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Your Fertile Window & Ovulation Date.

Enter the first day of your last period and your average cycle length to see your estimated ovulation day and 6-day fertile window on a visual calendar, using the standard calendar method (ovulation ≈ 14 days before the next period). It is an educational estimate — not a contraceptive method and not a substitute for medical advice.

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Fertile Window & Ovulation Calculator
Ovulation day, fertile window and next period — from your cycle data
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Your cycle data

days
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Your results

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estimated ovulation
Enter your data
Fertile window starts5 days before ovulation—
Fertile window endsday of ovulation—
Next expected period—
Days until ovulation—
Period Fertile Ovulation Regular

How it's calculated

Ovulation day and the 6-day fertile window

The fertile window calculator uses the calendar method to estimate your ovulation date and the 6 most fertile days of your cycle. For a 28-day cycle starting on June 1, ovulation is estimated on June 15, with the fertile window running June 10–15 and the next period expected July 1.

Ovulation occurs approximately 14 days before the next period, regardless of cycle length. This is because the luteal phase (from ovulation to next period) is relatively constant at around 14 days. The follicular phase (period to ovulation) varies between women and between cycles.

The fertile window spans 6 days: the 5 days before ovulation (sperm can survive 3–5 days in the reproductive tract) plus ovulation day itself (the egg is viable for 12–24 hours).

Ovulation Day = LMP + Cycle Length − 14
Fertile Window = Ovulation Day − 5 → Ovulation Day
Next Period = LMP + Cycle Length
  1. 1
    Identify your last menstrual period (LMP)
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  2. 2
    Calculate ovulation day (LMP + cycle − 14)
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  3. 3
    Determine fertile window (ovulation − 5 to ovulation)
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  4. 4
    Estimate next period (LMP + cycle length)
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Understand the terms

Ovulation
The release of a mature egg (oocyte) from the ovary. Occurs approximately 14 days before the next period. The egg survives 12–24 hours after release.
Fertile window
The 6-day window in a cycle during which intercourse can result in pregnancy: 5 days before ovulation plus ovulation day. The highest probability is the 2–3 days before ovulation.
Luteal phase
The second half of the menstrual cycle, from ovulation to the next period. Typically 12–16 days (average 14). Progesterone rises to prepare the uterine lining.
LH surge
A sharp rise in luteinizing hormone (LH) that triggers ovulation within 24–48 hours. Detected by ovulation predictor kits (OPKs).
Cycle length
The number of days from the first day of one period to the first day of the next. A typical range is 21–35 days; the average is 28 days.
See the full glossary →

🔢 Worked example

In a 28-day cycle, ovulation occurs around day 14; the fertile window runs from ~2 days before to 1 day after. It is not a reliable contraceptive method.

Disclaimer: estimation tool for informational and planning purposes. The calendar (rhythm) method is not reliable for contraception — typical-use failure rate is approximately 24% per year. Cycle length varies due to stress, illness, medication, and other factors. This tool does not replace medical advice. For contraception or fertility treatment, consult a qualified healthcare provider. If in doubt about your health, see a professional.

Frequently asked questions — Fertile Window

What is the fertile window?
The fertile window is the period in a menstrual cycle during which pregnancy is possible. It spans approximately 6 days: the 5 days before ovulation and the day of ovulation itself. This is because sperm can survive in the female reproductive tract for up to 5 days, while the egg is only viable for 12–24 hours after release.
How do I calculate when I ovulate?
Ovulation typically occurs 14 days before the next period. The formula is: Ovulation Day = First Day of Last Period + Cycle Length − 14. For example, if your last period started on June 1 and your cycle is 30 days, ovulation is estimated on June 17. This assumes a consistent luteal phase of 14 days.
Is the calendar method reliable for contraception?
No. The rhythm method has a typical-use failure rate of about 24% per year, meaning roughly 1 in 4 women using it as their only contraception will become pregnant within a year. Cycle length can vary significantly due to stress, illness, weight changes, and other factors, making prediction unreliable. Consult a healthcare provider for effective contraceptive options.
What is the luteal phase?
The luteal phase is the second half of the menstrual cycle, from ovulation until the next period. It typically lasts 12–16 days, averaging 14 days. During this phase, the corpus luteum produces progesterone to prepare the uterine lining for possible implantation. If no pregnancy occurs, progesterone levels drop and menstruation begins.
What is an LH surge and how does it relate to ovulation?
The LH (luteinizing hormone) surge is a sharp rise in LH levels that triggers the final maturation and release of the egg. Ovulation typically occurs within 24–48 hours of the LH surge. Ovulation predictor kits (OPKs) detect this surge in urine, providing a more accurate ovulation signal than calendar calculations alone. The LH surge usually lasts 12–48 hours.

📚 Learn more — official sources

🏛️ Organization WHO — Family Planning: A Global Handbook for Providers World Health Organization global reference on fertility awareness-based methods, including the calendar (rhythm) method and its effectiveness rates. See official source → 🏛️ Organization ACOG — Understanding Menstrual Cycles American College of Obstetricians and Gynecologists patient resource explaining the menstrual cycle, ovulation, and what is normal variation. See official source → 🏛️ Organization NHS — Natural Family Planning NHS (UK) guidance on fertility awareness methods, explaining how to identify fertile days using basal body temperature, cervical mucus, and calendar tracking. See official source → 🔬 Study Wilcox et al. — The timing of the "fertile window" in the menstrual cycle (BMJ 2000) Landmark study showing that the fertile window can occur on nearly any day of the cycle, and emphasising the limitations of the calendar method. See on PubMed →

🔗 Related calculators — Pregnancy & wellness

📅 Due date 🫃 Gestational age ⚖️ Gestational weight 🌙 Chronotype 😴 Sleep calculator

About the Fertile Window Calculator

The Fertile Window Calculator uses the standard calendar method to estimate your most fertile days from two simple inputs: the first day of your last menstrual period and your average cycle length. The underlying formula — Ovulation Day = LMP + Cycle Length − 14 — is based on the well-established 14-day luteal phase and is widely cited in reproductive medicine literature.

The visual cycle calendar helps you see at a glance which days are period days (red), fertile days (green), ovulation day (coral), and regular cycle days. This calculator is intended for informational and planning purposes. If you are trying to conceive, the due date calculator, gestational age calculator and gestational weight calculator pick up where this one leaves off. For fertility treatment, cycle tracking, or contraception, always work with a qualified healthcare provider and consider additional methods such as basal body temperature monitoring, ovulation predictor kits (OPKs), and cervical mucus observation.

By Vinicius Fonseca · Formulas checked against scientific sources · Updated Aug 2026 · How we calculate →