# Period/Menstrual Cycle Calculator

Track your menstrual cycle and predict your next period, fertile window, and ovulation day. Understand your cycle phases with our free period calculator.

## What this calculates

Understanding your menstrual cycle helps you anticipate your period, identify your fertile window, and recognize any irregularities. Enter your cycle details to see your current cycle phase, predicted next period, and estimated ovulation day.

## Inputs

- **Average Cycle Length** (days) — min 20, max 45 — Days from the first day of one period to the first day of the next
- **Average Period Duration** (days) — min 1, max 10 — Number of days your period typically lasts
- **Days Since Last Period Started** (days) — min 0, max 60 — Days since the first day of your most recent period

## Outputs

- **Current Cycle Phase** — formatted as text — Which phase of the menstrual cycle you are currently in
- **Days Until Next Period** — Estimated days until your next period begins
- **Fertile Window** — formatted as text — Cycle days when conception is most likely
- **Estimated Ovulation** — formatted as text — Expected day of ovulation in your cycle

## Details

The menstrual cycle consists of four main phases: menstruation (the period), the follicular phase, ovulation, and the luteal phase. A typical cycle lasts 28 days, but normal cycles can range from 21 to 35 days in adults. The cycle is counted from the first day of menstrual bleeding.

During the follicular phase (days 1-13 in a 28-day cycle), follicle-stimulating hormone (FSH) causes an egg to mature in the ovary. At ovulation (approximately day 14), the mature egg is released. The luteal phase follows (days 15-28), during which progesterone prepares the uterine lining for potential implantation. If fertilization does not occur, progesterone drops and the lining sheds as menstruation.

The luteal phase is relatively constant at 12-16 days (average 14), while the follicular phase varies more widely between individuals and cycles. This is why ovulation does not always occur on day 14 and why this calculator uses the cycle length to estimate ovulation by counting back 14 days from the expected next period. Tracking your cycle over several months improves prediction accuracy.

## Frequently Asked Questions

**Q: What is a normal menstrual cycle length?**

A: A normal menstrual cycle ranges from 21 to 35 days for adults and 21 to 45 days for adolescents. The average is 28 days, but many women have cycles that are shorter or longer. Cycles may vary by a few days from month to month, which is considered normal. If your cycle is consistently shorter than 21 days or longer than 35 days, or if your cycle length varies significantly, consult a healthcare provider.

**Q: What are the four phases of the menstrual cycle?**

A: The four phases are: (1) Menstruation (days 1-5 approx), when the uterine lining sheds. (2) Follicular phase (days 1-13), when estrogen rises and an egg matures. (3) Ovulation (around day 14), when the mature egg is released from the ovary. (4) Luteal phase (days 15-28), when progesterone rises to prepare for potential pregnancy. If pregnancy does not occur, hormone levels drop and the cycle restarts with menstruation.

**Q: Why do periods sometimes come early or late?**

A: Period timing can be affected by many factors including stress, travel, illness, significant weight changes, excessive exercise, hormonal contraceptives, breastfeeding, and perimenopause. Stress is one of the most common causes of temporary cycle irregularity as it affects the hypothalamus, which controls the hormones that regulate your cycle. Occasional irregularity is normal, but persistent changes should be discussed with a doctor.

**Q: How long is a normal period?**

A: A normal period lasts between 2 and 7 days, with the average being 3-5 days. Bleeding is typically heaviest during the first 1-2 days and tapers off. Total blood loss during a normal period is about 30-80 mL (2-5 tablespoons). If your period regularly lasts longer than 7 days, requires pad or tampon changes more than hourly, or includes large clots, consult your healthcare provider as these may indicate menorrhagia or other conditions.

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Source: https://vastcalc.com/calculators/health/period-cycle
Category: Health & Fitness
Last updated: 2026-04-21