How do I calculate when my next period is?

Add your average cycle length to the first day of your last period. For example, if your last period started March 1 and your cycle is 28 days, your next period is expected around March 29. This calculator does the math automatically and projects several future cycles.

What is a normal menstrual cycle length?

Normal menstrual cycle length ranges from 21 to 35 days, with the population average around 28 days. Cycles shorter than 21 days (polymenorrhea) or longer than 35 days (oligomenorrhea) warrant evaluation. Occasional variation is normal — persistent irregularity should be assessed clinically.

When is my period due? How accurate is the prediction?

The predicted period date is accurate to ±2–5 days for women with regular cycles. Accuracy decreases with irregular cycles. The prediction assumes future cycles will match your historical average, which is not always the case, especially with stress, illness, or hormonal changes.

How do I predict my period with an irregular cycle?

Use the average of your last 3–6 cycles as your cycle length input. Accept that predictions will have wider uncertainty (±5–7 days). Tracking actual start dates over time refines your average. Period-tracking apps can calculate this average automatically from historical data.

What is the fertile window relative to my period?

The fertile window is approximately cycle day 9–18 for a 28-day cycle — 5 days before ovulation through 1 day after. Ovulation typically occurs on cycle day 14 (28-day cycle). For other cycle lengths, ovulation is approximately cycle length minus 14 days from the period start.

How does cycle tracking support period and fertility planning?

By tracking your cycle start dates, you can predict upcoming periods for planning, identify your fertile window for conception or avoidance, and monitor cycle regularity as a general health indicator. Cycle irregularity can be an early sign of hormonal, thyroid, or reproductive health changes.

Can I get pregnant right before or during my period?

Pregnancy during menstruation is very unlikely but not impossible in women with very short cycles or long fertile windows. Ovulation can occasionally occur earlier than predicted, especially in shorter cycles. For reliable contraception, consult your provider rather than relying on period-based timing alone.

How does PCOS affect period prediction?

PCOS causes highly irregular cycles that are often longer than 35 days or absent. Calendar-based period prediction is unreliable in PCOS. Women with PCOS may need hormonal support to regulate cycles and predict ovulation. Consult an OB/GYN or endocrinologist for personalized management.

Can perimenopause cause irregular periods?

Yes. Perimenopause, which can begin 2–10 years before menopause (average onset in mid-40s), causes cycle length variability, skipped periods, heavier or lighter flow, and other changes. If you are in your 40s with new cycle irregularities, discuss perimenopause evaluation with your provider.

When should I take a pregnancy test if my period is late?

Take a home pregnancy test if your period is 5 or more days late and you have been sexually active. Most home pregnancy tests are highly accurate from the first day of a missed period. A blood hCG test at a clinic is even more sensitive and can detect pregnancy earlier.

What does it mean if my period is 5 days early or late?

Variation of ±5 days from your predicted period date is common and usually normal, especially with stress, travel, illness, or cycle length variation. Persistent significant variation (more than 7 days) or very irregular cycles warrant evaluation to rule out hormonal causes.

Can stress cause a late period?

Yes. Physical or emotional stress can delay ovulation by raising cortisol levels, which suppresses the GnRH pulse that triggers the LH surge. Since the period follows ovulation by ~14 days, delayed ovulation means a delayed period. Stress is one of the most common causes of a late period in otherwise healthy women.

Online Medical Tools — Period Calculator

Printed on 8/14/2026

For informational purposes only. This is not medical advice.

Obstetrics

Period Calculator

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 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

First Day of Last Period

Average Cycle Length

days

Average Period Length

days

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Keep a history of calculations, favorite tools, and access your dashboard anytime.

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How It Works

Enter your last menstrual period date and cycle length

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.

Calculate next period and subsequent cycles

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.

View your fertile window

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.

Who Uses the Period Calculator

Family Planning and Contraception Timing

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.

Period Symptom Management

Women with dysmenorrhea

Predict upcoming period dates to prepare for dysmenorrhea, PMS, or endometriosis symptoms. Advance notice allows proactive pain management and scheduling adjustments.

Gynecology Clinical Assessment

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 Awareness Natural Family Planning

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.

Endometriosis and PCOS Symptom Correlation

Endocrinologists

Track cycle timing alongside symptom diaries to identify patterns in endometriosis flares, PCOS-related irregularities, and hormonal symptom burden throughout the cycle.

Pro Tips

28 days is the average, not universal

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.

Track 3–6 months for better accuracy

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.

Measure cycle length correctly

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 is fixed; the follicular phase varies

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.

Signs of ovulation beyond calendar prediction

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.

PCOS causes highly irregular cycles

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.

Thyroid disease affects cycle regularity

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.

Perimenopause can begin 2–10 years before menopause

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).

Test for pregnancy if period is more than 5 days late

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 accuracy over time

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.

Common Questions About Your Results

Evidence-Based Methodology

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).

Clinical Content Trust

Last reviewed:: April 21, 2026
Guideline version:: General evidence framework v2026.04
Source set version:: Primary-source set v1

How to Interpret Your Result

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).

When to Use This Tool

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.

Limitations

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.

Changelog

April 21, 2026 · trust-baseline
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