Menstrual Cycle Calculator – Track Period, Ovulation & Fertile Window

  • Adam
  • November 1, 2025

Free menstrual cycle calculator. Track your period, predict next period date, calculate ovulation, and identify fertile window for family planning. Simple and accurate cycle tracking.

📅 🩸 📅

Menstrual Cycle Calculator

Track your menstrual cycle, predict future periods, calculate ovulation, and plan fertility with comprehensive cycle tracking and analysis tools.

📋 Quick Navigation

📅 Track your menstrual cycle! 🩸
When did your period start?
21-35 days (average 28)
2-7 days
Next Period Predicted
TBD
approximately

Next Period

—
date

Ovulation Date

—
date

Fertile Window

6
days

Cycle Length

28
days

📊 Your Cycle Schedule

Phase Start Date End Date Days Description
Menstruation — — — Bleeding phase
Follicular — — — Egg development
Fertile Window — — 6 High pregnancy chance
Ovulation — — 1 Egg released
Luteal — — — Hormone preparation

🔄 Menstrual Cycle Phases

Menstrual Phase (Days 1-5)

The first day of your period marks Day 1 of your menstrual cycle. During this phase, the uterus sheds its lining, resulting in bleeding. This typically lasts 2-7 days. Hormone levels (estrogen and progesterone) are at their lowest during this phase.

Follicular Phase (Days 1-13)

Overlaps with menstruation and continues until ovulation. The pituitary gland releases follicle-stimulating hormone (FSH), which causes the ovaries to produce estrogen and develop egg-containing follicles. Estrogen levels rise gradually as the follicles mature, creating a thickened uterine lining for potential pregnancy.

Ovulation (Around Day 14)

A surge in luteinizing hormone (LH) triggers the release of a mature egg from the ovary. This usually occurs approximately 12-16 days before your next period. The egg travels down the fallopian tube and can be fertilized by sperm within 12-24 hours. The ovulation window can vary by ±2-3 days from predicted date.

Luteal Phase (Days 15-28)

After ovulation, the empty follicle transforms into the corpus luteum, which produces progesterone. This hormone prepares the uterus for potential pregnancy by thickening the endometrium. If the egg isn't fertilized, progesterone and estrogen levels drop, triggering menstruation and starting the cycle again.

💡 Note: Individual cycles vary significantly. A regular cycle is one where the longest and shortest cycles vary by less than 8 days. Tracking multiple cycles helps identify your personal pattern.

🧮 Calculation Formulas

Next Period Calculation

Next Period Date = Last Period Date + Cycle Length

Example: Last period started Nov 2, cycle is 28 days
Next Period = Nov 2 + 28 days = Nov 30

For irregular cycles, use average cycle length
Track cycles for 3-6 months to calculate average

Ovulation Date Calculation

Ovulation Date = Last Period Date + (Cycle Length − 14)

For standard 28-day cycle (ovulation day 14):
Ovulation = Last Period + 14 days

Example: Last period Nov 2, 28-day cycle
Ovulation = Nov 2 + 14 = Nov 16

For 35-day cycle:
Ovulation = Last Period + 21 days

Fertile Window Calculation

Fertile Window Start = Ovulation Date − 5 days
Fertile Window End = Ovulation Date

Example: Ovulation Nov 16
Fertile Window = Nov 11 to Nov 16 (6 days total)
Peak Fertility = Nov 14 to Nov 16 (3 days)

Note: Most likely conception in 3 days before ovulation

Period End Date

Period End Date = Last Period + Period Duration − 1

Example: Last period Nov 2, duration 5 days
Period End = Nov 2 + 5 − 1 = Nov 6
Bleeding Days = Nov 2, 3, 4, 5, 6

Follicular Phase Duration

Follicular Phase = Ovulation Day − 1

For 28-day cycle (ovulation day 14):
Follicular Phase = 14 − 1 = 13 days

For 35-day cycle (ovulation day 21):
Follicular Phase = 21 − 1 = 20 days

Luteal Phase Duration

Luteal Phase = Cycle Length − Ovulation Day

For 28-day cycle (ovulation day 14):
Luteal Phase = 28 − 14 = 14 days

Note: Luteal phase typically 12-16 days
More consistent than follicular phase length

❓ Frequently Asked Questions

Can my cycle length change? +
Yes, cycle length can vary slightly month to month. A regular cycle is one where the longest and shortest cycles vary by less than 8 days. Factors affecting cycle length include stress, illness, exercise, diet, travel, and hormonal changes. Tracking for 3-6 months helps identify your average.
What if I have irregular periods? +
With irregular periods, use your shortest cycle length for the most accurate prediction. Track your cycles for 3-6 months to find the average. If cycles are very irregular or you miss periods, consult a healthcare provider. Conditions like PCOS, endometriosis, or thyroid issues can cause irregular periods.
How accurate is this calculator? +
Accuracy is best for regular cycles with known cycle length. For irregular cycles, accuracy decreases. Ovulation typically occurs ±2-3 days from predicted date. Using additional methods like cervical mucus observation, basal body temperature tracking, or ovulation predictor kits improves accuracy.
Can I get pregnant on my period? +
It's possible but rare. While pregnancy is unlikely during menstruation, sperm can survive up to 5 days. If you have a shorter cycle (21 days) or longer menstrual bleeding, ovulation could occur close to the end of your period, making pregnancy possible.
What causes irregular periods? +
Common causes include: stress, significant weight changes, excessive exercise, hormonal birth control changes, PCOS, endometriosis, thyroid disorders, and irregular sleep. Teen periods are often irregular while the body is establishing cycles. Menopause also causes irregularity. Consult a doctor if concerned.
Can I use this calculator for birth control? +
This calculator shows fertility patterns but is NOT reliable for birth control. The rhythm/fertility awareness method has a 24% failure rate. If preventing pregnancy, use proven contraceptive methods like condoms, birth control pills, IUDs, or other methods recommended by your healthcare provider.
When should I track my cycle? +
Track your cycle starting on the first day of your period (Day 1). Record the date it starts and when it ends. Note cycle length (first day of one period to first day of next). After 3-6 months of tracking, you'll have enough data to calculate an accurate average cycle length and predict future periods accurately.

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