BMI Calculator
Calculate your Body Mass Index — a simple screening tool that uses your height and weight to estimate whether you're in a healthy weight range.
Measurements
The weight range where your BMI stays within the WHO normal category
If you weighed the average of these four formulas, this is what your BMI would be
What Is BMI?
Body Mass Index (BMI) is a numerical value calculated from your weight and height. The formula divides your weight in kilograms by the square of your height in meters: BMI = weight (kg) ÷ height (m)². First developed by Belgian mathematician Adolphe Quetelet in the 1830s, it remains the most widely used screening tool for categorizing weight status in population studies and clinical settings.
BMI does not directly measure body fat, muscle mass, or body composition. It is a proxy — a quick, inexpensive way to flag potential weight-related health risks. Its strength is simplicity; its limitation is that it treats all weight equally regardless of whether it comes from muscle, fat, bone, or water.
WHO Weight Categories
The World Health Organization defines four primary BMI categories for adults aged 20 and older:
- Underweight (BMI < 18.5): May indicate nutritional deficiency, eating disorders, or underlying health conditions. Associated with weakened immunity, bone loss, and fertility issues.
- Normal weight (BMI 18.5–24.9): Generally associated with the lowest risk of weight-related health problems. The ideal range for most adults, though individual variation matters.
- Overweight (BMI 25–29.9): Elevated risk for cardiovascular disease, type 2 diabetes, and hypertension. Not all individuals in this range face the same risk — body fat distribution (waist circumference) is an important modifier.
- Obese (BMI ≥ 30): Substantially increased risk for multiple chronic conditions including heart disease, stroke, type 2 diabetes, certain cancers, and osteoarthritis. Further subdivided into Class I (30–34.9), Class II (35–39.9), and Class III (≥ 40).
BMI Prime Explained
BMI Prime is simply your BMI divided by 25 — the upper boundary of the normal range. A BMI Prime of 1.0 means you are exactly at the overweight threshold. A value of 0.85 means your BMI is 85% of the upper normal limit. Values above 1.0 indicate overweight or obesity; values below 0.74 indicate underweight.
The advantage of BMI Prime over raw BMI is that it works across populations without needing to remember different cutoff values. "10% above threshold" is universally understandable, whereas "BMI 27.5" requires knowing that 25 is the cutoff.
Limitations of BMI
BMI has well-documented limitations that are important to understand:
- Muscle vs. fat: Athletes and people who strength train regularly often register as overweight or obese by BMI despite having low body fat and excellent metabolic health. A 200-pound person at 12% body fat has a very different health profile than a 200-pound person at 35% body fat, but BMI treats them identically.
- Age: Older adults tend to have more body fat and less muscle at the same BMI as younger adults. A BMI of 24 may represent very different body compositions at age 25 versus age 65.
- Sex: Women naturally carry more body fat than men. A BMI of 24 typically corresponds to a higher body fat percentage in women than in men.
- Ethnicity: Health risk thresholds vary by ethnicity. Some Asian populations experience elevated diabetes risk at BMIs below 25, while some Pacific Islander populations maintain good health at BMIs above 25.
- Fat distribution: BMI cannot distinguish between visceral fat (around organs, metabolically dangerous) and subcutaneous fat (under skin, less risky). Waist circumference or waist-to-hip ratio provides better insight into fat distribution.
For a more complete picture, consider complementing BMI with body composition analysis, waist circumference, or a body composition test (DEXA scan, bioelectrical impedance). BMI is a useful first-pass screening tool, not a definitive health assessment.
Ideal Body Weight Formulas
The collapsible "Ideal Weight Formulas" section in the results shows estimates from four clinical formulas (Devine, Robinson, Miller, and Hamwi). Each uses only your height and sex to estimate the weight associated with the lowest health risk. The 4–8 kg spread between formulas reflects genuine uncertainty — treat the range as your target zone rather than fixating on a single number.
Your actual ideal weight depends on factors these formulas ignore: bone structure, muscle mass, body fat distribution, and fitness level. A muscular person's healthy weight may be 10–15% above the formula average. For a complete picture, combine these results with your body fat percentage and waist measurements.
Frequently Asked Questions
What is a healthy BMI range?
A BMI between 18.5 and 24.9 is considered normal weight by the WHO. Below 18.5 is underweight, 25–29.9 is overweight, and 30 or above is obese. These thresholds apply to adults aged 20 and older. For children and teens, BMI is assessed using age-specific percentile charts.
Is BMI accurate for muscular people?
No. BMI uses only height and weight, so it cannot distinguish muscle from fat. Athletes and people with significant muscle mass often score as overweight or obese despite having low body fat. For these individuals, body fat percentage or waist-to-hip ratio provides a more meaningful assessment.
What is BMI Prime?
BMI Prime is your BMI divided by 25 (the upper normal threshold). A value of 1.0 means you are at the overweight boundary. Below 1.0 is normal weight territory; above 1.0 is overweight. It provides an intuitive percentage-based view — a BMI Prime of 0.90 means your BMI is 90% of the threshold.
Does BMI apply to children?
Standard BMI categories apply only to adults aged 20 and older. For children and teens (ages 2–19), BMI is plotted on age- and sex-specific growth charts and interpreted as a percentile. A BMI at the 85th–94th percentile is considered overweight, and at or above the 95th percentile is considered obese. Consult a pediatrician for proper assessment.
How often should I check my BMI?
For most adults maintaining a stable weight, checking BMI once or twice a year during routine checkups is sufficient. If you are actively trying to lose or gain weight, monthly checks alongside waist measurements, progress photos, and how your clothes fit give a more complete picture than any single number.
What is ideal body weight?
Ideal body weight is a clinical estimate of the weight associated with the lowest health risk for your height and sex. The four formulas shown use only height and sex. Treat the range as a target zone rather than a single mandatory number — factors like muscle mass, bone structure, and fitness level all affect what weight is healthy for you.