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Cycle Length Average Calculator

Work out cycle length average instantly with clear inputs, formula shown and shareable results.

About Cycle Length Average Calculator

Averages the cycle lengths you have recorded, measures their spread and projects the next period from the last start date.

Formula

Cycle statistics

Average = sum of cycle lengths / number of cycles; variability = longest - shortest; next period = last start date + average length

Worked example

With the default values below, Cycle Length Average Calculator returns Average cycle length of 29.00. Change any input above and every figure here is recalculated in your browser.

Inputs used

Recent cycle lengths
28, 30, 27, 29, 31
First day of the last period
2026-09-01

Results

Average cycle length
29.00 days
Range between shortest and longest
4 days
Standard deviation
1.41 days
Next period expected
September 30, 2026

Step by step

  1. 1Values used
  2. 2Cycle statistics
  3. 3Average cycle length= 29.00 days
  4. 4Range between shortest and longest= 4 days
  5. 5Standard deviation= 1.41 days
  6. 6Next period expected= 2026-09-30

Assumptions and limits

  • The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or fees or charges into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Educational estimate only. Not a substitute for professional clinical judgement, and must not be used to make treatment or dosing decisions.
  • This is an estimate for general information and not professional medical advice. See the full disclaimer for the limits that apply to this kind of calculator.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Educational estimate only. Not a substitute for professional clinical judgement, and must not be used to make treatment or dosing decisions.

Frequently Asked Questions

What counts as a regular cycle?

Adult cycles of 24 to 38 days with year-to-year variation under 9 days are considered regular.

How many cycles should be tracked?

At least three, and ideally six to twelve, before drawing conclusions about regularity.

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