📅 Menstrual Cycle Calculator

Calculate your average cycle length, check regularity, and find your next period & ovulation date

Enter the start dates of your last 3–6 periods for the most accurate cycle length calculation.

1
2
3
💡 This lets us calculate your actual cycle length
Your Average Cycle Length
—
days
—
📊
—
Average Cycle
⬆️
—
Longest Cycle
⬇️
—
Shortest Cycle
📐
—
Variation

📈 Your Cycle Regularity Score

Irregular Moderate Very Regular

📋 Your Cycle History

🌙
Next Period Expected
—
Based on your average cycle length
🥚
Estimated Next Ovulation
—
Ovulation typically occurs 14 days before your next period
📌 Pin it f Share 💬 WhatsApp
⚕️ Medical Disclaimer: This calculator provides estimates based on your entered cycle data. Cycle predictions are not guaranteed and vary from month to month. This tool is for educational purposes only and should not be used as a contraceptive method or a substitute for medical advice. Consult your doctor about any concerns with your menstrual cycle.

How to Calculate Your Menstrual Cycle Length

Your menstrual 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 source of confusion.

For example: if your last period started on June 1 and your current period started on June 29, your cycle length is 28 days.

💡 For the most accurate average: Track at least 3 cycles. One cycle can vary significantly due to stress, illness, or travel. The more data points you have, the more reliable your average cycle length becomes.

What Is a Normal Menstrual Cycle?

A "normal" cycle can range from 21 to 35 days — there is no single right number. What matters most is consistency. If your cycles are consistently 32 days, that is your normal, even though it is longer than the textbook 28-day cycle.

Cycle LengthCategoryWhat It Usually Means
Under 21 daysShort (Polymenorrhea)Worth mentioning to your doctor — may affect fertility
21–24 daysShort-normalWithin range but on the shorter side
25–35 days✅ Normal rangeConsidered typical — includes the textbook 28-day cycle
36–45 daysLong (Oligomenorrhea)Infrequent periods — worth investigating if persistent
Over 45 days / absentAmenorrheaSee a doctor — may indicate PCOS, thyroid issues, or other conditions

What Affects Cycle Length?

🔄 Common Causes of Cycle Variation

It is completely normal for your cycle to vary by a few days from month to month. These are the most common reasons:

  • Stress: High cortisol can delay or suppress ovulation, lengthening your cycle
  • Illness or travel: Both can temporarily disrupt hormone patterns
  • Weight changes: Significant gain or loss affects estrogen production
  • Exercise: Very intense training can shorten or stop periods (athletic amenorrhea)
  • Sleep disruption: Poor sleep affects the hormones that regulate your cycle
  • Age: Cycles often shorten slightly in your 30s and can become irregular approaching perimenopause
  • Breastfeeding: Prolactin suppresses ovulation, which can delay or stop periods
  • Coming off hormonal contraception: Cycles can be irregular for 3–6 months after stopping the pill

The 4 Phases of Your Menstrual Cycle

🩸 Menstrual Phase (Days 1–5)

The uterine lining sheds as a period. Estrogen and progesterone are at their lowest. Energy is typically lowest now — rest is natural and healthy.

🌱 Follicular Phase (Days 1–13)

Overlaps with menstruation. FSH stimulates follicle growth. Estrogen rises, energy and mood lift. The uterine lining rebuilds.

🥚 Ovulation (Around Day 14)

A surge in LH triggers egg release. Your most fertile point. Libido often peaks. Lasts 12–24 hours but fertile window spans 5–6 days.

🌙 Luteal Phase (Days 15–28)

Progesterone rises to prepare the uterus. If no pregnancy, progesterone drops and the cycle restarts. PMS symptoms often appear here.

Irregular Periods — When to See a Doctor

🚨 See Your Doctor If You Notice:

  • Cycles consistently shorter than 21 days or longer than 35 days
  • Variation of more than 7–9 days between your shortest and longest cycles
  • Missed periods when not pregnant (more than 3 in a row)
  • Periods that suddenly become much heavier, lighter, or more painful
  • Bleeding between periods or after sex
  • Periods lasting longer than 7 days
  • Irregular cycles combined with other symptoms like excess hair growth, acne, or weight changes (may indicate PCOS)
💡 Trying to conceive with irregular cycles? Irregular ovulation is one of the most common causes of difficulty conceiving. Tracking with OPKs and BBT alongside this calculator gives you the clearest picture of when (or whether) you are ovulating each cycle.

How Cycle Length Affects Fertility

Understanding your cycle length directly impacts your ability to identify your fertile window. Ovulation typically happens 14 days before your next period — not 14 days after your last period. This matters most if your cycle is longer or shorter than 28 days.

Your Cycle LengthEstimated Ovulation DayFertile Window Opens
21 daysDay 7Day 2
24 daysDay 10Day 5
26 daysDay 12Day 7
28 daysDay 14Day 9
30 daysDay 16Day 11
32 daysDay 18Day 13
35 daysDay 21Day 16

These are estimates. Use our Ovulation Calculator for a personalized fertile window prediction.

Frequently Asked Questions

What is the average menstrual cycle length?
The commonly cited average is 28 days, but research shows the true average is closer to 29 days — and the normal range spans 21 to 35 days. Very few women have a consistent 28-day cycle every single month. Your personal average, tracked over several cycles, is far more useful than the textbook number.
How many days variation in cycle length is normal?
Variation of up to 7–9 days between your shortest and longest cycles is considered within the normal range. So if your shortest cycle is 26 days and your longest is 32 days, that is considered regular. Variation beyond 9 days suggests irregular cycles worth discussing with your doctor, especially if you are trying to conceive.
Can stress change my cycle length?
Yes — stress is one of the most common causes of cycle disruption. High cortisol levels can delay or suppress the LH surge that triggers ovulation, which pushes back your period and lengthens your cycle. Once the stressor passes, cycles typically return to normal within 1–2 months.
Is a 21-day cycle normal?
A 21-day cycle is at the very short end of the normal range. For some women this is simply their natural rhythm. However, very short cycles can sometimes indicate a short luteal phase (the time between ovulation and your period), which may affect fertility. If you have consistently short cycles and are trying to conceive, it is worth discussing with your doctor.
How is the menstrual cycle calculator different from the period tracker?
Our Period Tracker predicts your future periods based on one cycle length. This Menstrual Cycle Calculator analyzes multiple past periods to calculate your true average cycle length, measure regularity, and identify variation patterns. Use this calculator first to find your accurate average, then use the Period Tracker and Ovulation Calculator with that number for future predictions.
Why do my cycles vary so much?
Cycle variation is normal and common. The follicular phase (before ovulation) is where most variation occurs — stress, illness, travel, or hormonal fluctuations can all delay ovulation and push your period later. The luteal phase (after ovulation) is usually more consistent at 12–14 days. Tracking your ovulation with OPKs or BBT alongside cycle tracking gives you the clearest picture of what is happening each month.

Try Our Other Free Tools

🥚 Ovulation Calculator 🌙 Period Tracker 🌱 Implantation Calculator 📖 Why Are My Periods Irregular?