Health
Period Calculator
Predict your next period and track upcoming cycles from your last period date and cycle length.
How long your period typically lasts. Default is 5 days.
Next Period
Cycle Day Today
Current Phase
Period ends around
Duration
Upcoming Periods
| # | Starts | Ends | Days away |
|---|
Current Cycle
Cycle Phases
Menstrual
Days 1–5Uterine lining sheds. Estrogen and progesterone are at their lowest. Cramping and flow are normal. Duration varies 2–7 days.
Follicular
Days 1–13Overlaps with menstruation. FSH stimulates follicle growth; estrogen rises. Energy often increases as the week progresses.
Ovulation
Day ~14LH surge triggers egg release. Cervical mucus becomes egg-white-like. Basal body temperature rises slightly. Fertile window peaks.
Luteal
Days 15–28Corpus luteum produces progesterone. If pregnancy does not occur, progesterone drops, triggering menstruation. PMS symptoms common.
Cycle Length Reference
| Cycle | Classification | Notes |
|---|---|---|
| < 21 days | Short (Polymenorrhea) | Less common; may indicate hormonal imbalance |
| 21–24 days | Short-normal | Within clinical range; monitor for anemia |
| 25–30 days | Average | Most common range; 28 days is textbook |
| 31–35 days | Long-normal | Normal variant; ovulation occurs later |
| > 35 days | Long (Oligomenorrhea) | Fewer than 9 periods/year; evaluate for PCOS |
How It's Calculated
Period prediction uses simple cycle arithmetic:
The luteal phase (ovulation → period) is nearly always 14 days. Cycle length variation shifts the follicular phase, not the luteal.
Frequently Asked Questions
Add your average cycle length to the first day of your last period. If your last period started January 1 and your cycle is 28 days, your next period is expected January 29. If your cycle is 32 days, it falls February 2. The formula is: Next period = Last period start date + cycle length in days.
A normal cycle ranges from 21 to 35 days, measured from the first day of one period to the first day of the next. The average is 28 days, but only about 13% of women have exactly 28-day cycles. Cycle length naturally varies by a few days month to month — a variation of ±7 days is considered normal. Consistently shorter than 21 days or longer than 35 days warrants a conversation with your doctor.
Most periods last 2 to 7 days. The average is 3–5 days. The first 1–2 days tend to be the heaviest flow, tapering off toward the end. Period length can vary due to stress, hormonal changes, contraceptive use, and age. Perimenopause often brings longer, heavier, or more irregular periods.
Common causes of a late period include pregnancy, stress, significant weight change (gain or loss), intense exercise, thyroid disorders, polycystic ovary syndrome (PCOS), and hormonal contraceptives. A period is generally considered "late" after day 35 of your cycle (or if it is more than 5–7 days past your expected date for a regular cycle). One late period occasionally happens to most women. Consistently irregular cycles deserve medical evaluation.
The menstrual cycle has four phases: (1) Menstrual phase — bleeding, days 1–5 on average; (2) Follicular phase — estrogen rises, follicles develop, days 1–13; (3) Ovulation — egg release around day 14 in a 28-day cycle; (4) Luteal phase — progesterone dominates, days 15–28. If pregnancy does not occur, progesterone drops and the cycle restarts with menstruation.
Period calculators are most accurate for women with regular, consistent cycle lengths. Research shows that only about 30% of cycles fit the "textbook" 28-day pattern; most women experience natural variation of ±3–4 days. For irregular cycles, a calendar-based calculator provides a rough estimate only. Tracking several months of data — ideally with an app — improves prediction accuracy over time.
Last updated: July 18, 2026