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Menstrual Cycle Dates Calculator

Work out menstrual cycle dates instantly with clear inputs, formula shown and shareable results.

About Menstrual Cycle Dates Calculator

Cycle tracking projects forward by the average cycle length from the first day of the last period, counted as cycle day one. With a 28 day cycle starting 20 August 2026, the next period is expected on 17 September and the one after on 15 October. Real cycles vary by several days month to month, so the projection is indicative.

Formulas

Next period

next start = last start + cycle length

Period end

end = next start + period length - 1

Worked example

With the default values below, Menstrual Cycle Dates Calculator returns Cycle Length Used of 28. Change any input above and every figure here is recalculated in your browser.

Inputs used

First day of the last period
2026-08-20
Average cycle lengthaccepts 20–45 days
28 days
Period lengthaccepts 1–10 days
5 days

Results

Cycle Length Used
28 days
Next Period Expected To Start
2026-09-17
Next Period Expected To End
2026-09-21
Cycle After That Begins
2026-10-15

Step by step

  1. 1Values used
  2. 2Next period
  3. 3Period end
  4. 4Cycle Length Used= 28 days
  5. 5Next Period Expected To Start= 2026-09-17
  6. 6Next Period Expected To End= 2026-09-21
  7. 7Cycle After That Begins= 2026-10-15

Assumptions and limits

  • The result is derived only from the 3 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 whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Cycle predictions from average length are estimates only and must not be relied on for contraception or fertility decisions. Cycles vary between months and between individuals. Consult a healthcare professional for contraceptive, fertility or menstrual health advice.

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

Cycle predictions from average length are estimates only and must not be relied on for contraception or fertility decisions. Cycles vary between months and between individuals. Consult a healthcare professional for contraceptive, fertility or menstrual health advice.

Frequently Asked Questions

Is a 28 day cycle normal?

It is the average, but anything from 21 to 35 days is considered normal in adults, and variation of a few days between cycles is typical.

Can this be used for contraception?

No. Calendar methods have high failure rates because ovulation timing varies. Use a recognised contraceptive method.

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