Period Calculator | OmniCalculator

  • Adam
  • November 1, 2025

Free period calculator to predict your next period, ovulation date, and fertile window. Track your menstrual cycle with accurate predictions and fertility insights.

📅 🩸 📅

Period Calculator

Track your menstrual cycle, predict your next period, calculate ovulation, and identify your most fertile days for family planning.

📋 Quick Navigation

📅 Enter your menstrual information to calculate predictions! 🩸
When did your last period start?
21-35 days (average 28)
2-7 days
Next Period Date
TBD
estimated date

Your Cycle Visualization

Menstruation Fertile Window Ovulation Luteal Phase

Next Period

—
date

Ovulation Date

—
date

Fertile Window

—
to —

Period Duration

5
days

📊 Detailed Breakdown

Phase Start Date End Date Days Description
Menstruation — — — Period bleeding
Follicular — — — Egg development
Fertile Window — — 6 Most likely to conceive
Ovulation — — 1 Egg released
Luteal — — — Hormone preparation

🔄 Menstrual Cycle Explained

What is the Menstrual Cycle?

The menstrual cycle is a monthly process that prepares the body for pregnancy. It typically lasts 21-35 days, with 28 days being average. The cycle is controlled by the rise and fall of hormones including estrogen and progesterone.

Menstrual Phase (Days 1-5)

This is when bleeding occurs as the uterus sheds its lining. This phase typically lasts 2-7 days. Day 1 of your period marks Day 1 of your menstrual cycle.

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.

Ovulation (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 toward the uterus.

Luteal Phase (Days 15-28)

After ovulation, the empty follicle produces progesterone to prepare the uterus for pregnancy. If the egg isn't fertilized, hormone levels drop, triggering menstruation and starting the cycle again.

💡 Note: Individual cycles vary. Some people have shorter or longer cycles, which is normal. Tracking your cycle for 3-6 months helps identify your personal pattern.

🥚 Ovulation & Fertile Window

What is Ovulation?

Ovulation is when a mature egg is released from the ovary. This usually happens about 12-16 days before your next period (or around day 14 of a 28-day cycle). The egg survives for approximately 12-24 hours.

The Fertile Window

Your fertile window is approximately 5-6 days during your cycle when you can get pregnant. It includes the 5 days before ovulation and the day of ovulation itself. Sperm can survive up to 5 days, so intercourse before ovulation can result in pregnancy.

Most Fertile Days

You're most likely to conceive during the 3 days leading up to and including ovulation day. Having intercourse during these days maximizes your chances of pregnancy.

Signs of Ovulation

  • Cervical mucus changes: Becomes clear, stretchy, and slippery (like egg whites)
  • Slight temperature rise: Basal body temperature increases 0.3-0.5°C after ovulation
  • Mild cramping: Some experience slight pain (mittelschmerz) in lower abdomen
  • Breast tenderness: Slight soreness or sensitivity
  • Increased libido: Natural increase in sexual desire
📌 Important: These calculations are estimates. Ovulation timing can vary based on stress, illness, exercise, diet, and hormonal changes. If trying to conceive, consider using ovulation predictor kits for confirmation.

🧮 Calculation Formulas

Ovulation Date Calculation

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

Or approximately:
Ovulation Date = First Day of Last Period + 14 days (for average 28-day cycle)

Example: Last period started Nov 2, cycle is 28 days
Ovulation = Nov 2 + 14 = Nov 16

Next Period Calculation

Next Period Date = First Day of Last Period + Cycle Length

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

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)

Cycle Phases

Menstrual Phase: Days 1 to Period Duration
Follicular Phase: Day 1 to Ovulation Day
Luteal Phase: Ovulation Day + 1 to Day Before Next Period

Typical Breakdown (28-day cycle):
Menstruation: 5 days
Follicular: 9 days
Ovulation: Day 14
Luteal: 14 days

❓ 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 periods for 3-6 months to find the average. If periods are very irregular or missed, consult a healthcare provider. Conditions like PCOS, endometriosis, or thyroid issues can cause irregularity.
How accurate are these predictions? +
These calculations are estimates based on typical cycle patterns. Accuracy is best for regular cycles. Ovulation timing can vary ±2 days from predicted date. For guaranteed accuracy regarding ovulation, use ovulation predictor kits, which detect LH surge. Basal body temperature tracking also provides confirmation.
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 or longer menstrual bleeding, ovulation could occur closer 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.
Does ovulation pain mean I'm ovulating? +
Mittelschmerz (ovulation pain) occurs in some people (about 20%) and typically indicates ovulation is happening or about to happen. However, absence of pain doesn't mean you're not ovulating. Some ovulate without any noticeable pain. Track cervical mucus changes for additional confirmation.
When should I take a pregnancy test? +
Best results: 12-14 days after ovulation (about 1-2 days before expected period). Some sensitive tests detect pregnancy 10 days after ovulation. Testing too early gives false negatives. Accuracy increases closer to missed period date. Blood tests are more sensitive than urine tests.
Can birth control affect cycle predictions? +
Yes, hormonal birth control (pills, patches, injections) prevents ovulation and regulates cycles artificially. This calculator is for natural cycles. If on birth control, follow your pill pack schedule instead. After stopping hormonal birth control, it may take months for natural cycles to resume.

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