How long is a normal menstrual cycle?
The NHS gives 21 to 35 days as the broad typical range for a menstrual cycle, measured from the first day of one period to the day before the next — while emphasising that everyone differs and there's no single 'right' length. Your own usual pattern matters more than hitting an average, and a cycle outside this range isn't automatically a problem.
What the typical range means
A cycle is counted from the first day of bleeding to the day before your next period starts. The 21-to-35-day figure is a broad guide, not a rule — plenty of people sit naturally at one end or the other, and cycles can vary a little month to month. What's usually more telling than any single length is your own normal: a cycle that's stable for you, even at the edge of the range, is different from one that has recently changed or become unpredictable.
Why your own pattern matters most
Because 'normal' is broad and personal, the useful thing is to know your own baseline — and that comes from a record, not a memory. Tracking the interval between period starts shows where your cycles actually sit and whether they're settled or shifting. If they regularly run longer than around 35 days, or swing widely, that's a pattern worth raising with a GP, who reads cycle length alongside other signs rather than against a single cut-off.
PCOS Diary tracks the interval between your period starts, so your own normal — and any drift from it — reads clearly in a doctor-ready PDF.
Common questions
- Is a cycle outside 21 to 35 days abnormal?
- Not necessarily — the range is a broad guide and people differ. A cycle outside it, or one that's recently changed or become unpredictable, is worth recording and raising with a GP, who considers it alongside other signs rather than in isolation.
- How is cycle length actually counted?
- From the first day of one period to the day before the next begins. Measuring the interval between period starts is the simplest way to capture it accurately, especially if your cycles vary.
- How do I know my own normal rather than chasing an average?
- By recording where your cycles actually sit, not where the textbook says they should. A few months of period-start dates shows your typical interval and how much it moves — so if you naturally run at the long end of the range and stay steady there, you'll know that's your baseline. The useful comparison later isn't against 28 days; it's against the pattern that's been normal for you.
- What counts as my cycle drifting from that baseline?
- A run of cycles noticeably longer or shorter than your own usual, or timing that starts swinging when it used to be settled. A single off month rarely means much; it's a sustained shift away from your recorded pattern that's worth raising with a GP, who reads it alongside other signs rather than against a fixed cut-off.