Period Calculator
Predict your next 6 periods, see your current cycle phase, and find your fertile window — all from your last period date.
How to use this calculator
Enter the first day of your last period and your average cycle length. Your next period date, current cycle phase, and ovulation window appear instantly. The calculator also shows the next 6 predicted periods so you can plan ahead.
Understanding your cycle
A typical menstrual cycle is 21–35 days, with 28 days being the average. The cycle has four phases: menstrual (days 1–5), follicular (days 6–13), ovulation (around day 14), and luteal (days 15–28). Ovulation — when an egg is released — typically occurs 14 days before your next period, regardless of cycle length. Cycles vary naturally month to month.
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 |
Menstrual cycle facts
Understanding your cycle's normal variation helps distinguish typical fluctuation from patterns worth discussing with a doctor.
- ·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
- ·Periods typically last 3–7 days; flow is heaviest in days 1–2
- ·Ovulation occurs approximately 14 days before the next period (not necessarily day 14 of the cycle)
- ·For a 35-day cycle, ovulation is around day 21; for a 24-day cycle, it's around day 10
- ·Stress is the most common cause of delayed or missed periods — cortisol interferes with the LH surge needed for ovulation
- ·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 or after sex
- ·Trying to conceive for 12 months without success (6 months if over 35)
- ·Signs of perimenopause before age 45 (irregular cycles, hot flashes, night sweats)