Enter the first day of your last few periods to see how long your cycles really are, how much they vary, and how they compare with what the NHS counts as regular. It's free, and nothing you enter is saved or sent anywhere.
First day of each period
Start with your most recent periods. At least 3 dates gives a useful picture.
How This Calculator Works
Your cycle starts on the first day of your period and ends the day before your next period starts (NHS). The calculator counts the days between each period start date you enter, then shows your average and how much your cycles vary. Use the first day of real bleeding, not spotting. The more months you add, the clearer the pattern.
What Counts as a Regular Cycle?
The NHS puts the average gap between periods at around 28 days, and a cycle that runs a little shorter or longer now and then is normal. It describes periods as irregular when the gap between them is less than 21 days or more than 35 days (NHS).
Why Cycle Length Changes
The NHS lists these common reasons for irregular periods:
- Puberty and the years around menopause
- Pregnancy
- Hormonal contraception, such as the progestogen-only pill, the injection or a hormonal IUS
- Big changes in weight, stress and anxiety, or very intense exercise
- PCOS (now also called PMOS) and thyroid problems
Cycle Length and Perimenopause
Changing cycles are often the first sign of perimenopause. Mayo Clinic says that if your cycle length is consistently different by 7 days or more, you may be in early perimenopause, and that going 60 days or more between periods suggests late perimenopause (Mayo Clinic). Our guide to irregular periods in perimenopause explains what's normal at this stage, and the perimenopause self-check can help you sort through other symptoms.
When to See a GP
The NHS suggests seeing a GP if:
- Your periods are irregular, or your usual pattern changes
- Your periods last longer than 7 days
- You bleed between periods
- Irregular periods come with other symptoms such as weight gain, tiredness, facial hair growth, or dry or oily skin
- Your periods are irregular and you're struggling to get pregnant
Bring your dates with you. Our free symptom tracker lets you log period days and symptoms and print a summary for your appointment.
Free printable
Perimenopause symptom & cycle tracker
Track your periods, sleep, hot flashes and mood on one page, then bring it to your appointment. Enter your email to download it.
About the author: Jessica Bordo writes Daily Health Cycle out of personal interest in women's health, not as a clinician. Each guide summarizes what sources such as the NHS, NICE and published studies say, and links to them so you can check the claims yourself. Use these guides to prepare questions for your own clinician, not to replace their advice.
This tool gives estimates for general education and isn't medical advice or a method of contraception. See our full disclaimer.