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Period Calculator
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Period Calculator

Using the Period Calculator

The period calculator tells you when to expect your period. Enter information about your cycle and the calculator automatically counts the days for you. See where you are in your cycle and estimate when your period should happen.

You'll need some information about your cycle to use the period calculator:

  1. Enter the first day of your last period
  2. Enter the typical length of your period
  3. Enter your average cycle length. Count from the first day of one period up until the first day of the next.

When you do a calculation the period calculator tells you what cycle day you're currently on. It also estimates when your next period will begin. We provide a calendar feature that shows a preview of your expected period dates 3 months at a time, ahead or in the past.

Predict Your Next Period

You can easily calculate when to expect your next period. Use these steps to mirror how the calculator finds the dates.

  1. Recall the first day of your last period. Day 1 is the first day of actual flow.
  2. Find your average cycle length. Count from day 1 of one period up until day 1 of the next. Track 2–3 cycles to find your average cycle length.
  3. Add your cycle length in days to your last period start date. The result is the date of your next expected period.

Example period date calculation:

If your last period started on January 1 and your cycle is 29 days long, your next period should start around January 30.

Why Use a Period Calculator

Knowing when to expect your period can help you plan for upcoming events or travel. It can also help inform when you might ovulate. Typically ovulation occurs around 14 days before the start of your next period.

Planning for travel and events:

Know when to expect your period so you can pack supplies, plan activities and avoid surprises.

Understanding your body:

Notice patterns and symptoms. Knowing your typical cycle can help you recognize shifts or changes in what's normal for you.

Tracking fertility:

If you're trying to get pregnant, understanding your cycle helps you figure out when you're most likely fertile.

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Frequently Asked Questions

Period calculators are estimates based on your average cycle length. They work best for people with regular cycles. However, many factors can affect your cycle including stress, illness, travel, and hormonal changes.

If your cycle length varies, track at least 3-6 cycles to find your average. Use the average in the calculator. Keep in mind that variations are normal.

Hormonal birth control affects your natural cycle, so this calculator may not be accurate if you're on birth control. Birth control creates a withdrawal bleed rather than a natural period.

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