Period Calculator

This free period calculator predicts your next three periods, your ovulation day and your fertile window from the date of your last period and your average cycle length. Working as both a menstrual cycle calculator and a simple period tracker, it answers the question "when is my next period?" with a clear three-month calendar — entirely in your browser, with nothing saved. It is for planning and information only and is not a contraception method or a substitute for medical advice.

What Is a Period Calculator?

A period calculator predicts your menstrual cycle so you can anticipate when your period will start and end. While every woman's body is different, menstrual cycles tend to repeat in a regular rhythm. As a period date calculator, this tool helps you plan travel, events and special days with confidence, and identify your ovulation and fertile days for family planning.

It works on the last-period-date method: enter the first day of your last period and your average cycle length (e.g. 28 days), and the menstrual calculator instantly projects your upcoming period dates, how long each will last, and the days you are most likely to conceive — all in a clear calendar.

How the Menstrual Cycle Works

The menstrual cycle is the recurring process by which the female reproductive system prepares for a possible pregnancy and, if none occurs, sheds the uterine lining through menstruation. Key points:

• Cycle start (Day 1): The first day of bleeding is Day 1 of the new cycle.
• Cycle length: Most commonly 28 days, but anywhere from 21 to 35 days is medically normal.
• Ovulation: Occurs on average 14 days before the next expected bleed. (In a 30-day cycle, ovulation is around day 16; in a 28-day cycle, day 14.)
• Period length: A bleed of 3 to 7 days is considered healthy.

Our calculator uses the constant 14-day "luteal phase" (the time from ovulation to menstruation) to give an accurate projection of your ovulation and fertile window.

The Four Phases of the Menstrual Cycle

A typical 28-day cycle moves through four phases. This period cycle calculator uses these phases to place your period, ovulation and fertile days accurately:

What Is the Period Calendar For?

1. Planning: Knowing your PMS days in advance helps when scheduling a holiday, surgery, exam or sports event.
2. Health tracking: Noticing changes in your cycle length (shorter than 21 or longer than 35 days) can support early diagnosis.
3. Family planning: Seeing your fertile window is essential whether you are trying to conceive or using natural methods. The high-fertility window covers the 4 days before ovulation plus 24 hours after.

The Fertile Window Explained

The fertile window is when conception is most likely. Because sperm can survive in the body for up to 5 days and the released egg lives about 24 hours, the window spans roughly the 5 days before ovulation and the day of ovulation itself. This calculator marks these days for each upcoming cycle.

Late Period: What It Can Mean

If your period is overdue, this tool also works as a late period calculator: the projected first-day date and the days-left counter show at a glance how many days past the expected date you are. A period that is a few days late is common and usually harmless — stress, travel, illness or a change in routine can all delay it. However, a significantly late period can also be an early sign of pregnancy, so if there is any chance of conception, consider a pregnancy test.

What Causes Irregular Periods?

It is normal for predicted dates to shift by a day or two. But if you consistently see large differences (more than a week), several factors can be involved: stress, significant weight change, intense exercise, thyroid problems, PCOS, perimenopause and certain medications. If your cycle is regularly shorter than 21 or longer than 35 days, consult a healthcare provider. Remember that this calculator gives an estimate based on averages and should not be relied on for contraception. See the frequently asked questions below for more detail.