Fertility Window Calculator

Enter your last period start date and cycle length to see your fertile window.

Uses the standard estimate: ovulation ≈ cycle length − luteal length days after the first day of your last period. Fertile window ≈ ovulation − 5 days through ovulation + 1 day.

Info

What this tool estimates

The Fertility Window Calculator estimates your most fertile days, your likely ovulation date, and the next period based on your last period start date, average cycle length, and (optionally) luteal phase length. It’s an estimate to help with planning—not a diagnosis.

Cycles vary from month to month; results are approximate and may not fit irregular cycles.

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How the calculator works

  1. Select the first day of your last menstrual period.
  2. Enter your average cycle length (e.g., 28 days).
  3. Optionally adjust the luteal phase (default 14 days).
  4. Press Calculate to see fertile window, ovulation, and next period estimate.
  5. Use Copy/Share in the tool to save your results.
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Key concepts

  • Cycle length: Days from the first day of one period to the first day of the next.
  • Luteal phase: Days from ovulation to the next period (often ~14 days, typically 9–17).
  • Fertile window: The 6-day interval ending on ovulation day (ovulation −5 to ovulation +1).

Tip: If you don’t know your luteal phase length, leave it at 14 days (the default).

Formula

Method

Ovulation estimate

Ovulation ≈ LMP + (Cycle length − Luteal length)

Fertile window

Fertile days ≈ Ovulation − 5 → Ovulation + 1

Next period

Next period ≈ LMP + Cycle length

“LMP” = first day of your last menstrual period.

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What can change your dates

  • Irregular cycles or recent changes (stress, travel, illness, postpartum, perimenopause).
  • Hormonal birth control use or recent discontinuation.
  • Medical conditions (e.g., PCOS, thyroid disorders).
  • Short or long luteal phase compared with the 14-day default.
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Helpful tracking tips

  • Log period start dates for 3–6 months to find your true average cycle length.
  • Consider ovulation predictor kits (OPKs) for a LH surge indication.
  • Track basal body temperature (BBT) to confirm ovulation retrospectively.
  • Note cervical mucus changes (clear/stretchy often precedes ovulation).
Warning

Common mistakes

  • Using period end date instead of the first day as LMP.
  • Assuming every cycle is exactly 28 days.
  • Ignoring luteal phase variability (it isn’t always 14 days).
  • Relying on estimates for contraception (this tool is not for that).
Help

Frequently asked questions

My cycles are irregular—can I use this?

Yes, but accuracy may be limited. Use several months of logs and consider OPKs/BBT for better timing.

Is the fertile window always 6 days?

Fertility is highest in the 2 days before ovulation and the day of ovulation, but viable timing can span about 6 days due to sperm lifespan.

Can I use this to avoid pregnancy?

No. This is an estimate and not a reliable contraception method. Consult a professional for family planning advice.

What if I don’t know my luteal phase?

Leave it at 14 days to start. If you track BBT or use OPKs, you may discover your personal luteal length.

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Important note

This tool provides estimates for educational purposes and is not medical advice. If you are experiencing irregular cycles, fertility challenges, or have health concerns, speak with a qualified healthcare professional.