Your cycle length is not 28 days by default — it is whatever your own last few periods say. Enter their start dates below to get your real average, your shortest and longest cycles, and a regularity check based on FIGO parameters.
Day 1 of one period to day 1 of the next. Day 1 = first day of full flow, not spotting.
24–38 days (FIGO). For teens in the first years after menarche, 21–45 days (ACOG).
28 is an average, not a standard. Most real cycles do not land exactly on 28.
Cycle length is not how long your period lasts, and it is not "once a month." It is the number of days from day 1 of one period to day 1 of the next — and day 1 means the first day of full flow, not the light spotting that sometimes comes before. Period length (how many days you bleed, normally up to 8) is a separate measurement that happens to live inside the cycle count.
One cycle tells you almost nothing. Cycle length is a personal statistic: it only becomes useful as an average across several cycles, together with the spread between your shortest and longest.
The calculator gives you three numbers — average, shortest, longest — and the spread is the one most people should look at first:
| Your result | Meaning | Where to go next |
|---|---|---|
| Spread under 7 days | Regular — a single average predicts you well | Use the average in the late period calculator |
| Spread 7–9 days | Borderline — at the edge of FIGO's regularity band | Keep logging; a 4th–6th cycle usually settles it |
| Spread over 9 days | Irregular band — single-day predictions will keep failing you | Use the irregular period calculator with your shortest and longest |
| Average outside 24–38 days | Outside the adult normal frequency (teens: 21–45) | Worth a clinician conversation if persistent |
| Life stage | Normal cycle range | Notes |
|---|---|---|
| Teens, first years after first period | 21–45 days | Variable while the hormone axis matures; irregularity here is physiology, not pathology |
| Adults | 24–38 days | Steady matters more than the exact number; spread under 7–9 days counts as regular |
| Perimenopause (typically 40s) | Variable again — after decades of stable cycles, irregularity returns approaching menopause | New variability after years of stable cycles is the classic pattern |
Cycle length is the single input behind every prediction on this site. Expected next period = last period start + your cycle length. Estimated ovulation = cycle length − 14 days. Late-period detection = comparing today against that expected date. Get the cycle length wrong and every downstream date shifts with it.
That is why this page is the right first stop. Once you have your real average, the result card above hands it directly to the late period calculator, the missed period calculator, and the irregular period calculator — no retyping. For the full prediction view, the main period calculator combines next period, ovulation, and fertile window in one result.
Three cycles give a usable average; six or more make it reliable. Two rules of thumb: log the actual day-1 date each time (memory drifts more than cycles do), and do not reset your average over one unusual cycle — an outlier is noise, a repeated pattern is signal. The period tracker keeps the log on your device with no account, and computes this average automatically as you log.
Count days from day 1 of one period to day 1 of the next — day 1 being the first day of full flow, not spotting. Average the gaps across at least 3 cycles; one cycle tells you very little.
The first day of full menstrual flow. Light spotting the day or two before does not count.
No. 28 is a textbook average. FIGO's normal adult range is 24–38 days, and large-scale data shows most real cycles do not land exactly on 28.
Adults: any steady pattern in 24–38 days (FIGO). Teens: 21–45 days (ACOG). Consistency matters more than the number — a shortest-to-longest spread under 7–9 days.
Yes — the count runs from the first day of one period to the first day of the next, bleeding days included. Period length itself (normally up to 8 days) is a separate measurement.
Yes. Cycles typically become shorter and more regular from the teen years into adulthood (Mayo Clinic), stay steadiest through the 20s–30s, and become variable again approaching menopause. Stress, weight change, training, and stopping hormonal birth control can also shift it.
3 for usable, 6+ for reliable. One unusual cycle is noise — do not let it reset your whole average.
A shortest-to-longest spread beyond about 7–9 days across a year, or cycles persistently outside 24–38 days (FIGO). The irregular period calculator is built for exactly this case.
Every prediction derives from it: next period = last period + cycle length; ovulation ≈ cycle length − 14. A wrong cycle length shifts every downstream date.
A steady 35-day cycle sits inside FIGO's normal adult range of 24–38 days — if it is consistent, it is a normal personal pattern. A 40-day cycle is just outside that range: worth watching, and persistently longer than 38 days is a reasonable reason to talk to a clinician. Teens get a wider band: 21–45 days (ACOG).
Indirectly, yes. Cycle length shifts when you ovulate — estimated ovulation ≈ cycle length − 14, and real luteal phases range 10–15 days (600,000+ cycle study). Conception is only possible in the 6 days ending on ovulation day (Wilcox 1995, NEJM). Long or highly variable cycles usually mean late or unpredictable ovulation — that makes calendar timing unreliable, not pregnancy impossible.
Shorter cycles are a normal age trend — cycles typically become shorter and more regular from the teen years into adulthood (Mayo Clinic). A sudden persistent shortening can also follow stress, training load, or weight change. If cycles drop below 24 days repeatedly, or the change comes with much heavier bleeding, that crosses FIGO's normal band and merits a clinician visit.
See a clinician if cycles are persistently outside 24–38 days (teens: 21–45), your shortest-to-longest spread exceeds about 7–9 days across a year, or you go 90 days with no period. Persistent irregularity is worth checking because conditions like PCOS (WHO: 6–13% of women, up to 70% undiagnosed) often show up first as a cycle-length change.
From the start — day 1 is the first day of full flow, and the count runs to day 1 of the next period. The end date of your bleeding is not used anywhere in the calculation.
Check your logged dates first — one mistyped date can drag an average. If the pattern is real and persistent, it is worth a clinician visit (teens: the range is 21–45).