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Ovulation Prediction Calculator

Work out ovulation prediction instantly with clear inputs, formula shown and shareable results.

About Ovulation Prediction Calculator

Predicts ovulation as 14 days before the next expected period and widens the estimate by your recorded cycle variability.

Formula

Ovulation timing

Ovulation day = cycle length - 14; prediction window = ovulation day plus or minus recorded cycle variability

Worked example

With the default values below, Ovulation Prediction Calculator returns Most likely ovulation date of September 17, 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
Average cycle lengthaccepts 21–40 days
30 days
Cycle variabilityaccepts 0–7 days
2 days

Results

Most likely ovulation date
September 17, 2026
Earliest likely ovulation
September 15, 2026
Latest likely ovulation
September 19, 2026
Width of the prediction window
5 days

Step by step

  1. 1Values used
  2. 2Ovulation timing
  3. 3Most likely ovulation date= 2026-09-17
  4. 4Earliest likely ovulation= 2026-09-15
  5. 5Latest likely ovulation= 2026-09-19
  6. 6Width of the prediction window= 5 days

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 subtract 14 days?

The luteal phase is the most consistent part of the cycle, averaging about 14 days regardless of total cycle length.

What confirms ovulation?

A sustained basal temperature rise, a mid-luteal progesterone level or a positive LH surge test.

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Ovulation Prediction Calculator | Calcrivo