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Ovulation Calculator

Updated Sep 2026 Used 0 times
First day of the last menstrual period.
Average cycle length in days.

Guest calculations stay on this device. Signed-in results sync privately.

Your result

Ovulation

Enter your values, then calculate to see a verified result.

Inputs usedReview the information used for this result.
Full calculation and sourcesEstimated ovulation = LMP + cycle length - 14 days; the displayed possible conception window spans five days before through one day after.

Versioned calculationFormula v1.0.0

Formula definitionUnit-normalized calculationFormula v1.0.0

What this result means

Estimate ovulation and a broader possible conception window from LMP and cycle length with an explicit variability warning.

Method used

Estimated ovulation = LMP + cycle length - 14 days; the displayed possible conception window spans five days before through one day after.

Important limitation

Calendar prediction cannot confirm ovulation and is unreliable for irregular cycles; it must not be used as a guarantee of conception or contraception.

Result actions
1
Enter the inputsUse values with the units and definitions shown beside each field.
2
Apply the methodEstimated ovulation = LMP + cycle length - 14 days; the displayed possible conception window spans five days before through one day after.
3
Inspect the bridgeReview the primary result and supporting outputs before using it.
4
Check limitationsConfirm the assumptions, date, jurisdiction or population match your use case.

Interpretation

For a 28-day cycle beginning September 1, estimated ovulation is September 15.

Estimated ovulation = LMP + cycle length - 14 days; the displayed possible conception window spans five days before through one day after.

Calendar prediction cannot confirm ovulation and is unreliable for irregular cycles; it must not be used as a guarantee of conception or contraception.

Use this result

Keep exploring

2 available tools
Conception CalculatorEstimate conception timing from LMP, due date, conception date, ultrasound dating or IVF transfer and show a wider intercourse window without claiming paternity evidence.Pregnancy Due Date CalculatorEstimate a due date from LMP, conception, ultrasound dating, a known due date or IVF transfer with ACOG-aligned limitations.
Public verification recordFormula v1.0.0
Review scope
Ovulation Calculator Formula, Input Validation, Edge Behavior And Result Presentation.
Review team
CalculatorGeek Expert Review Team
Verified
2026-09-21
Next source review
2027-09-21
Automated fixtures
1 cases

Review method

Package reconciliation, primary-source mapping, competitor feature comparison, deterministic fixtures, PHP/JavaScript parity and responsive preview QA.

Known limitations

  • Calendar prediction cannot confirm ovulation and is unreliable for irregular cycles; it must not be used as a guarantee of conception or contraception.
  • Changing legal, medical, exam or statistical conventions must be checked at the source.

Primary sources

Latest update

2026-09-21 ·

On this page

Direct answer

Estimate ovulation and a broader possible conception window from LMP and cycle length with an explicit variability warning.

Continue with the Conception Calculator when that method matches your next question.

Continue with the Pregnancy Due Date Calculator when that method matches your next question.

Formula and methodology

Estimated ovulation = LMP + cycle length - 14 days; the displayed possible conception window spans five days before through one day after.

For a 28-day cycle beginning September 1, estimated ovulation is September 15.

Continue with Conception Calculator or Pregnancy Due Date Calculator for the next calculation.

Worked example

For a 28-day cycle beginning September 1, estimated ovulation is September 15.

The calculator retains working precision and rounds only the displayed values.

Interpret the result carefully

Method boundary: Calendar prediction cannot confirm ovulation and is unreliable for irregular cycles; it must not be used as a guarantee of conception or contraception.

Inputs, boundary behavior and JavaScript/PHP parity are covered by deterministic release tests.

Use the result for the named task only. Do not stretch it into a legal, clinical, safety or causal conclusion the formula does not support.

Inputs, outputs and result meaning

Calculates estimated ovulation date, possible window start, possible window end, and estimated next period from first day of last period and typical cycle length with visible method, validation, precision, and limitations.

Inputs

First day of last period
First day of the last menstrual period. The supported range is 1900-01-01 through 2100-12-31.
Typical cycle length
Average cycle length in days. The supported range is 21 through 45.

Outputs

Estimated ovulation date
This is the primary result. The visible value is displayed without decimal places.
Possible window start
The visible value is displayed without decimal places.
Possible window end
The visible value is displayed without decimal places.
Estimated next period
The visible value is displayed without decimal places.

The calculator applies the documented method after validating and normalizing the inputs. Keep the result label, unit, selected mode, and stated limitations together when sharing the answer.

Frequently asked questions

What method does this calculator use?

Estimated ovulation = LMP + cycle length - 14 days; the displayed possible conception window spans five days before through one day after.

How should I round the result?

Keep full precision during the calculation and round only the final display unless a governing standard says otherwise.

What is the main limitation?

Calendar prediction cannot confirm ovulation and is unreliable for irregular cycles; it must not be used as a guarantee of conception or contraception.

Can I reproduce the answer?

Yes. The formula, inputs, supporting outputs and worked example are displayed on the page.

Sources

Source research refreshed September 21, 2026. No rankings, review credentials or official affiliation are implied.