Period Calculator

About the Period Calculator

A period calculator (menstrual cycle calculator) predicts the dates of your next several periods and maps out your complete cycle phases based on your last period start date and average cycle length. Knowing when your next period is expected helps you plan around it, prepare for PMS symptoms, understand your cycle's impact on energy, mood, and athletic performance, and identify irregularities that may warrant medical attention. A normal menstrual cycle ranges from 21 to 35 days, with 28 days being the textbook average (though research shows only about 13% of women actually have a 28-day cycle). Our free period calculator predicts your next 6-12 periods, shows the dates for each cycle phase (menstruation, follicular, ovulatory, luteal), calculates your average cycle length from historical data, and provides a colour-coded calendar view. It also flags cycle lengths outside the 21-35 day normal range and shows how your cycle length compares to population norms.

How It Works

Period prediction: next period start date = last period start date + cycle length. For a 29-day cycle with last period starting March 5: next period = March 5 + 29 = April 3. Following period = April 3 + 29 = May 2, and so on. Cycle phases based on 29-day cycle: Menstruation (period) = days 1-5 (average duration 3-7 days). Follicular phase = days 1-13 (overlaps with menstruation; follicles develop in ovaries). Ovulation = approximately day 15 (cycle length minus 14). Luteal phase = days 15-29 (after ovulation until next period). Average cycle length from history: sum all cycle lengths, divide by the count. For cycles of 27, 29, 31, 28, 30 days: average = 145/5 = 29 days. Variation of up to 7-8 days from your personal average is generally considered within normal range.

Tips & Best Practices

• ✓Track 3-6 months of cycles before relying on predictions — your personal average cycle length is far more accurate for predicting future periods than the textbook 28-day assumption.
• ✓Cycle tracking apps (Clue, Flo, Natural Cycles) store historical data and improve prediction accuracy over time using machine learning on your personal cycle pattern.
• ✓Premenstrual syndrome (PMS) symptoms typically begin in the luteal phase, 1-2 weeks before the period, and resolve within a few days of menstruation starting. The period calculator shows when your luteal phase begins so you can anticipate and plan around PMS timing.
• ✓Late period definition: a period is considered late if it has not started 5 or more days past your expected date. Missing a period entirely (more than 5-7 days late) warrants a pregnancy test and potentially a healthcare consultation if negative.
• ✓Perimenopause typically begins in the mid-40s: the first sign is often irregular cycles — lengthening, shortening, or becoming unpredictable. Tracking cycles helps identify this transition and distinguish it from other causes of irregularity.
• ✓Cycle length and health: consistently very short cycles (under 21 days) or very long cycles (over 35 days) can indicate hormonal imbalances including PCOS, thyroid dysfunction, or anovulation and warrant medical evaluation.
• ✓Athletic performance and cycle phases: research shows performance, recovery, and strength vary across cycle phases. Many athletes now track their cycle to optimise training periodisation — scheduling high-intensity training in the follicular phase and prioritising recovery in the late luteal phase.
• ✓Birth control effects: hormonal contraceptives (the pill, patch, ring) suppress natural cycles — withdrawal bleeds are not true periods. The period calculator is most accurate for women not using hormonal contraception.

Who Uses This Calculator

Women and people who menstruate use the period calculator to plan around their cycle for major events, travel, athletic competitions, and social occasions where period management is inconvenient. People managing PMS or PMDD use it to anticipate symptom windows and prepare coping strategies or schedule fewer obligations during the luteal phase. People with dysmenorrhea (painful periods) use it to ensure they have appropriate pain management supplies available ahead of time. Healthcare providers use cycle tracking information during consultations about menstrual irregularity, fertility concerns, and hormonal health. Women approaching perimenopause use the calendar to track cycle length changes over time and discuss them with their OB-GYN. Competitive athletes use cycle phase tracking to align training intensity with the phases associated with highest energy and performance. Partners of people who menstruate use period calculators to better understand cycle-related mood and energy changes and support their partners accordingly.