Next Period & Fertile Window
Last reviewed: January 2026
Calculate your next period date, ovulation day, and fertile window based on your average cycle length. This calculator runs entirely in your browser — your data stays private, and no account is required.
The average cycle is 28 days but normal ranges from 21–35 days. Ovulation typically occurs 14 days before the next period — not necessarily on day 14 of your cycle. If your cycle is 32 days, you likely ovulate around day 18. The fertile window is approximately 6 days: the 5 days before ovulation and the day of ovulation. Sperm can survive up to 5 days in the reproductive tract, which is why the fertile window extends before ovulation.
| Phase | Days (28-day cycle) | Hormones | Key Events |
|---|---|---|---|
| Menstrual | 1–5 | Low estrogen/progesterone | Period bleeding |
| Follicular | 1–13 | Rising estrogen, FSH | Egg develops in ovary |
| Ovulation | ~14 | LH surge, peak estrogen | Egg released |
| Luteal | 15–28 | Rising progesterone | Uterine lining thickens |
The menstrual cycle is a complex orchestration of hormonal signals between the brain and reproductive organs, cycling through four distinct phases over an average of 28 days — though normal cycles range from 21 to 35 days. Understanding each phase helps with fertility planning, symptom management, and identifying irregularities that may warrant medical attention.
Menstrual phase (days 1–5): The uterine lining (endometrium) sheds, producing menstrual bleeding that typically lasts 3–7 days with average blood loss of 30–80 mL per cycle. Hormone levels are at their lowest point. Many women experience cramping caused by prostaglandins — chemicals that trigger uterine contractions to expel the lining. Follicular phase (days 1–13): Overlapping with menstruation, the pituitary gland releases FSH, stimulating 5–20 ovarian follicles to begin maturing. Rising estrogen from the developing follicles thickens the uterine lining. One dominant follicle emerges by day 7–10, and the others regress. Estrogen peaks just before ovulation, triggering the LH surge. Ovulatory phase (day 14): The LH surge causes the dominant follicle to rupture, releasing a mature egg. Some women feel a twinge of pain called mittelschmerz on the side of ovulation. Cervical mucus becomes stretchy and clear, facilitating sperm transport. The egg survives 12–24 hours if not fertilized. Luteal phase (days 15–28): The ruptured follicle becomes the corpus luteum, producing progesterone to maintain the uterine lining. If implantation does not occur, the corpus luteum degrades after about 12–14 days, progesterone drops, and the cycle begins again with menstruation.
Cycle length variation of up to 7–9 days between cycles is considered normal, especially in adolescents (first 2–3 years after menarche) and women approaching perimenopause (typically ages 40–51). However, consistently irregular cycles may indicate underlying conditions. Oligomenorrhea (cycles longer than 35 days) can signal PCOS, thyroid disorders, or hypothalamic dysfunction. Polymenorrhea (cycles shorter than 21 days) may indicate luteal phase deficiency or anovulation. Amenorrhea (absence of periods for 3+ months in previously cycling women) requires evaluation — common causes include pregnancy, extreme exercise, low body weight, stress, and hormonal disorders.
Dysmenorrhea (painful periods) affects 50–90% of reproductive-age women. Primary dysmenorrhea results from prostaglandin-driven uterine contractions and responds well to NSAIDs and hormonal contraceptives. Secondary dysmenorrhea indicates underlying pathology — most commonly endometriosis (affecting 10% of reproductive-age women), adenomyosis, or uterine fibroids. Menorrhagia (heavy menstrual bleeding) is defined as blood loss exceeding 80 mL per cycle or bleeding lasting more than 7 days. Signs include soaking through a pad or tampon every hour for several consecutive hours, passing clots larger than a quarter, or needing double protection (pad plus tampon). Heavy bleeding can cause iron deficiency anemia, producing fatigue, shortness of breath, and pale skin.
Tracking your cycle provides valuable data beyond fertility awareness. Patterns in mood, energy, sleep, appetite, skin changes, and exercise performance often correlate with cycle phases. Many women report peak energy and confidence during the late follicular phase (rising estrogen), while the late luteal phase (falling progesterone) brings premenstrual symptoms in 75% of women. Recognizing these patterns enables proactive management — scheduling demanding tasks during high-energy phases, planning extra rest during premenstrual days, and adjusting exercise intensity to match hormonal fluctuations rather than fighting them.
Seek medical evaluation if you experience periods lasting longer than 7 days, cycles consistently shorter than 21 or longer than 35 days, bleeding between periods, severe pain unresponsive to over-the-counter medication, sudden cycle changes after years of regularity, or absence of periods for three or more months (when not pregnant or breastfeeding). Basic evaluation includes hormone blood tests (day 3 FSH, estradiol, LH, thyroid, prolactin, androgens), pelvic ultrasound, and sometimes endometrial biopsy or hysteroscopy depending on symptoms and age.
Hormonal birth control methods (combined pills, progestin-only pills, IUDs, implants, patches, rings) alter the natural menstrual cycle by suppressing ovulation and thinning the endometrium. The "period" on combined hormonal contraceptives is actually a withdrawal bleed triggered by the hormone-free interval — not a true menstruation. Many modern contraceptive regimens allow extended or continuous use, eliminating withdrawal bleeds entirely. After discontinuing hormonal contraceptives, normal ovulatory cycles typically resume within 1–3 months, though some women experience a delay of 6–12 months, especially after long-acting methods like Depo-Provera injections.
If your cycles remain irregular more than six months after stopping hormonal contraception, consult your healthcare provider for evaluation — underlying conditions like PCOS may have been masked by hormonal birth control rather than treated.
A "normal" menstrual cycle ranges from 21 to 35 days, with 28 days being the statistical average but not the personal norm for many women. Cycle length often varies by 1–5 days month to month, and variations of up to 7–9 days are considered within normal limits. Factors that affect cycle length include stress, significant weight changes, intense exercise, travel across time zones, illness, and perimenopause (which typically begins in the mid-40s). Consistently irregular cycles (varying by 10+ days), cycles shorter than 21 days, or cycles longer than 35 days warrant discussion with a healthcare provider. Tracking your cycle over 3–6 months reveals your personal pattern and helps identify meaningful changes. Estimate your ovulation window based on your average cycle length.
See also: Ovulation Calculator · Pregnancy Due Date Calculator · Due Date Calculator
→ Ovulation occurs ~14 days before your next period, not 14 days after your last one. This distinction matters for irregular cycles. A woman with a 35-day cycle likely ovulates around day 21 (not day 14). A woman with a 24-day cycle likely ovulates around day 10. The luteal phase (ovulation to period) is consistently 12–16 days; the follicular phase (period to ovulation) varies. Use our Ovulation Calculator for detailed estimates.
→ Cycle length varies naturally — a few days variation is normal. Your cycle might be 27 days one month and 30 the next. Variation of ±3–4 days is considered regular. Consistently irregular cycles (varying by 7+ days) or cycles shorter than 21 or longer than 35 days may warrant evaluation by a gynecologist.
→ Stress, travel, illness, and weight changes can shift your cycle. The hypothalamic-pituitary-ovarian axis is sensitive to physical and emotional stress. Jet lag, heavy exercise, significant weight loss, and emotional stress can delay ovulation, causing a longer-than-usual cycle. The period doesn't "skip" — ovulation was delayed, pushing everything later.
→ Tracking additional signs improves prediction accuracy. Basal body temperature (BBT) rises 0.2–0.5°F after ovulation and stays elevated until your period. Cervical mucus becomes clear, stretchy, and "egg-white" during the fertile window. Combining calendar, BBT, and mucus observations (the symptothermal method) is much more accurate than calendar-only tracking. See our Due Date Calculator for pregnancy dating.
See also: Ovulation Calculator · Due Date Calculator · Fertility Calculator · Pregnancy Calculator