wondered whether that is normal. This guide explains how to measure a cycle, why reputable sources give slightly different “normal” ranges, what a long or short cycle can mean, and when the length of your cycle is worth raising with a doctor.
How to measure your cycle
A cycle runs from the first day of one period to the first day of the next.[1] Count every day up to, but not including, the first day of your next period, and that number is your cycle length. One cycle tells you very little; three to six give you a picture of your own range.
What counts as normal
The 28-day cycle is an average, not a rule. In an analysis of more than 600,000 cycles from app users, the average was 29.3 days and only about 13% of cycles were exactly 28 days long.[2] The ranges that health bodies call normal differ a little:
| Source | Typical or normal range |
|---|---|
| U.S. Office on Women’s Health | 24 to 38 days[1] |
| StatPearls (NIH medical reference) | 24 to 38 days[3] |
| U.S. National Institutes of Health (NICHD) | 21 to 35 days[4] |
| NHS | Periods count as irregular when the gap is under 21 or over 35 days[5] |
The difference is mostly about where the line is drawn, not a disagreement about bodies. A cycle that falls inside the narrower 21 to 35 day range is normal by every definition above. A cycle of 36 to 38 days is normal by some definitions and on the long side by others.
Is a 40-day cycle normal?
A 40-day cycle is longer than all of the ranges above. A single long cycle can have ordinary causes; the NHS lists stress, sudden weight loss, doing a lot of exercise and some types of hormonal contraception among the reasons a period can be late.[6] If your cycles are regularly longer than 38 days, the Office on Women’s Health suggests talking to a doctor or nurse.[1] The exception is the first few years after periods start, when cycles longer than 38 days are common.[1]
Irregular periods are also one of the signs the NHS lists for polyendocrine metabolic ovarian syndrome (PMOS, previously called PCOS), though long cycles have many other possible causes; there is more in PCOS is now called PMOS.[7]
Is a 24-day cycle normal?
Yes. A 24-day cycle is inside every range in the table. Cycles shorter than 24 days are on the short side by the Office on Women’s Health’s definition, and it suggests talking to a doctor or nurse if your cycles are regularly shorter than that.[1] Cycles also tend to get slightly shorter with age; in the large app-user analysis, average cycle length fell by about 0.18 days a year between the ages of 25 and 45.[2]
Does it matter if my cycle length changes?
Some variation is normal for everyone. Most of it comes from the first half of the cycle, the time before ovulation, which can stretch or shrink from month to month while the second half stays fairly steady.[2] What is worth noticing is a change in your own pattern: cycles that used to be steady and now swing widely, or that get much longer or shorter and stay that way.
How Bloom works out your cycle length
Bloom predicts from the median of your last six cycles, so one unusual month doesn’t drag every prediction with it. It keeps any cycle from 15 to 90 days long in your record instead of discarding long ones as mistakes, and after three logged cycles it shows you the rhythm it has actually observed, including your average cycle and period length. That gives you real numbers to compare with the ranges above, or to show a doctor. Bloom can’t tell you why your cycle is the length it is, and it is not a fertility tool or contraception. For cycles that won’t sit still, see a period tracker for irregular periods.
Tips for getting a reliable number
- Log the first day of every period, even if you are not sure it has really started; you can correct it later.
- Look at several cycles, not one. Your own range matters more than any single month.
- Note big changes such as illness, travel, starting or stopping hormonal contraception, or a new training plan, so you can tell an outlier from a new pattern.
When to see a doctor
Talk to a doctor, nurse or midwife if:
- your cycles are regularly longer than 38 days or shorter than 24 days
- your usual pattern changes and stays changed
- you have had no period for three months and you are not pregnant
- you are trying to get pregnant and your cycles are irregular
If you are in severe pain, bleeding very heavily, or think you could be having an emergency, get urgent medical help.
Questions people ask
Is a 40-day cycle normal?
It is longer than the ranges health bodies call normal. One 40-day cycle can happen for ordinary reasons such as stress or illness, but if your cycles are regularly longer than 38 days, talk to a doctor or nurse. In the first few years after periods start, long cycles are common.
What is a normal menstrual cycle length?
The U.S. Office on Women’s Health says 24 to 38 days; the NIH and NHS use 21 to 35 days as the typical range. A cycle inside 21 to 35 days is normal by every one of these definitions.
Is it normal for my cycle length to change every month?
Some change is normal for everyone, and most of it comes from the part of the cycle before ovulation. A lasting change in your own pattern is the thing worth noticing.
Bloom is a wellness and planning app. It is not a medical device, it does not diagnose or treat any condition, and it is not a method of contraception. This page is general information, not medical advice. If something about your cycle worries you, talk to a doctor, nurse or midwife.
Sources
- Office on Women’s Health, U.S. Department of Health and Human Services. Your menstrual cycle.
- npj Digital Medicine (Bull JR et al., 2019), via PubMed Central. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles.
- StatPearls, NCBI Bookshelf (NIH). Physiology, menstrual cycle.
- Eunice Kennedy Shriver National Institute of Child Health and Human Development (NIH). What are menstrual irregularities?.
- NHS. Irregular periods.
- NHS. Stopped or missed periods.
- NHS. Polyendocrine metabolic ovarian syndrome (PMOS).