Enter the first day of full flow for each period. The calculator lists every interval between those dates, your average cycle length, and the shortest-to-longest spread in those entries. Two dates give one interval; additional dates show how the gaps differ.
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 the number of days from day 1 of one period to day 1 of the next — day 1 being the first day of full flow, not spotting. It is not how long you bleed (period length, normally up to 8 days), and it is not “once a month”:
One pair of start dates gives one complete interval. Additional start dates let you compare intervals and calculate their average.
| Measurement | Count between | Demo |
|---|---|---|
| Cycle length | One period start and the next period start | Jan 1 → Jan 29 = 28 days |
| Period length | The first and last bleeding days, both included | Jan 1–5 = 5 bleeding days; how to log the last day |
Read the average together with the shortest, longest and individual gaps. The spread is longest minus shortest; a few entries describe your recent record, not a full year:
| Your result | Meaning for these entries | Next step |
|---|---|---|
| One interval | Two dates give one gap; there is no spread to compare yet | Add the next actual start date when it happens |
| Spread under 7 days | Small variation in these entries | Read the listed gaps alongside the average |
| Spread 7–9 days | Moderate variation in these entries | Keep logging to see your longer-term pattern |
| Spread over 9 days | Wide variation in these entries | Try a window using your shortest and longest in the irregular period calculator |
| Average outside 24–38 days | Outside the adult frequency reference range (teens: 21–45) | Check the dates; discuss a persistent pattern with a clinician |
Most cycles are not 28 days. In 612,613 tracked cycles only 13% were exactly 28 days long; two-thirds fell between 25 and 30:
So a 31- or 33-day average is common, not a red flag. What matters clinically is the range below and whether your own spread stays small.
| 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 |
Within the normal range, the average drifts down with age:
Cycle length is the input behind every prediction on this site: next period = last start + cycle length; ovulation ≈ cycle length − 14 days; “late” = today compared with that expected date. The reason the ovulation formula works is in the data — a longer cycle is longer almost entirely before ovulation:
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.
Each cycle is the gap between two start dates, so N dates give N − 1 cycles — the calculator takes up to 6 dates (5 cycles) and shows every gap, the average and the spread. Here is the calculation it runs on five Demo dates (four cycles):
Jun 3, Jul 2, Aug 3, Aug 31, Oct 1.
29, 32, 28 and 31 days.
(29 + 32 + 28 + 31) ÷ 4. Shortest 28, longest 32, spread 4 — small variation in these four recorded intervals; this is not a year-long regularity assessment.
Next period ≈ Oct 1 + 30 = Oct 31; ovulation ≈ Oct 1 + (30 − 14) = Oct 17, cycle day 17.
Demo — a decimal average: start dates Jan 1, Jan 29 and Feb 27, 2026 give gaps of 28 and 29 days. (28 + 29) ÷ 2 = 28.5 days, which the average card displays. Forecasts and links to other calculators round that to 29 days; Feb 27 + 29 days = Mar 28.
Log the actual day-1 date each time — memory drifts more than cycles do — and don’t drop a cycle from your dates just because it was unusual; the list of gaps shows if one stands apart. The period tracker keeps the log on your device and computes this average as you go.
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 between your logged start dates — the more cycles you include, the more your average reflects your usual pattern rather than one month.
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: the reference frequency range is 24–38 days (FIGO); teens: 21–45 days (ACOG). A few start dates show the intervals entered here. Clinical regularity considers the shortest-to-longest variation across a year, with age-specific parameters.
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.
Two start dates give one interval. Each additional date adds one more; this calculator takes up to six dates (five intervals). Read the individual gaps and average together. FIGO’s regularity assessment looks across a year, so a few gaps show a recent pattern rather than establishing annual regularity.
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).