Period Calculator
Enter the first day of your last period and your average cycle length to predict your next period and the following six cycles. Includes estimated ovulation dates.
| # | Period Starts | Period Ends | Est. Ovulation | Days Away |
|---|---|---|---|---|
| 1 | May 11, 2026 | May 15, 2026 | May 25, 2026 | In 28 days |
| 2 | Jun 8, 2026 | Jun 12, 2026 | Jun 22, 2026 | In 56 days |
| 3 | Jul 6, 2026 | Jul 10, 2026 | Jul 20, 2026 | In 84 days |
| 4 | Aug 3, 2026 | Aug 7, 2026 | Aug 17, 2026 | In 112 days |
| 5 | Aug 31, 2026 | Sep 4, 2026 | Sep 14, 2026 | In 140 days |
| 6 | Sep 28, 2026 | Oct 2, 2026 | Oct 12, 2026 | In 168 days |
Disclaimer: This is an estimate based on average cycle patterns. Actual dates may vary due to stress, illness, hormonal changes, and other factors. Not suitable for contraception. Consult a healthcare professional for medical advice.
Understanding the Menstrual Cycle
The menstrual cycle is the monthly series of changes the body goes through to prepare for possible pregnancy. It is controlled by a complex interplay of hormones produced by the hypothalamus, pituitary gland, and ovaries. The cycle begins on the first day of menstruation (a period) and ends the day before the next period starts.
The average cycle length is 28 days, but "normal" spans a wide range from 21 to 35 days. Even within a single person, cycle length can vary by a few days each month due to stress, illness, travel across time zones, changes in exercise or diet, and other lifestyle factors.
The Four Phases of the Menstrual Cycle
Phase 1: Menstruation (Days 1–5)
The first day of menstruation is Day 1 of the cycle. During menstruation, the uterine lining (endometrium) that built up during the previous cycle sheds because fertilization did not occur. Prostaglandins cause uterine contractions that help expel the lining, which is why cramping is common. Menstruation typically lasts 3–7 days.
Phase 2: Follicular Phase (Days 1–13)
The follicular phase begins at Day 1 (overlapping with menstruation) and ends at ovulation. The pituitary gland releases follicle-stimulating hormone (FSH), which stimulates several follicles in the ovary to develop. One dominant follicle emerges and secretes increasing amounts of estrogen. Rising estrogen causes the uterine lining to thicken in preparation for a potential embryo.
Phase 3: Ovulation (Around Day 14)
A surge in luteinizing hormone (LH) — triggered by peak estrogen levels — causes the dominant follicle to rupture and release a mature egg (ovum). This is ovulation. The egg travels down the fallopian tube toward the uterus. The egg is viable for fertilization for approximately 12–24 hours, but sperm can survive in the reproductive tract for up to 5 days, making the fertile window wider.
Phase 4: Luteal Phase (Days 15–28)
After ovulation, the ruptured follicle becomes the corpus luteum, which secretes progesterone (and some estrogen). Progesterone prepares the uterine lining for embryo implantation and suppresses further ovulation. If fertilization does not occur, the corpus luteum degenerates after about 14 days, progesterone drops, and menstruation begins — starting the next cycle. The luteal phase is the most consistent phase in length: approximately 14 days for most people.
Estimating Ovulation
This calculator estimates ovulation using the calendar method:
Estimated ovulation = first day of next period − 14 days
This works because the luteal phase (from ovulation to the next period) is consistently around 14 days for most people. The follicular phase (from period start to ovulation) varies more, which is why cycle length varies — it reflects differences in the follicular phase.
Calendar-based ovulation estimation is approximate. More reliable methods include:
- LH surge testing — over-the-counter ovulation predictor kits (OPKs) detect the LH surge 24–48 hours before ovulation
- Basal body temperature (BBT) — temperature rises 0.2–0.5°C after ovulation; charting over months identifies patterns
- Cervical mucus monitoring — mucus becomes clear and stretchy (egg-white consistency) around ovulation
- Transvaginal ultrasound — most accurate, used in fertility treatment to confirm ovulation
Period Length and Variations
Normal period duration is 3–7 days. Normal blood loss is 30–80 mL per period. Variations commonly observed:
| Term | Definition | Possible Causes |
|---|---|---|
| Amenorrhea | Absence of periods | Pregnancy, excessive exercise, low body weight, PCOS, menopause |
| Oligomenorrhea | Infrequent periods (>35 day cycles) | PCOS, thyroid disorders, stress, perimenopause |
| Polymenorrhea | Frequent periods (<21 day cycles) | Hormonal imbalance, thyroid disorders, endometriosis |
| Menorrhagia | Heavy periods (>80 mL loss) | Fibroids, endometriosis, hormonal imbalance, IUDs |
| Dysmenorrhea | Painful periods | Primary (prostaglandins) or secondary (endometriosis, fibroids) |
PMS and PMDD
Premenstrual syndrome (PMS) refers to physical and emotional symptoms that occur in the luteal phase (1–2 weeks before menstruation) and resolve with the onset of the period. Common symptoms include bloating, breast tenderness, mood changes, irritability, fatigue, and food cravings. Up to 75% of people who menstruate experience some PMS symptoms.
Premenstrual dysphoric disorder (PMDD) is a severe form of PMS characterized by debilitating mood symptoms including depression, anxiety, and irritability that significantly impair daily functioning. PMDD affects about 3–8% of people who menstruate and is recognized as a psychiatric condition treatable with SSRIs, hormonal contraceptives, and other interventions.
Frequently Asked Questions
How does the period calculator work?
The calculator takes the first day of your last menstrual period (LMP) and adds your cycle length to predict future periods. For example, if your last period started on March 1 and your cycle is 28 days, your next period is predicted for March 29. The ovulation estimate is calculated as cycle start date plus (cycle length − 14 days), since ovulation typically occurs 14 days before the next period.
What is a normal menstrual cycle length?
A menstrual cycle is measured from the first day of one period to the first day of the next. The average is 28 days, but normal cycles range from 21 to 35 days for most adults. Cycle length can vary month to month even in people with generally regular cycles. Consistently shorter than 21 days or longer than 35 days may warrant discussion with a healthcare provider.
What causes irregular periods?
Irregular periods (oligomenorrhea) can be caused by many factors: polycystic ovary syndrome (PCOS), thyroid disorders, significant weight loss or gain, excessive exercise, chronic stress, perimenopause, hormonal birth control, certain medications, eating disorders, and breastfeeding. A period that varies by 1–7 days each month is generally considered within normal range. Greater variability or missed periods should be evaluated by a healthcare provider.
Can I use this calculator for contraception?
No. This calculator is not suitable for contraception. The fertile window prediction is based on averages and does not account for individual variation. Ovulation timing can shift significantly due to stress, illness, travel, and other factors. For contraception, use medically validated methods and consult a healthcare provider. Fertility awareness methods require several months of tracking and instruction from a trained practitioner.
What are the four phases of the menstrual cycle?
The menstrual cycle has four phases: (1) Menstruation (days 1–5 on average) — the uterine lining sheds as a period. (2) Follicular phase (days 1–13) — overlaps with menstruation; the pituitary releases FSH, stimulating follicle growth in the ovary; estrogen rises. (3) Ovulation (around day 14) — a surge in LH triggers release of a mature egg. (4) Luteal phase (days 15–28) — the empty follicle becomes the corpus luteum and releases progesterone to prepare the uterine lining for potential implantation.
Medical Disclaimer: For informational purposes only. Period and ovulation predictions are estimates based on averages and may not reflect your actual cycle. This tool is not suitable for contraception or fertility planning. Consult a qualified healthcare professional for medical advice about your menstrual health, irregular cycles, or fertility concerns.
Related Calculators
- Ovulation Calculator — fertile window based on cycle length
- Pregnancy Calculator — due date and trimester timeline from LMP
- Age Calculator — calculate exact age in years, months, and days
- BMI Calculator — Body Mass Index with healthy weight range