Is your cycle changing? Cycle length calculator

A cycle runs from the first day of one period to the first day of the next. The American College of Obstetricians and Gynecologists puts a typical cycle at 24 to 38 days. In perimenopause that number changes. Cycles usually get shorter first, then start to vary by a week or more from one month to the next, and later there can be gaps of 60 days or longer. The calculator below does the counting and tells you which of those patterns your dates show. It is an estimate from dates you type in, and it cannot diagnose anything.

How the calculator reads your dates

It sorts the dates, counts the days between each start and the next, and gives you four numbers: the average cycle, the shortest, the longest, and the spread between those two. Then it compares each cycle with three published yardsticks.

ACOG's typical range. Cycles of 24 to 38 days are typical, according to ACOG. A cycle outside that range is flagged and linked to the article that explains it. Its page on abnormal uterine bleeding lists periods that start less than 21 days apart as a pattern a doctor should check, and the calculator flags any cycle that short.

The early transition marker. The STRAW+10 staging system dates the start of the early menopausal transition to the point where two cycles in a row differ in length by 7 days or more, and the same happens again within the next 10 cycles. The calculator compares every cycle with the one before it and counts how often that happens. One swing is shown as one swing. Two or more inside 10 cycles match the STRAW+10 description.

The late transition marker. STRAW+10 marks the late transition by a stretch of 60 days or more without a period. The calculator looks for it in the gaps between your dates, and also counts the days from your most recent date to today, in case the current gap is already that long. A full year with no period is the definition of menopause. If your most recent period was more than a year ago, the result says so, because any bleeding after menopause should be checked by a doctor.

All of this rests on dates alone. The calculator knows nothing about flow, pain or bleeding between periods. For those, use the bleeding checker.

A worked example

Say your last five periods started on 2 March, 28 March, 22 April, 27 May and 20 June. That gives four cycles: 26, 25, 35 and 24 days. The average is 27.5 days, the shortest 24, the longest 35, so the spread is 11 days. Every cycle sits inside ACOG's 24 to 38, which on its own looks ordinary.

The pairs tell a different story. The jump from 25 days to 35 is 10 days, and the drop from 35 to 24 is 11. That is two swings of a week or more within four cycles, which matches the STRAW+10 description of the early transition. Now suppose the next period does not come until 25 August. That cycle is 66 days long, past the 60-day line, and the result switches to the 60-day marker.

Shorter, longer, or closer together

Readers tend to notice one of two things first. Periods arriving closer together, sometimes twice in a month, usually come first. STRAW+10 lists shorter cycles as a sign of the late reproductive years, before the transition officially begins. Our page on two periods in one month covers that stage. Longer cycles and skipped months tend to come later, as ovulation becomes less regular. Our guide to irregular periods in perimenopause explains what each pattern means and when to see a doctor about it.

The order is a tendency, not a timetable. The NHS says a change in your periods does not necessarily mean there is a problem, but it does need to be investigated. It also says bleeding between periods, after sex or after menopause should always be checked by a doctor.

When the numbers mislead

Hormonal contraception is the big one. A combined pill, patch or ring gives you withdrawal bleeds on a schedule set by the packet, and a hormonal coil can make bleeding light or stop it. Dates recorded while you use them say little about your own cycle. The NHS notes that hormonal contraception can hide the signs of perimenopause. A pregnancy can also explain a long gap, and our page on pregnancy in perimenopause explains why a test is still worth doing.

Spotting entered as a period is the other common slip. Two dates a few days apart produce a very short "cycle" that is really one episode. If the result shows a cycle under 10 days, look back at whether that second date was a new period.

My take

I'd put in every date you can find, from a calendar, an old diary or a period app, and go back at least six months. Three dates give two cycles, which is enough to see a 7-day swing but too few to tell a one-off from a pattern. I'd judge the result by the pairs more than by the average: an average of 28 can hide a 22-day cycle next to a 34-day one. Keep writing new dates in the printable symptom diary, and take the diary and these results to your appointment. Our guide to perimenopause symptoms lists the other changes worth noting alongside them.