Predict a realistic window for your next period when your cycles are irregular, using your shortest and longest recent cycle lengths instead of one average.
| Input | Result |
|---|---|
| Last Mar 1, 2026 · shortest 25 · longest 32 | window Mar 26 – Apr 2 (7-day span) |
| Last Mar 1, 2026 · shortest 21 · longest 40 | window Mar 22 – Apr 10 (19-day span) |
| Last Mar 1, 2026 · shortest 28 · longest 28 | single date: Mar 29 |
| Last Mar 1, 2026 · shortest 26 · longest 30 | window Mar 27 – Mar 31 (4-day span) |
An average is only a good predictor when the thing being averaged is stable. When recent cycles ran 25, 31 and 27 days, the average of 27.7 promises a precision your body does not have — the honest answer is a range. This calculator takes your shortest and longest recent cycles as the two edges: the next period is likely to start no earlier than last + shortest and no later than last + longest.
The common, usually benign drivers are stress, significant weight change, hard exercise training, travel across time zones and broken sleep — all of which act by delaying ovulation. Life-stage transitions do the same: cycles are naturally irregular in the first years after the first period, while breastfeeding, and in the years approaching menopause. Persistent irregularity can also signal conditions worth checking, such as thyroid problems or PCOS. None of that can be told from a calendar; if cycles stay outside the 21–38 day band, disappear for three months, or change suddenly from your established pattern, that is the point to see a clinician rather than a calculator.
| Measure | Typical adult range | Source basis |
|---|---|---|
| Cycle length (day 1 to day 1) | 24–38 days | ACOG guidance for adult cycles; adolescents run wider (roughly 21–45 days) in the first years |
| Cycle-to-cycle variation | Up to about 7–9 days | FIGO/ACOG regularity definitions, age-dependent |
| Bleeding length | Up to 8 days | ACOG normal menstruation parameters |
| Luteal phase (ovulation to next period) | About 14 days on average | The convention this calculator uses; real cycles vary roughly 10–16 days |
Working definitions from ACOG and FIGO: cycles consistently shorter than 21 days or longer than about 35 to 38 days, or a cycle length that varies by more than roughly a week from one cycle to the next.
Track the start dates of your last four to six periods and count the days between each pair. The smallest gap is your shortest cycle, the largest your longest. The cycle length calculator on this site does the counting for you.
Only roughly. This page shows the ovulation range implied by your window, but when the window is two weeks wide the estimate is nearly useless. Ovulation predictor kits or basal body temperature tracking respond to the actual cycle and work far better for irregular cycles.
Usually not. One long or short cycle after an illness, a stressful month or long-haul travel is expected physiology. The pattern matters, not the exception.
Common referral triggers: no period for three months, cycles persistently under 21 or over 38 days, bleeding lasting more than about eight days, bleeding between periods, or a sudden persistent change from your established pattern.