Menstrual Cycle Length Calculator
Calculate mean, median and observed variation in cycle length from recorded period-start dates without predicting fertility or future dates.
YOUR CALCULATION
Result
—
Calculation steps
Work through one step at a time or reveal the complete calculation. The steps use your current inputs.
How to calculate
Subtract consecutive recorded start dates and summarize only the complete observed calendar intervals.
Observed cycle length=next recorded period-start date−previous recorded start date
Worked example
Starts on January 1, January 29, February 26 and March 28 in 2026 give intervals 28, 28 and 30 days: mean 28⅔ days and median 28 days.
Assumptions and input details
2–50 increasing Gregorian dates, years 1900–2199. Dates remain in the browser. Missing records are not detected and can lengthen an apparent interval. This is a descriptive log calculation, not a next-period prediction, ovulation calculator, contraceptive method or assessment of irregular bleeding. No clinical normal-range label is assigned.
References: NHS: first-day cycle-counting convention
Using this calculator
Can I enter my own problem?
Yes. Replace the example values with your own inputs. The result, calculation steps, tables and applicable diagrams update from those inputs. Blank or invalid values clear the result; example values are never silently substituted for missing inputs.
Are the results exact?
Results use floating-point arithmetic and displayed rounding. Read the formula, assumptions and any stated data precision before interpreting the result.
Is my input uploaded?
The calculation runs in your browser. No account, file upload or external AI service is required. Inputs are not saved between page reloads.
Count calendar days between two Gregorian dates, optionally including both endpoints. View the direction, week breakdown and Monday–Friday versus weekend counts.
Estimate a due date and gestational calendar age from a last-period start or known conception date, with clear dating assumptions.
Enter a list of numbers to calculate mean, median, modes, quartiles, population and sample standard deviation, with a frequency histogram.
Reserved advertising space.