Menstrual Cycle Variability Analyzer
Enter three to twenty-four actual period-start dates in increasing order.
Your cycle history inputs
Historical intervals do not predict ovulation, contraceptive safety, pregnancy or a diagnosis.
Cycle history report
29 days · mean recorded cycle interval
Historical next-start scenario 2026-09-24 to 2026-09-26
Full result table
| Recorded start | Next recorded start | Calendar days |
|---|---|---|
| 2026-06-01 | 2026-06-29 | 28 |
| 2026-06-29 | 2026-07-29 | 30 |
| 2026-07-29 | 2026-08-27 | 29 |
Historical intervals do not predict ovulation, contraceptive safety, pregnancy or a diagnosis. The next-date range is a scenario based on recorded extremes, not a confidence interval.
Let’s work it out.
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How to use Menstrual Cycle Variability Analyzer
- Enter three to twenty-four actual period-start dates in increasing order.
- The mean uses every completed interval; the median is the central sorted interval, averaging the two middle values when needed.
- Adding the shortest and longest recorded intervals to the latest start gives a transparent date range.
Worked example
The three example intervals are 28, 30 and 29 days, averaging 29 days.
Formula, accuracy & limitations
Calculate consecutive calendar-day intervals between recorded start dates. Report mean, median, observed minimum/maximum and population CV. Add observed minimum and maximum intervals to the last start date as a historical scenario.
Historical intervals do not predict ovulation, contraceptive safety, pregnancy or a diagnosis. The next-date range is a scenario based on recorded extremes, not a confidence interval.
The primary result is rounded to two decimal places. Calculation tables show up to four and supporting metrics up to six. Very small nonzero values use scientific notation. Display rounding is not fed back into the arithmetic.
Record starts rather than guessed cycle lengths
Enter three to twenty-four actual period-start dates in increasing order. Consecutive starts define the completed intervals. A date with uncertain meaning should not be relabelled automatically; the calculation cannot distinguish spotting, missing entries or a different recording convention.
Compare the centre and the spread
The mean uses every completed interval; the median is the central sorted interval, averaging the two middle values when needed. Minimum, maximum and population variation show different aspects of the same history. None is converted into a normality score or a fertility probability.
Treat future dates as historical scenarios
Adding the shortest and longest recorded intervals to the latest start gives a transparent date range. It is not a statistical prediction interval and does not know whether the next cycle will resemble the past. A single outlying interval remains visible rather than being removed without explanation.
Protect the scope of the tool
This page does not infer ovulation, provide contraception guidance or diagnose a cycle condition. Hormonal treatment, pregnancy and other context are not modelled. The useful output is a clear summary of the supplied dates and an auditable explanation of the arithmetic.
Frequently asked questions
How many starts make three cycle intervals?
Four recorded starts produce three completed intervals. The final start begins an interval that has not yet been completed.
Is the next-start range an ovulation window?
It adds observed minimum and maximum intervals to the last start. It does not estimate ovulation or contraceptive safety.
Why are the mean and median different?
The mean uses every interval’s numerical size. The median uses the centre of the sorted intervals, so an unusually long interval can affect them differently.
Are uncertain dates corrected automatically?
Dates are checked for validity and order. The tool cannot decide what an uncertain entry represented or replace it with an assumed start.
