What is a 'normal' cycle length?

The medical definition of a normal menstrual cycle is 21–35 days, measured from the first day of one period to the first day of the next. Cycles shorter than 21 days (polymenorrhea) or longer than 35 days (oligomenorrhea) may warrant evaluation. 'Normal' for an individual can vary — consistent cycles of 25 or 33 days are normal for those women.

How accurate are period predictions?

For women with very regular cycles (cycle length varies by 1–2 days), predictions are accurate to within a few days. For women with more variation (5–7 day variation), predictions become a window rather than a specific date. Tracking actual period start dates and updating your average cycle length over 3–6 months improves prediction accuracy.

My period is late — am I pregnant?

A late period can result from pregnancy, stress, illness, significant weight changes, intensive exercise, hormonal fluctuations, or thyroid issues. If your period is more than a week late and you're sexually active, a home pregnancy test is the most straightforward next step. Occasional late periods are common; consistently irregular cycles deserve medical evaluation.

What causes cycle length to vary month to month?

The follicular phase (period to ovulation) varies most between cycles; the luteal phase is relatively stable at 12–16 days. Factors that extend the follicular phase (delaying ovulation) include stress, illness, travel across time zones, significant diet changes, and intensive new exercise programs. This extends cycle length, making the next period arrive later than predicted.

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When Will Your Next Period Arrive? Period Tracker & Predictor

When will your next period arrive?

What This Does

Knowing when your next period will arrive helps with planning, managing symptoms, and understanding your overall menstrual health. The menstrual cycle is counted from the first day of one period to the first day of the next — typically 21 to 35 days, with 28 days being the well-known average that many women don't actually experience. Period prediction is straightforward for regular cycles: add your cycle length to the first day of your last period to find when the next one begins. But most women experience some variation — cycle lengths fluctuate by 2–7 days from month to month due to stress, sleep, diet, exercise, hormonal fluctuations, and life events. This means predictions become less certain the further ahead you project. Beyond period start dates, understanding your cycle phases helps explain the physical and emotional patterns many women notice throughout the month: the follicular phase (period through ovulation) is associated with rising energy and mood; ovulation marks peak fertility and often a brief energy peak; the luteal phase (ovulation through next period) is when PMS symptoms typically emerge; and menstruation itself completes the cycle. This calculator predicts your next period date, duration, and end date, then projects the next 6 cycles — useful for travel planning, event scheduling, and health tracking.

When Should You Use This?

→Predicting when your next period will start for scheduling or travel planning
→Tracking whether your period is late or early relative to your average cycle
→Understanding your cycle phases and when to expect hormonal changes
→Planning ahead with a 6-month period calendar
→Checking if your cycle length falls within the normal range

Example Scenario

Keisha has a 29-day cycle with periods that typically last 5 days. Her last period started February 10. Next period prediction: February 10 + 29 days = March 11, ending March 15. The six-month forecast shows all predicted period windows through August, color-coded by cycle phase. She can now plan her beach vacation in May to avoid her predicted period window and identify the days when she will likely feel best during the trip.

Period Calculator

Next Period · Cycle Phases · Fertile Window · 6-Cycle Forecast

Results update in real time as you adjust any input.

First Day of Last Period

Cycle Length (days)

Normal range: 21–35 days

Period Duration (days)

Typical: 2–7 days

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Enter the first day of your last menstrual period to predict your next period and see your full cycle analysis.

About This Calculator

This period calculator predicts your next period date, current cycle phase, fertile window, and next ovulation from the first day of your last menstrual period (LMP), cycle length, and period duration. All results update in real time as you type — no button needed. Enter any date and see your next period, the days remaining, and your position in the current cycle immediately.

The Overview tab renders a fertility area chart showing estimated fertility probability across all days of your cycle, plus a phase composition bar chart showing how many days of your cycle fall in each phase (Menstrual/Follicular/Ovulation/Luteal). The Phases tab provides an interactive colour-coded cycle bar with your current position marked, plus four detailed phase cards explaining symptoms, hormones, and tracking tips for each phase. The Forecast tab shows a bar chart of days until each of the next 6 periods, plus a full calendar list with fertile window and ovulation dates for each cycle.

Dynamic accent colours reflect your cycle status: rose (currently in period), orange (period imminent, ≤3 days), amber (period approaching, ≤7 days), indigo (mid-cycle). Accuracy flags appear automatically for cycle lengths outside 21–35 days, periods longer than 7 days, or periods shorter than 2 days. Export to PDF generates a complete report with the 6-cycle forecast table and key insights. Canvas download creates a shareable card with your cycle phase bar and key dates.