Period Calculator
Predict your next 6 periods, see which phase of your cycle you're in today, and track your menstrual health over time — all from your last period date.
How to use this calculator
Enter the first day of your last period, your average cycle length, and how many days your period usually lasts. Your next period date and current cycle phase appear instantly, along with the next 6 predicted periods so you can plan ahead and watch for changes in your pattern over time.
Understanding your cycle
Your menstrual cycle is counted from the first day of one period to the first day of the next, and a length of 21–35 days is considered normal, with 28 days being the average. Tracking your cycle over several months reveals your personal baseline — how long your periods last, how heavy they are, what symptoms recur, and how much your timing varies. That baseline is what makes it possible to notice meaningful changes: a cycle that suddenly runs much shorter or longer, a period that becomes far heavier, or new pain. Day-to-day variation of a few days is expected and rarely a concern; it is consistent shifts away from your own pattern that merit attention.
Frequently asked questions
The four phases of the menstrual cycle
The menstrual cycle is divided into four distinct phases driven by changing hormone levels. Each phase has characteristic symptoms, energy levels, and fertility implications.
| Phase | Days (28-day cycle) | Key hormones | Characteristics |
|---|---|---|---|
| Menstrual | Days 1–5 | Estrogen and progesterone low | Uterine lining sheds; cramping common; energy often lower |
| Follicular | Days 6–13 | Estrogen rising | Energy and mood improve; follicles mature in ovary |
| Ovulation | Day 14 (approx) | LH surge, estrogen peaks | Egg released; most fertile day; may feel peak energy and libido |
| Luteal | Days 15–28 | Progesterone rises then drops | PMS symptoms may appear; appetite increases; temperature slightly elevated |
Tracking your cycle and reading the patterns
Keeping a simple record of your start dates, flow, and symptoms turns a sense of "something feels off" into something concrete you can act on. The notes below cover normal variation, common premenstrual symptoms, and the changes to expect as you approach menopause.
- ·Average cycle length is 28 days, but 21–35 days is considered normal range
- ·Cycle length varies month to month — variation of ±3–5 days is typical for most women
- ·PMS (premenstrual syndrome) — bloating, irritability, breast tenderness, food cravings, and fatigue in the week or so before a period — affects a large share of menstruating people and usually eases once bleeding begins
- ·PMDD (premenstrual dysphoric disorder) is a severe, less common form in which mood symptoms such as marked depression, anxiety, or anger seriously disrupt daily life; tracking symptoms against cycle dates is key to recognizing it, and effective treatments exist
- ·Perimenopause, the transition that often begins in the 40s, brings increasingly irregular cycles, changes in flow, and symptoms like hot flashes and night sweats; cycles may grow longer or shorter and more unpredictable for several years before periods stop
- ·Periods typically last 3–7 days; flow is heaviest in the first 1–2 days, then tapers
- ·It is normal for cycle length and flow to change across your lifespan — the years after your first period and the years approaching menopause tend to be the most irregular
- ·Tracking at least 3 consecutive cycles gives a far more reliable picture than any single month
- ·Logging symptoms alongside dates (cramps, mood, breast tenderness, headaches, bloating) helps you and your doctor spot patterns such as PMS or PMDD
- ·Stress is one of the most common reasons a period arrives late or is skipped, because elevated cortisol can temporarily disrupt the hormonal signals that govern the cycle
- ·Significant weight change (gain or loss) can disrupt cycle timing and regularity
- ·Hormonal contraceptives (pill, IUD, implant) regulate or suppress natural cycle variation
What's in a normal period?
Average blood loss during a period is 30–80 mL (about 2–5 tablespoons). What looks like a lot is often mixed with uterine lining tissue and discharge. Menstrual flow over 80 mL is considered heavy menstrual bleeding (menorrhagia) and warrants medical evaluation.
| Feature | Normal range | May warrant evaluation |
|---|---|---|
| Duration | 3–7 days | <2 days or >7 days consistently |
| Cycle length | 21–35 days | <21 days or >35 days consistently |
| Blood loss | 30–80 mL total | >80 mL (soaking >1 pad/hour for 2+ hours) |
| Color | Bright red to dark brown | Consistently gray or unusual colors |
| Clots | Small clots (quarter-size) are normal | Clots larger than a quarter consistently |
| Pain | Mild to moderate cramping, days 1–2 | Severe pain interfering with daily life |
When to see a doctor about your period
- ·Cycles consistently shorter than 21 days or longer than 35 days
- ·Missing 3 or more periods in a row (when not pregnant)
- ·Bleeding so heavy you soak through a pad or tampon every hour for 2+ hours
- ·Periods suddenly becoming much heavier, longer, or more painful than usual
- ·Severe cramping (dysmenorrhea) that doesn't improve with over-the-counter pain relief
- ·Bleeding between periods, after sex, or any bleeding after menopause
- ·Periods that stopped for several months and have not returned (when not pregnant or breastfeeding)
- ·Premenstrual mood symptoms severe enough to disrupt work, relationships, or daily life (possible PMDD)
- ·Periods that started before age 9 or had not begun by age 15