The formula
How to calculate cycle length
A menstrual cycle is counted from the first day of one period to the day before the next begins. The textbook 28 days is an average rather than a norm — anything from 21 to 35 days is considered within the typical range for adults.
Averaging three or more recent cycles gives a more usable figure than any single one, and the variation between them is as informative as the average itself.
Fill in the following:
- Most recent cycle (days)
- Previous cycle (days)
- The one before that (days)
Results appear immediately — there is nothing to submit. Changing a field rewrites the link, so you can share the exact scenario you are looking at.
Consistency in how the underlying measurements are taken matters more than precision to the last decimal — measuring at the same time of day, in the same state of rest or hydration, makes readings taken weeks apart genuinely comparable in a way that a single very precise reading on its own is not.
Why cycle length matters
This kind of calculation gets used both out of general interest and ahead of a specific conversation — with a trainer, a midwife or a GP — where having an actual number ready, rather than a description, makes that conversation more concrete and specific.
Because the calculation depends on a formula rather than a device, it is useful anywhere a scale, a tape measure or a stopwatch is available, without needing specialist equipment — which is also exactly why it should be treated as an estimate rather than a clinical measurement.
None of the figures produced on this page are intended to replace a proper assessment, and nothing here should be read as a diagnosis or a treatment recommendation. What a calculator like this is good for is preparation — arriving at an appointment with an actual number already in hand, rather than a vague sense of a change, tends to make that conversation more useful for everyone involved.
A calculator like this one is frequently used to prepare a specific number for an upcoming appointment, so that a description like "I think it has changed" can be replaced with an actual figure and a date it was measured on. Clinicians and trainers generally find a concrete number, however approximate, more useful to work from than a general impression.
Worked example
Take the figures the calculator starts with:
- Most recent cycle: 29 days
- Previous cycle: 27 days
- The one before that: 30 days
That gives:
- Average cycle length: 28.7 days
- Variation between longest and shortest: 3 days
- Likely ovulation day, average cycle: 14.7
Those starting figures exist only to show the calculation working, not as a target or a reference value to compare yourself against — replace them with your own measurements in the calculator above and the same formula applies exactly as shown.
Reading the result
Variation of up to seven or eight days between cycles is common and not usually a concern. Consistent cycles shorter than 21 days or longer than 35, or a sudden change in a previously regular pattern, are worth discussing with a GP.
Where this goes wrong. Cycle length varies mostly in the follicular phase, before ovulation. The luteal phase afterwards stays close to constant, which is why a long cycle means later ovulation rather than a longer second half.
As with any general-purpose formula, the result here is an estimate derived from population averages, not a measurement specific to you — a reasonable starting point for a conversation with a qualified professional, not a replacement for one. Nothing on this page is medical advice.
It falls within the range generally considered normal for adults. What matters more is consistency — a regular 35-day cycle is less concerning than one that swings between 24 and 40.
Stress, significant weight change, intense exercise, thyroid problems, PCOS and perimenopause are among the common causes. Persistent irregularity is worth investigating with a GP.
It returns average cycle length. With 29 days most recent cycle, 27 days previous cycle and 30 days the one before that, that comes to 28.7 days. Change any field and the figure moves with it.
It uses a published formula applied exactly as defined, so the arithmetic itself is precise — but the formula is a population-level estimate, and individual results can vary from it for reasons the formula has no way to account for. Treat the output as a reasonable estimate rather than a measured or diagnostic value.
For most people, yes — a single reading is a snapshot, while the same measurement repeated consistently over weeks or months shows a trend, which is generally more informative than any one figure taken in isolation.
Small differences between calculators usually come down to which published formula or reference dataset each one uses — several equations in this area exist in more than one accepted version. This page states the specific formula used in the section above, so it is possible to check directly whether another source is using the same one.