Pregnancy & Family

Fertile days calculator

Fertile window around ovulation as a date range.

Updated on Apr 21, 2026 Calculator, calculation path and examples on one page

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Calculator

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Calculation path

Step by step

  1. Estimate ovulation

    Result: Apr 21, 2026

  2. Divert windows to the front and rear

    Result: April 16, 2026 to April 22, 2026

Instructions

How to use this calculator correctly

How to use the calculator

  • Enter the date of the last period and cycle length.
  • Read the window directly as a date range.

Limits of the calculation

  • The calculation is only an approximation without temperature or LH measurement.

Common mistakes

  • Treat ovulation and fertile days as identical.

Related guide: Better understand week of pregnancy, ovulation and fertile days

Examples

Typical calculations

28 days.

Typical cycle

Fertile window of opportunity: April 16, 2026 to April 22, 2026

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31 days.

Longer cycle

Fertile window of opportunity: April 13, 2026 to April 19, 2026

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25 days.

Shorter cycle

Fertile window of opportunity: April 16, 2026 to April 22, 2026

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FAQ

Frequently asked questions

What are fertile days?

This is the time window around ovulation when pregnancy may be more likely.

Why does the window start before ovulation?

Sperm can survive in the body for several days.

Why does the window after ovulation end?

The egg cell can only be fertilized for a short time, so a short follow-up period is often taken into account.

Is the estimate equally good for all cycles?

No. If the cycle length fluctuates, the estimate becomes more uncertain.

Is the calculator suitable for contraception?

No. It is not safe enough for this and is only used for orientation.

Related calculators

Continue seamlessly

Ovulation calculator

Calculate ovulation and fertile phase approximately from cycle start and cycle length.

Sources and notes

Rule status and context

methodology
Fertile window = approximately 5 days before to 1 day after ovulation.