Cycle Length Calculator

A "normal" cycle length falls anywhere between 21 and 35 days — there's a pretty wide range. The textbook 28-day cycle gets all the attention, but only about 13% of people actually have one. Most cycles land somewhere between 25 and 30 days. What matters more than hitting a specific number is whether your cycles are consistent for you. If yours is reliably 32 days, that's your normal.

Count from the first day of one period (day 1 of bleeding) to the day before your next period starts. That's one cycle. So if your period starts on March 1 and your next period starts on March 29, your cycle length is 28 days. The key is tracking several cycles — at least 3 — because a single cycle doesn't tell you much. Your average gives you a much clearer picture.

Absolutely. A variation of 1-4 days from cycle to cycle is completely normal and nothing to worry about. Your body isn't a clock — things like stress, travel, sleep changes, and even intense exercise can nudge your cycle a day or two in either direction. If your cycles swing by more than 7-8 days regularly, that's worth looking into with your doctor.

Lots of things can throw off your cycle. The most common culprits are stress (your body literally delays ovulation under stress), significant weight changes, over-exercising, thyroid issues, and PCOS. Starting or stopping hormonal birth control can cause irregularity for a few months too. Less common causes include uterine fibroids, endometriosis, and pituitary disorders. If your periods have been irregular for more than 3 months, it's a good idea to check in with your healthcare provider.

It depends on the situation, but here are some clear signals: your cycle is consistently shorter than 21 days or longer than 35 days, you've gone 90+ days without a period (and you're not pregnant), your cycle length varies by more than 7-8 days month to month, you're bleeding for more than 7 days, or you're experiencing very heavy bleeding. Bringing your tracked data to the appointment makes it way easier for your doctor to spot patterns.

Yes, and this is totally normal. In your teens and early twenties, cycles tend to be longer and more variable as your body is still finding its rhythm. Your late twenties through mid-thirties are usually the most regular years. Then in your late thirties and forties, cycles often start getting shorter and more variable again as you approach perimenopause. Eventually cycles get longer and further apart before stopping altogether at menopause. Tracking helps you see these shifts as they happen.