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Period Calculator — Track Your Menstrual Cycle

📝 Reviewed by Health Management Editorial Team 🕒 Last updated: 23 July 2026 ⏱ 3 min read
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What Is a Period Calculator?

A period calculator predicts when your next menstrual period is likely to start and end, based on the first day of your last period and your average cycle length. It also helps you track which day of your cycle you're currently on, and estimate your PMS window.

How This Calculator Works

Enter the first day of your last period, your average cycle length (the number of days from the start of one period to the start of the next), and your typical period duration. This calculator projects your next period date, then continues the pattern forward across upcoming cycles.

MetricFormula
Next PeriodLast Period + Cycle Length (repeated until a future date)
Period EndNext Period Start + Period Duration − 1
Current Cycle Day(Days Since Last Period) mod Cycle Length + 1
PMS WindowApproximately 5 days before the next period

Phases of the Menstrual Cycle

PhaseTypical DaysWhat's Happening
Menstrual PhaseDay 1–5Uterine lining sheds; period occurs
Follicular PhaseDay 1–13Follicles develop; estrogen rises
OvulationAround Day 14Egg is released; most fertile phase
Luteal PhaseDay 15–28Uterine lining thickens; PMS symptoms may appear

For a detailed fertility-focused breakdown, see our Ovulation Calculator.

Understanding PMS

Premenstrual syndrome (PMS) refers to physical and emotional symptoms that often appear in the days leading up to a period, typically during the luteal phase. Common symptoms include mood changes, bloating, breast tenderness, fatigue, and food cravings. Symptoms usually ease once the period begins.

💡 Tip Tracking your cycle for 3–6 months gives a much more reliable average than a single cycle, since natural month-to-month variation is completely normal.

What Counts as an Irregular Cycle?

A typical menstrual cycle ranges from 21 to 35 days. Cycles are generally considered irregular if they vary significantly in length from month to month, are consistently shorter than 21 days or longer than 35 days, or if periods are missed for several months without pregnancy or menopause explaining it.

Tips for Tracking Your Cycle

  • Note the first day of your period each month, not the last
  • Track period duration and flow intensity for a fuller picture
  • Record any notable symptoms (cramps, mood changes, etc.)
  • Use a consistent method — app, calendar, or journal — for reliable trends

When to Consult a Doctor

  • Periods are consistently irregular or unpredictable
  • Very heavy bleeding or severe pain that disrupts daily life
  • Missed periods without pregnancy, for several months
  • Sudden, significant changes to your usual cycle pattern
⚠️ Not medical advice This calculator provides a general prediction only and does not replace professional medical advice. Consult a gynaecologist for persistent irregularities or concerns about your menstrual health.
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Frequently Asked Questions

It provides a reasonable prediction based on your average cycle, but actual timing can shift due to stress, illness, travel, or hormonal changes.
A typical menstrual cycle ranges from 21 to 35 days, with 28 days often cited as an average — though healthy cycles vary quite a bit between individuals.
Stress, illness, travel, significant weight changes, and hormonal fluctuations can all shift the timing of a period from the average prediction.
It's the days leading up to your period, typically the last part of the luteal phase, when premenstrual symptoms like mood changes and bloating commonly appear.
Yes — tracking helps you notice subtle changes early, plan around your cycle, and provides useful information if you ever need to discuss your cycle with a doctor.
Missing three or more periods in a row (without pregnancy or menopause explaining it) is generally worth discussing with a doctor.
Yes — the 6-cycle projection table can help with planning travel, events, or simply being prepared with supplies ahead of time.
Yes — significant stress can affect the hormones that regulate your cycle, sometimes delaying or occasionally accelerating your period.
This tool focuses on predicting your period and cycle day; the Ovulation Calculator focuses specifically on your fertile window for conception planning.
Yes — hormonal birth control often creates a more regular, predictable cycle (or stops periods altogether), which can make calendar-based predictions less relevant.

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