Period Calculator
Free Period Calculator with medically-sourced formulas. Enter your measurements for personalized, accurate health insights.
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Your next period is predicted by adding your average cycle length to the first day of your last period. Ovulation is estimated at (Cycle Length - 14) days after period start, based on the relatively consistent 14-day luteal phase. The fertile window spans 5 days before ovulation through 1 day after. These are estimates — actual cycles can vary.
Last reviewed: January 2026
Worked Examples
Example 1: Regular 28-Day Cycle Prediction
Example 2: Shorter 25-Day Cycle
Background & Theory
The Period Calculator applies the following established principles and formulas. Health and medicine calculators are grounded in validated physiological measurement methods established through decades of clinical research. Body Mass Index, or BMI, is calculated by dividing weight in kilograms by height in meters squared (kg/m²), a formula originating from Adolphe Quetelet's 19th-century statistical work and later codified by the WHO into standard classifications: underweight below 18.5, normal weight 18.5 to 24.9, overweight 25 to 29.9, and obese at 30 and above. Basal Metabolic Rate quantifies the minimum energy required to sustain life at rest. The Mifflin-St Jeor equation, published in 1990 and widely regarded as the most accurate for most adults, calculates BMR as (10 × weight in kg) + (6.25 × height in cm) − (5 × age) ± sex adjustment. The older Harris-Benedict equations, revised in 1984 by Roza and Shizgal, remain in common use. Total Daily Energy Expenditure is derived by multiplying BMR by a physical activity factor ranging from 1.2 for sedentary individuals to 1.9 for extremely active ones, following the methodology validated by doubly labeled water studies. Body fat percentage can be estimated without laboratory equipment using the U.S. Navy circumference method, which uses neck, waist, and hip measurements, or via BMI-derived equations adjusted for age and sex. The Jackson-Pollock skinfold method offers higher precision with calipers. Blood pressure classification, according to the American College of Cardiology and the 2017 ACC/AHA guidelines, defines normal as below 120/80 mmHg, elevated as 120 to 129 systolic, and hypertension stage 1 as 130 to 139 systolic or 80 to 89 diastolic. Target heart rate zones for aerobic exercise are derived from maximum heart rate estimates, most commonly using the formula 220 minus age in years, with moderate-intensity training typically defined as 50 to 70 percent of maximum heart rate and vigorous intensity at 70 to 85 percent, consistent with CDC and American Heart Association guidelines. These thresholds guide safe and effective cardiovascular conditioning.
History
The history behind the Period Calculator traces back through the following developments. The history of health measurement stretches back to ancient Greece, where Hippocrates around 400 BCE laid the foundation for observational medicine by systematically recording patient symptoms, diet, and environment. His humoral theory, though scientifically superseded, established the principle that the body operates as an interconnected system subject to measurable imbalance. The transformation toward modern medicine accelerated in the 19th century. Louis Pasteur and Robert Koch developed germ theory in the 1860s and 1870s, identifying microorganisms as disease agents and enabling targeted interventions. Florence Nightingale, working during the Crimean War in the 1850s, introduced statistical analysis to nursing practice, demonstrating through data visualization that sanitation reduced mortality. Her work is foundational to evidence-based health measurement. The discovery of vitamins in the early 20th century, beginning with Casimir Funk's coinage of the term in 1912 and culminating in the isolation of vitamins A through K, created the field of nutritional science and gave rise to dietary reference intake frameworks. The World Health Organization, founded in 1948, subsequently established global standards for health metrics, disease classification through the International Classification of Diseases, and recommended daily allowances. The BMI as a clinical screening tool gained traction in the 1970s through Ancel Keys' large-scale epidemiological work, which validated Quetelet's index as a population-level obesity indicator. Through the 1980s and 1990s, the Framingham Heart Study produced landmark data linking cholesterol, blood pressure, and lifestyle factors to cardiovascular disease risk, directly shaping the numeric thresholds still used in health calculators. The evidence-based medicine movement, formalized by Gordon Guyatt and colleagues at McMaster University in the early 1990s, demanded that all health recommendations derive from systematically graded clinical evidence. The digital health era beginning in the 2000s brought these formulas to consumer devices, wearable sensors, and smartphone applications, expanding access to health self-monitoring on a global scale and enabling population-level data collection that continues to refine clinical reference ranges.
Frequently Asked Questions
Formula
Next Period = Last Period Start + Cycle Length
Your next period is predicted by adding your average cycle length to the first day of your last period. Ovulation is estimated at (Cycle Length - 14) days after period start, based on the relatively consistent 14-day luteal phase. The fertile window spans 5 days before ovulation through 1 day after. These are estimates — actual cycles can vary.
Worked Examples
Example 1: Regular 28-Day Cycle Prediction
Problem: Last period started March 1. Cycle length is 28 days, period duration is 5 days. Predict the next 3 periods and fertile windows.
Solution: Cycle 1: Period March 29 - April 2\nOvulation: April 12 (Day 14)\nFertile window: April 7-13\n\nCycle 2: Period April 26 - April 30\nOvulation: May 10\nFertile window: May 5-11\n\nCycle 3: Period May 24 - May 28\nOvulation: June 7\nFertile window: June 2-8
Result: Next 3 periods: Mar 29, Apr 26, May 24 | Periods per year: ~13 | ~65 bleeding days/year
Example 2: Shorter 25-Day Cycle
Problem: Last period started March 5. Cycle length is 25 days, period duration is 4 days. When are the next periods?
Solution: Ovulation day = 25 - 14 = Day 11\n\nCycle 1: Period March 30 - April 2\nOvulation: April 10\nFertile window: April 5-11\n\nCycle 2: Period April 24 - April 27\nOvulation: May 5\nFertile window: April 30 - May 6\n\nPeriods per year = 365 / 25 ≈ 15
Result: Next periods: Mar 30, Apr 24, May 19 | Ovulation on Day 11 | ~15 periods/year
Frequently Asked Questions
How long does a normal period last?
A normal period (menstruation) lasts between 3 and 7 days, with the average being 5 days. Bleeding is typically heaviest during the first 2-3 days and gradually lightens. Normal blood loss during a period is 30-80 mL (about 2-5 tablespoons) total. If you soak through a pad or tampon every hour for several hours, pass clots larger than a quarter, or your period lasts more than 7 days, you may have menorrhagia (heavy menstrual bleeding), which affects about 1 in 5 women. Light periods (fewer than 2 days or very light flow) can also indicate hormonal issues. Period duration and flow can change with age, after pregnancy, with certain medications (especially hormonal contraceptives), and during perimenopause. Keeping a period diary helps you notice changes and provides valuable information for your healthcare provider.
When should I see a doctor about my period?
Consult a healthcare provider if you experience: Absent periods (amenorrhea) — no period for 3+ months without pregnancy; could indicate hormonal imbalances, PCOS, thyroid issues, or premature ovarian failure. Very heavy bleeding — soaking through products every 1-2 hours, periods lasting more than 7 days, or passing large clots. Severe pain — cramps that don't respond to OTC pain relievers or significantly impact daily activities (could indicate endometriosis or fibroids). Irregular cycles — cycles consistently shorter than 21 days or longer than 35 days, or significant variation between cycles. Bleeding between periods — spotting or bleeding outside your normal period. Post-menopausal bleeding — any bleeding after 12 months without a period. Sudden changes — significant changes in flow, duration, or symptoms from your normal pattern. These symptoms may indicate conditions like PCOS, endometriosis, fibroids, thyroid disorders, or other treatable conditions.
How accurate are period prediction calculators?
Period prediction calculators provide estimates based on your average cycle length and are most accurate for women with regular cycles that vary by only a few days each month. For someone with a consistent 28-day cycle, predictions can be accurate within one to two days. However, many factors can cause deviations including stress, illness, travel, changes in exercise or diet, and hormonal fluctuations. Studies show that only about 13 percent of women have a perfect 28-day cycle. For the most accurate predictions, track your cycle for at least three to six months to establish your personal average cycle length.
What is the difference between spotting and a period?
Spotting refers to light bleeding or brown discharge that occurs outside of your regular menstrual period. It is typically much lighter than a period, often requiring only a panty liner rather than a pad or tampon. Spotting can occur mid-cycle around ovulation due to the drop in estrogen, during early pregnancy as implantation bleeding, as a side effect of hormonal contraceptives, or due to cervical irritation. A period involves heavier bleeding that lasts three to seven days and follows a predictable cyclical pattern. If you experience frequent or unexplained spotting between periods, it is worth discussing with your healthcare provider to rule out underlying conditions.
Is my data stored or sent to a server?
No. All calculations run entirely in your browser using JavaScript. No data you enter is ever transmitted to any server or stored anywhere. Your inputs remain completely private.
How do I verify Period Calculator's result independently?
The Formula section on this page shows the equation used. You can reproduce the calculation manually or in a spreadsheet using those steps. Compare your answer against the worked examples in the Examples section, which use known reference values so you can confirm the calculator is behaving as expected.
References
Reviewed by Rahul Singh, Health & Wellness Specialist · Editorial policy