Period Calculator: Track Your Cycle and Predict Ovulation

How to Use Period Calculator: Track Your Cycle and Predict Ovulation

1. Step 1: Select last period date

   Choose the first day of your last period in the date picker.

2. Step 2: Enter cycle length

   Type your average cycle length in days (commonly 21–35 days).

3. Step 3: Enter period length

   Add how many days your period usually lasts (e.g., 5).

4. Step 4: Add variability (optional)

   If cycles vary, set ± days to see a confidence range.

5. Step 5: Calculate and review

   Tap Calculate to see your next period, ovulation, and fertile window.

Key Features

• Next period date prediction
• Estimated ovulation day
• Fertile window estimation
• Adjustable cycle variability
• Mobile-first calendar view

Understanding Results

Formula

We project your next period by adding your average cycle length to the first day of your last period. From that expected start date, we estimate ovulation by subtracting the luteal phase (default ≈14 days, adjustable). The fertile window is defined as the five days before ovulation plus ovulation day because sperm can survive in fertile cervical mucus up to roughly five days.

Variables in plain English: last period start (LMP), average cycle length (days between first day of one period and the day before the next), period length (days of bleeding), luteal phase (ovulation → next period). You can set variability (± days) to reflect how much your cycle typically shifts month to month and view a confidence range around the next period date.

Reference Ranges & Interpretation

Typical cycles for many adults fall between 21–35 days with 3–7 days of bleeding. The luteal phase commonly clusters near 14 days, but values between about 11 and 16 days occur in healthy cycles. If your cycle length is consistent, the prediction will cluster tightly around the estimated date. If your length varies widely, rely on the confidence range and consider complementary methods like ovulation tests or basal body temperature to narrow fertile days if you are trying to conceive.

Use the “Upcoming cycles” list and the calendar view to plan travel, training, or work. The period range spans your selected period length (e.g., 5 days). The fertile window highlights days with higher conception chances; peak probability tends to occur in the two days before ovulation and on ovulation day.

Assumptions & Limitations

Predictions are estimates—not medical advice. Illness, stress, travel, sleep disruption, and hormonal contraception can shift timing. Postpartum, perimenopause, PCOS, thyroid conditions, or recent birth control changes can increase variability. If cycles are frequently shorter than 21 days or longer than 35 days, very heavy, or accompanied by severe pain, consult a clinician. For conception planning, combine this estimate with your body’s signs and—if desired—ovulation testing.

Complete Guide: Period Calculator: Track Your Cycle and Predict Ovulation

Use our period calculator to predict your next period, ovulation day and fertile window from your cycle and period length. Mobile-friendly, private and free.

This tool estimates your next period, ovulation day, and fertile window using your last period date, average cycle length, and typical period length. It’s designed for quick, on‑the‑go use—especially on mobile—while keeping your data on your device.

How the period calculator works

Menstrual cycles have two main phases: the follicular phase before ovulation and the luteal phase after ovulation. The length of the luteal phase tends to be more consistent for many people, averaging about 14 days. Our calculator uses your average cycle length to project the next period start and subtracts the luteal‑phase length to estimate the ovulation day. The fertile window is the five days before ovulation plus ovulation day, because sperm can survive in cervical mucus for up to about five days.

Practically, we add your average cycle length to the first day of your last period to estimate the next cycle start. We then subtract your luteal‑phase setting (default 14 days) to estimate ovulation. Finally, we set the fertile window as ovulation minus five days through ovulation day. You can optionally enter variability in days to see a confidence range for the next period.

The calendar view highlights predicted period days (based on your period length), the fertile window, and ovulation. Because this is a projection, treat the highlights as guides. After your next cycle, update your inputs to keep the forecast aligned with reality.

Example timelines and planning

Example A: Suppose your last period started on June 2 and your average cycle length is 28 days with a 5‑day period. Your next period is projected to start around June 30 and last through July 4. Ovulation is estimated about 14 days before June 30, which is June 16. Your fertile window spans June 11–16. If you add ±2 days of variability, your next period confidence range would be roughly June 28 to July 2.

Example B: With a 32‑day cycle and a 6‑day period, your ovulation may occur around day 18 (32 − 14). If your last period started on July 7, ovulation lands near July 25 and the fertile window runs July 20–25. Your next period would be predicted for around August 8–13 depending on variability. Notice how a longer cycle shifts ovulation and the fertile window later.

Example C: If your cycles vary between 26 and 34 days, enter an average of 30 with a variability of ±4. Your calendar will display a broader confidence range and a wider spread of potential fertile days. To refine, consider a month or two of ovulation testing; once you learn your typical luteal‑phase length, enter it to fine‑tune ovulation estimates.

If you would like to focus solely on conception timing without period predictions, try our ovulation calculator. If you only need the next date quickly, we also offer a streamlined next period calculator and a simpleperiod tracker with basic logging.

What is a normal cycle?

For many, a normal cycle length ranges from about 21 to 35 days, measured from the first day of one period to the day before the next one starts. Period length commonly lasts 3–7 days. That said, “normal” varies across life stages and individuals. Adolescents often have longer, more irregular cycles. In your 20s and 30s, cycles may become more regular. Approaching perimenopause, cycle length can shorten, lengthen, or fluctuate.

Consistency matters more than any single number. If your cycle length tends to cluster around a value—say, 28–30 days—the calculator will provide a reliable estimate. If lengths vary widely month to month (for example, 24, then 41, then 29), treat any prediction as a broad range and consider additional tracking methods such as basal body temperature or ovulation predictor kits.

Lifestyle factors can influence timing. Travel across time zones, strenuous training, illness, and poor sleep can delay or advance ovulation by a few days. Nutritional changes or sudden weight shifts may affect hormones as well. When these events occur, expect a little drift and revisit the calculator after your next period to bring averages back in line.

If you’re actively planning for pregnancy, complement this tool with ourpregnancy due date calculator (EDD) and thepregnancy test timing calculator to better understand when to test and how a positive might translate into an estimated due date.

Ovulation and the fertile window

Ovulation is when an ovary releases an egg, which can be fertilized for roughly 12–24 hours. Because sperm can live in the reproductive tract for several days under fertile cervical mucus conditions, the fertile window spans the five days before ovulation and the day of ovulation itself. Intercourse on those days carries the highest probability of conception, with a peak on the two days before ovulation and on ovulation day.

Our calculator estimates ovulation by subtracting your luteal‑phase length from the expected next period. For many people the luteal phase is near 14 days, but values between about 11 and 16 days are common. If you know yours differs, adjust the advanced setting to refine the estimate. For more targeted conception planning, combine predictions with physical signs such as fertile‑type cervical mucus or a positive ovulation test.

If you prefer a tighter conception‑focused workflow, try the dedicated ovulation calculator which centers timing and fertile‑window specifics.

Cervical mucus often becomes clear, stretchy, and slippery near ovulation—sometimes compared to raw egg whites. Basal body temperature typically rises slightly after ovulation. These signals can validate the window predicted by the calculator, especially for users whose cycles vary by more than a few days.

Irregular cycles: how to use this tool

Irregular cycles can still benefit from structured tracking. Start by entering the last period date and your current best estimate for average length. Use the variability field to add a plus‑minus range. The tool will display a confidence range around the next period prediction. Over several months, update your average and variability based on actual patterns rather than a single month’s outlier.

You can also track signs like cervical fluid quality, ovulation test results, mid‑cycle pain, or basal body temperature shifts. While we don’t store your data on our servers, you can keep personal notes alongside the dates in your own device or companion apps. For simple date predictions without symptom detail, ourperiod tracker is another quick option.

If your period vanishes for several months or becomes extremely frequent, check in with a clinician. Thyroid issues, elevated prolactin, polycystic ovary syndrome (PCOS), and other conditions can affect ovulation and bleeding patterns. Treatment—when appropriate—often improves predictability over time.

Tips to improve accuracy

Aim for three to six months of consistent data. Each time you log a period, update your average cycle length in the calculator. If cycles drift shorter or longer, adjust the average—small changes add up to more precise predictions over time. If you know your luteal phase length from temperature charts or confirmed ovulation tests, use the advanced setting to refine the ovulation estimate.

• Use the same definition of “cycle start” (first day of full flow) each month.
• Track period length consistently; predictions show a range across those days.
• If you miss a period or have a much longer cycle, re‑estimate your average rather than keeping the old value.
• Combine predictions with ovulation tests or temperature tracking for conception planning.

Consider the context of your goals. If you want to avoid pregnancy, treat the fertile window conservatively: assume it may be a little wider than predicted. If you are trying to conceive, time intercourse across the fertile days rather than one exact date, emphasizing the two days before the projected ovulation.

For a broader planning toolkit, bookmark ourall‑in‑one pregnancy planner and thepregnancy week calculator to translate conception timing into week‑by‑week milestones.

Birth control, postpartum, and perimenopause

Hormonal contraception can alter bleeding patterns and suppress ovulation. If you recently started, switched, or stopped a method, expect predictions to be less stable for a few months. With IUDs, implants, or extended‑cycle pills, the concept of a “typical cycle” may be less meaningful, so treat outputs as general guidance rather than a precise schedule.

After childbirth, cycles often take time to resume, especially with lactation. Postpartum ovulation may occur before your first period returns, which is why contraception is still relevant if you wish to avoid pregnancy. During perimenopause, ovulation can be sporadic and luteal phases may shorten, making estimates more variable. In all of these contexts, the variability field becomes especially useful.

Certain medications—such as some steroids, antipsychotics, and thyroid drugs—can influence cycle timing. If you start or change a medication and notice cycle shifts, re‑tune the calculator after a couple of months to reflect the new pattern.

Understanding the luteal phase setting

The luteal phase spans ovulation to the start of the next period. Many people cluster around 14 days, but healthy values vary. If your luteal phase is known—perhaps from a basal body temperature chart or lab‑confirmed ovulation—enter it in the advanced setting for a more personalized ovulation prediction. If you are unsure, the default is a reasonable starting point.

Keep in mind that stress, illness, travel, and medications can shift timing modestly. The follicular phase (before ovulation) is typically the more variable phase across cycles, which is why adjusting the luteal phase alone can improve ovulation estimates even when the overall cycle length fluctuates.

If your luteal phase is consistently shorter than about 10–11 days, bring it up with a clinician. A very short luteal phase can make conception less likely and may warrant further evaluation in the context of your overall health history and goals.

Common myths and facts

Myth: “Everyone ovulates on day 14.” Fact: Day 14 is an average based on a 28‑day cycle with a 14‑day luteal phase. Many ovulate earlier or later, and both are normal.

Myth: “You can’t get pregnant on your period.” Fact: If you have a short cycle, sperm can survive long enough that intercourse late in a period could lead to fertilization soon after bleeding ends. Probability is lower, but not zero.

Myth: “Cycle apps guarantee exact dates.” Fact: Predictions are estimates informed by your data. They’re most helpful when you keep them updated and combine them with your body’s signs.

Myth: “Irregular means abnormal.” Fact: Some variability is common, especially during adolescence and perimenopause. Persistent extremes or sudden changes deserve a check‑in, but not every irregular month signals a problem.

When to see a clinician

Seek medical care if you experience very heavy bleeding, cycles consistently shorter than 21 days or longer than 35 days (and not typical for your age), periods that frequently last more than seven days, severe pain, bleeding between periods, or if you have gone months without a period and are not pregnant. If you are trying to conceive and have not been successful after 6–12 months (depending on age), or have any concerns about thyroid, prolactin, or other conditions, discuss options with a clinician.

For reference information on normal cycles and ovulation, see the American College of Obstetricians and Gynecologists (ACOG) guidance.ACOG offers accessible patient education on menstrual health.

If you get a positive pregnancy test, switch to tracking with prenatal tools. Our pregnancy week calculator and EDD calculator can translate that date into milestones and timelines for appointments.

Privacy and data handling

This calculator runs in your browser. We do not store your inputs on our servers. If you choose to save notes or dates, do so locally on your device. You can revisit this tool anytime without an account. For a full overview of site practices, visit our privacy policy.

Ready to plan ahead? Use the calculator at the top of the page, then explore related tools like the ovulation calculator,next period calculator, andperiod tracker to round out your planning.

Frequently Asked Questions