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PERIOD
CALCULATOR

Periods are a natural part of every menstruating person’s life, marking an important cycle of reproductive health. Each month, the body prepares for a possible pregnancy, and when it doesn't happen, the uterine lining is shed as a period. While this process is natural, tracking it can help you better understand your body and health patterns. A period calculator makes this easier by predicting your next period, fertile window, and ovulation days based on your cycle. Whether you're planning, preventing, or just staying prepared, it’s a smart tool to stay in sync with your body. Simple inputs, clear insights — that’s the power of a period calculator.

Last Period Start Date

How long did it last?
(In days)

Average Cycle Length
(In days)

Next Period Date

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Ovulation Window

​

Period Start
Period End
Ovulation Window

How to use Period Calculator?

Select the Date of Your Last Period:  

  • Click the calendar and choose the first day of your last period.

  • Example: If your last period started on April 1st, select "01-Apr-2025".

Enter Your Average Cycle Length:

  • This is the number of days from the start of one period to the start of the next.

  • Most people have a cycle length between 26–32 days. The default is set to 28.

Enter Your Average Period Length:

This is how many days your period usually lasts (e.g., 4–7 days).

View Your Predictions Instantly:

​As soon as you enter your details, the calculator will show:

  • Your next expected period

  • Your ovulation window

  • A table with your next 10 predicted cycles including:

    • Period start and end dates

    • Ovulation window for each cycle

What is the menstrual cycle, and how does it work?

The menstrual cycle is a complex biological process that prepares the female body for potential pregnancy every month. It is regulated by intricate hormonal changes involving the brain (hypothalamus and pituitary gland), the ovaries, and the uterus.

 

A complete menstrual cycle starts on the first day of a period and ends the day before the next period begins. The average cycle lasts about 28 days, but cycles ranging from 21 to 35 days are considered normal.

 

Here’s a breakdown of the four phases:

  • Menstrual Phase (Days 1–5):
    This is when bleeding occurs. The uterus sheds its inner lining (endometrium) because no fertilized egg has implanted. The blood consists of the lining tissue, mucus, and blood, typically totaling 30–80ml.

  • Follicular Phase (Days 1–13):
    Begins on the first day of your period and overlaps with the menstrual phase. The brain releases follicle-stimulating hormone (FSH), prompting the ovaries to develop follicles (each containing an immature egg). One becomes dominant and matures.

  • Ovulation (Day 14, approx.):
    Triggered by a surge in luteinizing hormone (LH), the mature egg is released from the ovary into the fallopian tube. This is the fertile window — the best time to conceive. Ovulation usually occurs 14 days before the next period.

  • Luteal Phase (Days 15–28):
    After ovulation, the empty follicle turns into the corpus luteum, which produces progesterone. This hormone maintains the uterine lining. If the egg isn’t fertilized, the corpus luteum disintegrates, hormone levels drop, and the cycle restarts.

 

The menstrual cycle is not just about bleeding — it's a whole-body process that can influence mood, energy, skin, digestion, and more.

What is considered a "normal" period, and how long should it last?

A normal period can look very different from one person to another, but most periods:

  • Last between 3 to 7 days

  • Happen every 21 to 35 days

  • Cause a blood loss of about 30 to 80 ml (roughly 2 to 6 tablespoons)

 

Here’s what “normal” might look like:

  • Color: Ranges from bright red to dark brown. Darker shades toward the end are normal.

  • Flow: Some people have heavy flow for a couple of days and then taper off; others have light bleeding throughout.

  • Consistency: Small clots are common, especially during heavy flow.

 

That said, "normal" is personal. What’s important is knowing what’s normal for you and spotting changes. Factors like stress, diet, exercise, illness, travel, and weight changes can all affect your period. Irregular or unusually heavy periods may be signs of conditions like PCOS, endometriosis, or thyroid disorders.

Why do periods sometimes become irregular?

Periods can become irregular due to a wide variety of physical, emotional, and lifestyle-related reasons. Some common causes include:

  • Hormonal Imbalances: Often due to puberty, perimenopause, or conditions like PCOS (polycystic ovary syndrome), thyroid disorders, or high prolactin levels.

  • Stress: Chronic stress can interfere with the brain's hormonal signals to the ovaries, delaying or even stopping ovulation.

  • Sudden Weight Changes: Rapid weight gain or loss, or eating disorders, can disrupt normal hormone function.

  • Intense Exercise: Athletes or those with very intense workout routines may experience missed or delayed periods.

  • Medical Conditions: Diabetes, celiac disease, liver disorders, and reproductive issues can all impact cycles.

  • Birth Control: Hormonal contraceptives can cause lighter, irregular, or absent periods — sometimes for months after stopping them.

 

An irregular period every once in a while isn’t usually a cause for concern, but frequent irregularities should be discussed with a healthcare provider.

What is ovulation and when does it happen in the cycle?

Ovulation is the process where a mature egg is released from one of the ovaries. It’s a crucial part of the menstrual cycle and the key to understanding fertility.

 

Here’s how it works:

  • Typically occurs around 14 days before your next period. In a 28-day cycle, that’s around Day 14.

  • The egg travels down the fallopian tube, where it may meet sperm and get fertilized.

  • If fertilization occurs, the egg implants into the uterine wall. If not, hormone levels drop and menstruation begins.

 

Ovulation signs may include:

  • Mild pelvic pain or twinges (called mittelschmerz)

  • Clear, stretchy, egg-white-like cervical mucus

  • Increased libido

  • Slight rise in body temperature (detectable with a basal thermometer)

  • Heightened sense of smell or taste

 

The fertile window includes the 5 days before and the day of ovulation, as sperm can survive for several days in the body. Tracking ovulation helps with both conception and natural family planning.

Is it normal to have symptoms like cramps, mood swings, or fatigue during a period?

Yes — it’s entirely normal to experience a variety of physical and emotional symptoms before and during your period. This collection of symptoms is known as PMS (premenstrual syndrome) and can begin up to 1–2 weeks before menstruation.

 

Common symptoms include:

  • Cramps (dysmenorrhea): Caused by the uterus contracting to shed its lining.

  • Mood swings: Hormonal fluctuations, especially drops in estrogen and progesterone, can affect serotonin (the mood-regulating chemical).

  • Bloating: Water retention and digestive changes.

  • Breast tenderness

  • Fatigue or insomnia

  • Headaches or migraines

  • Food cravings or appetite changes

  • Acne or skin changes

 

While PMS is common, severe symptoms could point to PMDD (premenstrual dysphoric disorder) — a more intense form of PMS that can seriously impact quality of life and may need medical treatment.

 

For cramps, remedies like heat pads, gentle yoga, hydration, magnesium-rich foods, and over-the-counter meds like ibuprofen can help.

How does the period calculator work, and how accurate is it?

The period calculator works by using your input — the start date of your last period, your average cycle length, and your period duration — to predict your next periods, fertile window, and ovulation days.

 

Here’s what it does step-by-step:

  1. It takes the last period start date as a reference point.

  2. It adds your cycle length (e.g., 28 days) to calculate when your next period is expected.

  3. It also estimates your period end date by adding your period duration (e.g., 5 days).

  4. It subtracts about 14 days from your next period to estimate ovulation — the most fertile time in your cycle.

  5. It repeats this process to show you upcoming cycles, usually 5–10 months ahead.

 

Accuracy:
The calculator is as accurate as your inputs. If your periods are regular, predictions are usually close. But your cycle can change due to stress, travel, illness, weight shifts, or hormonal imbalances. The tool is a great way to plan, but it’s not a substitute for medical advice or contraception.

Track your cycle, understand your body, and take charge of your health — one period at a time.

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