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Fertility Window Calculator

Work out fertility window instantly with clear inputs, formula shown and shareable results.

About Fertility Window Calculator

Counts forward from the first day of the last period to estimated ovulation using the luteal phase length, then marks the five days before ovulation and the day after as the fertile window.

Formula

Fertile window

Ovulation day = cycle length - luteal phase; fertile window = ovulation day minus 5 to ovulation day plus 1

Worked example

With the default values below, Fertility Window Calculator returns Estimated ovulation date of September 15, 2026. Change any input above and every figure here is recalculated in your browser.

Inputs used

First day of the last period
2026-09-01
Usual cycle lengthaccepts 21–40 days
28 days
Luteal phase lengthaccepts 10–17 days
14 days

Results

Estimated ovulation date
September 15, 2026
Fertile window opens
September 10, 2026
Fertile window closes
September 16, 2026
Ovulation cycle day
15

Step by step

  1. 1Values used
  2. 2Fertile window
  3. 3Estimated ovulation date= 2026-09-15
  4. 4Fertile window opens= 2026-09-10
  5. 5Fertile window closes= 2026-09-16
  6. 6Ovulation cycle day= 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.
  • 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

Why five days before ovulation?

Sperm can survive up to five days in the female reproductive tract, while the egg is viable for about 24 hours.

How accurate is calendar prediction?

Only moderately. Ovulation timing varies between cycles, so tracking basal temperature or LH gives a tighter estimate.

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Fertility Window Calculator | Calcrivo