Period Calculator — Free Menstrual Cycle Predictor
Predict your next six periods based on your cycle length and last period start date. See predicted start and end dates for each upcoming period with a countdown to your next one.
Quick Summary
Period Predictions
Next Period Starts In
18 days
Sunday, March 15, 2026
Upcoming 6 Periods
Summary: Based on your last period starting Sunday, February 15, 2026 with a 28-day cycle and 5-day period, your next period is expected around Sunday, March 15, 2026 (18 days from now). These predictions assume a regular cycle. If your cycle length varies significantly, consult your healthcare provider.
How to Use the Period Calculator
- Enter the first day of your last period: Use the date picker to select when your most recent period started. This is the first day of bleeding, not spotting. The calculator accepts dates within the past 90 days. If your last period was more than 90 days ago, you may want to consult your healthcare provider about the delay.
- Set your average cycle length: Enter the typical number of days between the start of one period and the start of the next. The default is 28 days. Normal cycles range from 21 to 35 days. If your cycle length varies, average your last 3 to 6 cycles. The calculator accepts values from 20 to 45 days.
- Set your average period length: Enter how many days your period typically lasts. The default is 5 days, and most periods last 2 to 7 days. This value determines the end date for each predicted period. If your flow duration varies, use the average or the most common duration.
The results instantly display a countdown to your next period and the predicted start and end dates for your next 6 periods. The first prediction is highlighted for quick reference. All predictions assume a regular, consistent cycle length. If your cycles are irregular, predictions further into the future become less reliable.
Period Prediction Formula
Next Period Start:
Next Period = Last Period Start + Cycle Length Period End Date:
Period End = Period Start + (Period Length - 1) days Future Periods:
Period N Start = Last Period Start + (Cycle Length x N) Variables Explained
- Last Period Start: The first day of your most recent menstrual bleeding. This is the anchor date for all predictions. Accuracy depends on correctly identifying this date — use the first day of actual flow, not spotting.
- Cycle Length: The total number of days from the first day of one period to the first day of the next. This includes the period itself plus the interval between periods. A 28-day cycle means 28 days from start to start.
- Period Length: The number of days of menstrual bleeding. Subtracting 1 from the period length gives the number of days to add to the start date for the end date, since the start day itself counts as day 1.
Step-by-Step Example
Predict the next 3 periods for someone with a 30-day cycle, 5-day period, and last period starting February 1, 2026:
- Period 1: February 1 + 30 = March 3 — March 7, 2026
- Period 2: February 1 + 60 = April 2 — April 6, 2026
- Period 3: February 1 + 90 = May 2 — May 6, 2026
- Days until next period (from Feb 16): 15 days
Practical Examples
Example 1: Laura Planning a Vacation
Laura, 25, has consistent 28-day cycles with 4-day periods. Her last period started February 8, 2026. She is planning a beach vacation for mid-March and wants to check if it will overlap with her period.
- Next period: February 8 + 28 = March 8-11, 2026
- Following period: April 5-8, 2026
- Vacation dates: March 14-21
Laura's next period is predicted to end on March 11, three days before her vacation begins. She feels confident planning her trip for mid-March. If she wanted extra certainty, she could talk to her healthcare provider about options to adjust timing, though in most cases simply knowing the prediction is enough for planning.
Example 2: Nina Tracking Cycle Changes
Nina, 38, has noticed her cycles getting shorter over the past year, from 30 days to 26 days. Her last period started February 10, 2026, with a 6-day duration. She wants to plan ahead.
- Next period (26-day): March 8-13, 2026
- Period 2: April 3-8, 2026
- Period 3: April 29 — May 4, 2026
- Periods per year with 26-day cycle: about 14
Nina discusses the shortening cycle trend with her gynecologist, who explains that cycles naturally become shorter in the late 30s as ovarian reserve decreases. This is a normal part of reproductive aging, though the change should be gradual. Sudden shortening by more than a few days warrants further evaluation. Nina uses the predictions to keep supplies stocked and plans appointments around her expected period dates.
Example 3: Sophia Preparing for an Athletic Event
Sophia, 22, is a competitive swimmer with 32-day cycles and 3-day periods. Her last period started January 30, 2026. She has a major swim meet on March 3 and wants to know if her period will coincide.
- Next period: January 30 + 32 = March 3-5, 2026
- Swim meet: March 3
- Overlap: Yes, day 1 of period
Unfortunately, Sophia's next period is predicted to start on the day of her swim meet. She discusses options with her sports medicine physician, who may recommend strategies such as menstrual products designed for swimming, or if medically appropriate, short-term hormonal management. Many elite athletes compete during their periods without performance issues, but advance knowledge allows Sophia to prepare physically and mentally. She uses the ovulation calculator to understand her full cycle timing for future event planning.
Menstrual Cycle Phase Reference
| Phase | Days (28-day cycle) | Hormones | What Happens |
|---|---|---|---|
| Menstruation | Days 1-5 | Low estrogen, low progesterone | Uterine lining sheds, menstrual bleeding occurs |
| Follicular Phase | Days 1-13 | Rising estrogen, FSH stimulates follicles | Egg-containing follicles mature, lining rebuilds |
| Ovulation | Day 14 | LH surge, peak estrogen | Mature egg released from ovary |
| Luteal Phase | Days 15-28 | Rising progesterone, moderate estrogen | Lining thickens for potential implantation |
| Premenstrual | Days 24-28 | Falling progesterone and estrogen | PMS symptoms may occur, cycle prepares to restart |
Tips and Complete Guide to Menstrual Tracking
Building a Period Tracking Habit
Consistent tracking produces the most useful predictions. Mark the first day of each period and count the total days of flow. After tracking 3 to 6 cycles, you will have a reliable average cycle length and period duration. Note any unusual symptoms, flow changes, or cycle length variations. This data is invaluable during healthcare appointments, providing your provider with accurate menstrual history that can help diagnose conditions, evaluate fertility, and assess hormonal health. Even simple tracking (just start dates) across multiple months reveals patterns that single-cycle predictions cannot.
Understanding Cycle Phases and Symptoms
Many individuals experience predictable symptoms tied to specific cycle phases. During the follicular phase (after menstruation ends, before ovulation), energy and mood often improve as estrogen rises. Around ovulation, some people notice mild pelvic pain, increased energy, and changes in cervical mucus. During the luteal phase (after ovulation, before the next period), progesterone dominates and may cause breast tenderness, bloating, mood changes, and food cravings. Premenstrual syndrome (PMS) typically occurs in the last 5 to 7 days before menstruation. Recognizing these patterns can help you anticipate and manage symptoms proactively.
When to Adjust Your Cycle Length Input
If your cycle length has changed consistently (not just one aberrant cycle), update the calculator input to reflect your new average. Common reasons for cycle length changes include age-related hormonal shifts, weight changes of more than 10 to 15 pounds, starting or stopping birth control, significant lifestyle changes (new exercise routine, major stress), and medical treatments. A single unusual cycle does not necessarily indicate a change — stress, illness, or travel can cause a one-time variation. Wait for 2 to 3 consistent cycles at the new length before updating your average.
Period Health Beyond Tracking
Period tracking is one component of menstrual health. Maintaining regular exercise, a balanced diet rich in iron and B vitamins, adequate hydration, and consistent sleep supports healthy cycles. Omega-3 fatty acids may help reduce menstrual cramps. Our calorie calculator can help ensure you are meeting nutritional needs that support hormonal health. If you experience significant pain, heavy bleeding, or cycle irregularity, these are not just inconveniences but potential symptoms of treatable conditions — discuss them with your healthcare provider.
Common Mistakes to Avoid
- Counting from the end of your period instead of the start: Cycle length is always measured from the first day of one period to the first day of the next. Using the end date gives an incorrect cycle length.
- Treating predictions as guarantees: Even regular cycles can vary by a few days. Predictions are estimates, especially those more than 2 to 3 months into the future.
- Ignoring significant changes in your cycle: A sudden shift in cycle length, flow heaviness, or period duration that persists for 3 or more cycles warrants medical evaluation.
- Assuming irregular cycles are always normal: While some variation is expected, consistently irregular cycles (varying by more than 7 to 9 days) may indicate hormonal issues like PCOS or thyroid disorders that benefit from treatment.
- Not tracking symptoms alongside dates: Recording symptoms (cramps, mood, flow heaviness) alongside dates provides much richer health information than dates alone, helping both you and your provider identify patterns.
Frequently Asked Questions
A normal menstrual cycle ranges from 21 to 35 days, measured from the first day of one period to the first day of the next. The average cycle length is 28 days, but cycles between 24 and 32 days are very common. Cycle length can vary slightly from month to month — a variation of up to 7 to 9 days is considered within normal limits. Adolescents often have longer or more irregular cycles in the first few years after menarche, gradually becoming more regular over time. Cycles tend to become shorter and more consistent between ages 25 and 35, then may become irregular again as perimenopause approaches, typically in the late 40s.
A normal period lasts between 2 and 7 days, with the average being about 5 days. The flow is typically heaviest during the first 2 to 3 days and gradually lightens toward the end. Total blood loss during a normal period is about 30 to 80 milliliters (roughly 2 to 6 tablespoons). If your period consistently lasts more than 7 days, if you soak through a pad or tampon every hour for several consecutive hours, or if you pass blood clots larger than a quarter, consult your healthcare provider. These could be signs of conditions like uterine fibroids, polyps, endometriosis, or hormonal imbalances that may need evaluation.
Period irregularity can result from many factors. Hormonal causes include polycystic ovary syndrome (PCOS), thyroid disorders, elevated prolactin, and perimenopause. Lifestyle factors such as significant weight changes, extreme exercise, high stress, travel across time zones, and shift work can disrupt cycle regularity. Medical causes include uterine fibroids, polyps, endometriosis, pelvic inflammatory disease, and certain medications (including some antidepressants and blood thinners). Cycles are commonly irregular during the first few years after menarche and in the years approaching menopause. If your cycles are consistently shorter than 21 days or longer than 35 days, or if you miss periods entirely, see your healthcare provider for evaluation.
Yes, stress can absolutely delay or even skip a period. The hypothalamic-pituitary-ovarian (HPO) axis, which regulates the menstrual cycle, is directly affected by stress hormones. When cortisol and adrenaline levels are elevated, the hypothalamus can reduce or stop the production of gonadotropin-releasing hormone (GnRH), which in turn suppresses the hormonal cascade needed for ovulation. Without ovulation, the period is delayed because the hormonal signals that trigger menstruation do not follow their normal pattern. Acute stress may delay a period by a few days to a few weeks, while chronic stress can cause prolonged amenorrhea (absence of periods). Managing stress through exercise, sleep, and relaxation techniques can help restore regular cycles.
This calculator uses a fixed cycle length to generate predictions, which works best for individuals with regular cycles. If your cycles vary in length, enter the average of your last 3 to 6 cycles for the most useful predictions. Keep in mind that each prediction becomes less reliable the further it extends into the future because natural cycle variation accumulates. For highly irregular cycles (varying by more than 7 days), the predictions serve as rough estimates rather than precise dates. In such cases, consider supplementing with ovulation tracking or period tracking apps that can adapt predictions based on multiple previous cycles.
Tracking your period provides valuable health insights and practical benefits. It helps you predict upcoming periods for planning purposes, identify patterns or changes that may indicate health issues, prepare for fertility-related decisions, and provide your healthcare provider with accurate menstrual history during appointments. Period tracking can also reveal correlations between your cycle and symptoms like headaches, mood changes, skin breakouts, and energy levels. If you are trying to conceive, tracking periods alongside ovulation helps identify your fertile window. If using fertility awareness for contraception, period tracking is essential. Even if you are not planning pregnancy, changes in your period pattern can be an early signal of conditions like thyroid disorders, PCOS, or premature ovarian insufficiency.
While some discomfort during menstruation is normal, certain symptoms warrant medical evaluation. See your healthcare provider if you experience periods lasting more than 7 days, bleeding so heavy you soak through a pad or tampon every hour for several consecutive hours, blood clots larger than a quarter, periods that come more frequently than every 21 days or less frequently than every 35 days, severe pain that prevents normal activities despite over-the-counter pain relief, bleeding between periods or after menopause, or a sudden significant change in your established cycle pattern. Additionally, missed periods for 3 or more consecutive cycles (when not pregnant, breastfeeding, or approaching menopause) should be evaluated.
Different forms of birth control affect periods in different ways. Hormonal birth control pills typically make periods lighter, shorter, and more regular, and may reduce cramps and PMS symptoms. Some formulations allow you to skip periods entirely. The hormonal IUD (such as Mirena) often reduces menstrual flow significantly, and some users stop having periods altogether after several months. The copper IUD may make periods heavier and longer, especially in the first few months. The implant and hormonal shot can cause irregular bleeding patterns, including infrequent periods or no periods at all. After stopping hormonal birth control, it may take 1 to 3 months for regular cycles to resume, though some people see their period return within weeks.
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Disclaimer: This calculator is for informational purposes only and does not constitute medical advice. Consult a qualified healthcare professional for medical guidance.
Last updated: February 23, 2026
Sources
- American College of Obstetricians and Gynecologists — Menstruation in Girls and Adolescents: acog.org/clinical/clinical-guidance
- National Institute of Child Health and Human Development — Menstruation: nichd.nih.gov/health/topics
- World Health Organization — Sexual and Reproductive Health: who.int/publications