Cycle length calculator: is my cycle normal?
Your average cycle: 28.7 days
Cycles: 29, 28, 29 days. That's a regular pattern (cycles within 7 days of each other).
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Your cycle length is the number of days from the first day of one period to the first day of the next. Enter the start dates of at least two periods, ideally four or more, and the calculator lists each cycle, gives your average and tells you whether your cycles fall within 7 days of each other, the usual definition of regular.
How to use the cycle length calculator
- Find the first day of bleeding for your recent periods in a diary, calendar or old app.
- Enter them in the date boxes. Order doesn't matter; the calculator sorts them.
- Add more with the button. Six months of dates gives a much truer picture than two.
- Read your average cycle length, each individual cycle, and whether your pattern counts as regular.
Your dates are never sent or stored. Close the page and they're gone.
What's a normal cycle length?
For adults, a cycle anywhere from 21 to 35 days is normal. In the first few years after periods start, cycles of 21 to 45 days are common, and they often get longer and more variable again in perimenopause. The famous 28 days is just an average: in a study of more than 600,000 cycles, only about 13% were exactly 28 days, and the average was 29.3.
| Your average | What it usually means |
|---|---|
| Under 21 days | Short. Worth mentioning to a doctor, especially if it's new. See short cycle. |
| 21 to 35 days | Within the normal adult range. |
| 36 to 45 days | Long for an adult, common in teens. See long cycle. |
| Over 45 days, or 3+ months with no period | See a doctor. Pregnancy, PCOS, thyroid issues and other causes are worth checking. |
Regular or irregular?
Cycle length varying a little is completely normal. The calculator calls your cycles regular when the shortest and longest are within 7 days of each other, which matches common clinical guidance (some doctors allow 7 to 9 days). Bigger swings count as irregular. Irregular doesn't automatically mean something is wrong, but it's worth tracking and, if it continues, mentioning to a doctor.
Most of the variation comes from when you ovulate. The luteal phase after ovulation is fairly steady at around 12 to 14 days, so a long cycle almost always means a late ovulation. Stress, illness, travel, big changes in exercise or eating, and coming off the pill are common reasons for a one-off long cycle.
What your cycle length tells you
- Your next period: add the average to your last start date, or use the period calculator.
- Your likely ovulation day: subtract 14 from your cycle length. The ovulation calculator does it for you.
- When PMS usually starts: most premenstrual symptoms begin 5 to 10 days before your period, so a steady cycle length makes your harder days easier to see coming.
- Changes over time: a cycle that's suddenly shorter or longer than your usual is useful information for you and your doctor.
Lulu recalculates your cycle length every time you log a period, uses your real range instead of assuming 28 days, and links each phase to how you usually feel.
When to talk to a doctor
See a doctor if your cycles are regularly shorter than 21 days or longer than 35, if they vary by more than 7 to 9 days for several months, if you go 3 months without a period, or if irregular cycles come with excess hair growth, acne, heavy bleeding or pelvic pain.
Frequently asked questions
How do you calculate cycle length?
Count from the first day of one period to the day before the next period starts. The number of days is your cycle length.
Is a 21-day cycle normal?
It's at the short end of normal for adults. If it's new or shorter, mention it to a doctor. See is a 21-day cycle normal.
Is a 35-day cycle normal?
Yes, it's at the long end of the adult range. See is a 35-day cycle normal.
How many cycles do I need to calculate an average?
Three gives a first estimate. Six or more shows your true range.
Sources
- NHS: Periods
- ACOG: Your First Period
- Bull JR et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. npj Digital Medicine (2019)
- NHS: Irregular periods
This page is for information only and isn鈥檛 medical advice. If something worries you, please talk to a doctor.
Keep reading
Period calculator: when is my next period?Free period calculator: enter the first day of your last period and your cycle length to see your next 6 periods. Private, instant, nothing is stored.
How long is a normal menstrual cycle?How long is a normal menstrual cycle? Anywhere from 21 to 35 days in adults. What's average, how much variation is fine, and when to see a doctor.
Irregular periods: what counts, common causes and what helpsIrregular periods explained: what counts as irregular, why cycles vary, common causes like PCOS, thyroid and perimenopause, and when to see a doctor.How to track irregular periods (and make sense of them)How to track irregular periods: what counts as irregular, what to log, how to read your range instead of an average, and when irregular cycles need a doctor.
Ovulation calculator: when do I ovulate?Free ovulation calculator: enter your last period and cycle length to see your likely ovulation day for the next 4 cycles. Private, instant, estimates only.