Ovulation Calculator

Estimate your ovulation date and fertile window from your last period and cycle length.

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  • Accurate results
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  • Works on all devices
Enter your last period date and cycle length to estimate your ovulation date and fertile window.
days

Your result

29-08-2026

Estimated ovulation date

Fertile window (start)24-08-2026
Fertile window (end)30-08-2026
Next expected period12-09-2026

AI explanation

Formula

Ovulation date = last period date + (cycle length − 14) days

Worked example

Last period 2026-01-01, 28-day cycle

Worked example: Last period 2026-01-01, 28-day cycle
FieldValue
First day of last period01-01-2026
Average cycle length28
Estimated ovulation date15-01-2026
Next expected period29-01-2026

Assumptions

  • Assumes ovulation occurs 14 days before the next expected period (a relatively fixed luteal-phase length), regardless of total cycle length.
  • The fertile window spans roughly 5 days before to 1 day after ovulation, reflecting sperm survival time and egg viability.
  • Informational only — cycle length and ovulation timing can vary month to month, especially with irregular cycles.

Frequently asked questions

How is ovulation date estimated?

By counting 14 days before your next expected period — since the luteal phase (after ovulation) is typically a consistent 14 days regardless of total cycle length.

What is the fertile window?

The days in your cycle when pregnancy is possible — typically the 5 days before ovulation (since sperm can survive that long) plus the day of ovulation itself.

What if my cycle length varies from month to month?

This calculator uses your average cycle length, but if your cycles are irregular, actual ovulation timing can shift — tracking basal body temperature or using ovulation predictor kits gives more precise, cycle-specific timing.

Related calculators

Period CalculatorPregnancy Conception Calculator

Related articles

Health & Fitness1 min read

How an Ovulation Date and Fertile Window Are Calculated

Ovulation is estimated by counting backward 14 days from the next expected period, since the post-ovulation luteal phase is a relatively fixed length — and a longer overall cycle shifts that ovulation date later, not the 14-day countdown itself.

Read guide
Health & Fitness2 min read

Why the Pregnancy Due Date Formula Already Assumes a Specific Ovulation Day

Feeding the same last-period date into both the ovulation calculator and the pregnancy calculator shows exactly why Naegele's 280-day rule works for a typical 28-day cycle but drifts for a longer one — the due-date formula silently assumes ovulation on day 14, every time.

Read guide

Sources

This calculator provides a general estimate only and does not constitute personalized medical advice, and should not be relied on as a method of contraception.

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