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Printed on 6/29/2026
For informational purposes only. This is not medical advice.
This period calculator estimates your next menstrual period start date from the first day of your last period, your average cycle length, and your typical period length. It also shows your current cycle day, estimated ovulation timing (use [Ovulation Calculator](/tools/ovulation-calculator) for detailed fertility planning), and a projected fertile window for planning and cycle awareness.
Formula: Next period start = LMP + n x cycle length; ovulation estimate = next period - 14 days
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Enter the first day of your most recent period and your average cycle length (21–35 days, average 28). Cycle length is the number of days from the first day of one period to the first day of the next.
The calculator predicts your next period date by adding your cycle length to your LMP date. Subsequent periods are projected every cycle-length days. Periods typically last 3–7 days.
Your fertile window is estimated from 5 days before ovulation through 1 day after. Ovulation is calculated at cycle length minus 14 days from LMP, reflecting the fixed ~14-day luteal phase.
Women planning pregnancy
Track your menstrual cycle to identify fertile windows for conception planning or understand cycle timing for natural family planning and contraception awareness.
Women with dysmenorrhea
Predict upcoming period dates to prepare for dysmenorrhea, PMS, or endometriosis symptoms. Advance notice allows proactive pain management and scheduling adjustments.
Gynecologists
Establish cycle regularity at clinical visits. Track LMP and cycle length as a baseline for evaluating menstrual irregularities, hormonal disorders, and reproductive health concerns.
Fertility counselors
Support the fertility awareness method (FAM) by correlating predicted fertile windows with observed cervical mucus changes, basal body temperature, and other cycle signs.
Endocrinologists
Track cycle timing alongside symptom diaries to identify patterns in endometriosis flares, PCOS-related irregularities, and hormonal symptom burden throughout the cycle.
Most women have cycles ranging 21–35 days — 28 days is the population average, not a standard all women should match. Enter your actual average cycle length for the most accurate predictions.
Irregular cycles make predictions less accurate. Track 3–6 months of cycle start dates to calculate a reliable average. Even a rough average is better than using the default 28-day assumption.
Cycle length is measured from the FIRST DAY of one period to the FIRST DAY of the next — not from the end of one period to the start of the next. This is a common measurement error.
The luteal phase (ovulation to period) is typically fixed at approximately 14 days. It is the FOLLICULAR phase (period to ovulation) that varies between women and cycles. This is why ovulation timing differs even in women with regular cycles.
Cervical mucus becomes egg-white (clear, stretchy) near ovulation. You may notice mild pelvic pain (Mittelschmerz) and a slight basal body temperature rise of 0.2–0.5°C after ovulation. These signs confirm calendar predictions.
Polycystic ovary syndrome (PCOS) causes cycles that are often longer than 35 days or absent. If your cycles are consistently longer than 35 days or you miss periods without pregnancy, discuss PCOS evaluation with your provider.
Hypothyroidism can cause heavy or irregular periods. If cycle changes accompany fatigue, weight gain, or cold intolerance, request thyroid function testing (TSH, free T4) at your next visit.
Irregular cycles in your 40s may signal perimenopause. Cycle changes — shorter or longer cycles, heavier or lighter flow — can begin 2–10 years before the final menstrual period (menopause at average age 51).
If your period is more than 5 days later than predicted and you have been sexually active, take a home pregnancy test. Most tests are accurate from the first day of a missed period.
Period tracker apps improve predictions with continuous data input. For fertility planning, consider hormone monitoring tests (LH strips, progesterone tests) to supplement calendar-based predictions.
Menstrual cycle physiology: Treloar et al. (J Reprod Med 1967) documented cycle length distribution from 2700 women — average 28.9 days (SD 3.5). Luteal phase fixity at approximately 14 days: Lenton et al. (Clin Endocrinol 1984). PCOS menstrual irregularity: Azziz et al. (J Clin Endocrinol Metab 2004). Fertility awareness method efficacy and cycle tracking: Fehring et al. (J Midwifery Womens Health 2004). Period tracking app validation: Symul et al. (NPJ Digit Med 2019).
Your result provides an estimated next period date and your current cycle day. If your cycles are usually regular, the prediction can be useful for planning. If your cycles vary month to month, focus on the approximate window rather than a single exact date.
The tool also estimates ovulation and the fertile window from cycle timing assumptions. If your goal is conception timing, use the [Ovulation Calculator](/tools/ovulation-calculator) for focused fertile-window planning. If pregnancy is possible, the [Pregnancy Due Date Calculator](/tools/pregnancy-due-date) can estimate gestational timing from your last period, and track progress with [Gestational Age Calculator](/tools/gestational-age).
Use this tool for cycle planning, symptom tracking, travel planning, or understanding where you are in your cycle today. It is useful for people with regular or moderately variable cycles who want a quick estimate without a full tracking app.
It can also support preconception awareness when combined with an [Ovulation Calculator](/tools/ovulation-calculator), especially if you are trying to understand when fertility is most likely in upcoming cycles.
This calculator assumes that future cycles resemble your average past cycle length and that ovulation occurs about 14 days before the next period. Real cycles can vary, especially with irregular menstruation, polycystic ovary syndrome (often associated with obesity - assess with [BMI Calculator](/tools/bmi-calculator)), recent postpartum changes, perimenopause, or hormonal contraception changes. Optimize reproductive health with cardiovascular assessment ([ASCVD Risk Calculator](/tools/ascvd-risk)).
Predictions are not diagnostic. Missed periods, very frequent periods, heavy bleeding, or persistent cycle irregularity should be evaluated clinically rather than managed with prediction tools alone.
Disclaimer: This tool is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider with questions about your health.
April 21, 2026 · trust-baseline
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