BMI Calculator
Calculate Body Mass Index (BMI) for adults or children/teens from height and weight.
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For adults (20+), BMI categories use the same thresholds regardless of sex. For children and teens, age- and sex-specific BMI percentiles are required.
Mode
Height (cm)
Weight (kg)
Result
BMI:
Category:
About BMI Calculator
This BMI calculator computes Body Mass Index and classifies results for adults and for children/teens. BMI serves as a population-level screening tool for weight-related health assessment.
BMI Formula and Calculation
Metric formula:
BMI = weight (kg) / height (m)² Example calculation: Height: 170 cm = 1.70 m Weight: 70 kg BMI = 70 / (1.70)² = 70 / 2.89 = 24.22
Imperial formula:
BMI = 703 × weight (lbs) / height (in)² Example calculation: Height: 5'9" = 69 inches Weight: 154 lbs BMI = 703 × 154 / (69)² = 108,262 / 4,761 = 22.74 Note: 703 is the conversion factor from imperial to metric units
WHO BMI Categories (Adults)
| Category | BMI Range | Health Risk Level |
|---|---|---|
| Underweight | < 18.5 | Increased (malnutrition, osteoporosis) |
| Normal weight | 18.5 - 24.9 | Lowest risk (reference category) |
| Overweight | 25.0 - 29.9 | Moderate (increased CVD, diabetes risk) |
| Obese Class I | 30.0 - 34.9 | High (significantly elevated health risks) |
| Obese Class II | 35.0 - 39.9 | Very high (severe health risks) |
| Obese Class III | ≥ 40.0 | Extremely high (morbid obesity) |
Child & Teen BMI-for-Age Categories (2.0-19.9 years)
Children and teens must be evaluated by BMI-for-age percentile, which depends on both age and sex.
| Category | Percentile Range | Notes |
|---|---|---|
| Underweight | < 5th percentile | Below expected range for age/sex |
| Healthy weight | 5th to < 85th percentile | Reference healthy range |
| Overweight | 85th to < 95th percentile | Elevated risk; monitor and assess lifestyle |
| Obesity | ≥ 95th percentile | High risk category |
| Severe obesity | ≥ 120% of 95th percentile or BMI ≥ 35 | Very high risk category |
Health Implications by Category
- Underweight (< 18.5): May indicate malnutrition, eating disorders, hyperthyroidism, or chronic disease. Associated with increased risk of osteoporosis, anemia, weakened immune function, and fertility issues.
- Normal weight (18.5-24.9): Associated with lowest all-cause mortality and morbidity in population studies. Maintain through balanced nutrition and regular physical activity.
- Overweight (25-29.9): Increased risk of cardiovascular disease, type 2 diabetes, hypertension, and certain cancers. Lifestyle modifications (diet, exercise) typically recommended.
- Obese (≥ 30): Significantly elevated risk of heart disease, stroke, type 2 diabetes, sleep apnea, osteoarthritis, fatty liver disease, and multiple cancer types. Medical intervention and structured weight management may be beneficial.
BMI Limitations and Considerations
While useful for population screening, BMI has important limitations for individual assessment:
- Does not measure body composition: Cannot distinguish between fat mass and lean mass (muscle, bone, organs)
- Athletes and muscular individuals: May be classified as overweight/obese despite low body fat percentage
- Elderly: May have normal BMI despite elevated body fat due to age-related muscle loss (sarcopenia)
- Ethnic variations: Health risk thresholds may differ—Asian populations have elevated risk at lower BMIs
- Fat distribution not considered: Visceral fat (abdominal) is more metabolically dangerous than subcutaneous fat
- Pregnancy: BMI categories do not apply during pregnancy
- Children and adolescents: Require age and sex-specific percentile charts, not adult thresholds
Better Health Indicators (Alongside BMI)
For comprehensive health assessment, consider these additional measures:
| Measure | What It Assesses | Risk Thresholds |
|---|---|---|
| Waist circumference | Abdominal/visceral fat | Men: > 40" (102cm); Women: > 35" (88cm) |
| Waist-to-hip ratio | Fat distribution pattern | Men: > 0.90; Women: > 0.85 |
| Body fat percentage | Actual fat vs lean mass | Men: 8-19%; Women: 21-33% (healthy ranges) |
| Blood pressure | Cardiovascular health | Normal: < 120/80 mmHg |
| Blood markers | Metabolic health | Cholesterol, HbA1c, triglycerides |
Example BMI Calculations
| Height | Weight | BMI | Category |
|---|---|---|---|
| 160 cm (5'3") | 50 kg (110 lbs) | 19.5 | Normal |
| 170 cm (5'7") | 70 kg (154 lbs) | 24.2 | Normal |
| 175 cm (5'9") | 85 kg (187 lbs) | 27.8 | Overweight |
| 180 cm (5'11") | 100 kg (220 lbs) | 30.9 | Obese Class I |
| 165 cm (5'5") | 45 kg (99 lbs) | 16.5 | Underweight |
| 178 cm (5'10") | 120 kg (265 lbs) | 37.9 | Obese Class II |
Unit Conversions
Height Conversions: inches → centimeters: multiply by 2.54 feet + inches → cm: (feet × 12 + inches) × 2.54 centimeters → inches: divide by 2.54 Example: 5'9" = (5 × 12 + 9) = 69 inches = 69 × 2.54 = 175.26 cm Weight Conversions: pounds → kilograms: divide by 2.205 (or multiply by 0.454) kilograms → pounds: multiply by 2.205 Example: 165 lbs = 165 / 2.205 = 74.8 kg
Asian BMI Thresholds
Research indicates Asian populations experience elevated health risks at lower BMIs. Some health organizations recommend adjusted thresholds:
| Category | Standard WHO | Asian-adjusted |
|---|---|---|
| Overweight threshold | BMI ≥ 25 | BMI ≥ 23 |
| Obese threshold | BMI ≥ 30 | BMI ≥ 27.5 |
How to Calculate Your BMI
- Select mode: Choose Adult (20+) or Child / Teen (2.0-19.9).
- Enter height and weight: Input height in centimeters and weight in kilograms.
- If Child / Teen mode: Enter age and select sex for percentile-based classification.
- Calculate: Click "Calculate BMI" to compute BMI and category.
- Review results: Adult mode shows BMI category; Child / Teen mode also shows percentile.
Important Notes
- BMI is a screening tool, not a diagnostic measure of individual health
- Results may be inaccurate for athletes, elderly, pregnant women, and children
- Consult healthcare providers for personalized health assessment
- Consider additional measures (waist circumference, body fat %, blood markers) for comprehensive evaluation
Frequently Asked Questions
- What is BMI and how is it calculated?
- BMI (Body Mass Index) is a screening tool that estimates body fat based on height and weight. The formula is: BMI = weight (kg) / height (m)². For imperial units: BMI = 703 × weight (lbs) / height (in)². Results categorize individuals as underweight, normal weight, overweight, or obese.
- What are the WHO BMI categories?
- World Health Organization BMI categories for adults: Underweight (< 18.5), Normal weight (18.5-24.9), Overweight (25-29.9), Obese Class I (30-34.9), Obese Class II (35-39.9), Obese Class III (≥ 40). These thresholds are based on epidemiological data linking BMI to health outcomes.
- What are the limitations of BMI?
- BMI does not distinguish between muscle and fat mass, so muscular athletes may be misclassified as overweight. It does not account for fat distribution (visceral fat is more dangerous than subcutaneous fat), age-related muscle loss, bone density, sex differences, or ethnic variations in health risk thresholds.
- Is BMI accurate for all populations?
- BMI has varying accuracy across populations. Asian populations have higher health risks at lower BMIs (overweight threshold often adjusted to 23). Elderly individuals may have normal BMI despite high body fat due to muscle loss. Athletes and bodybuilders often have elevated BMI from muscle mass, not fat.
- What health risks are associated with high BMI?
- Elevated BMI is associated with increased risk of cardiovascular disease, type 2 diabetes, hypertension, certain cancers (breast, colon, endometrial), sleep apnea, osteoarthritis, and metabolic syndrome. However, BMI is a screening tool, not a diagnostic measure—individual health assessment requires additional factors.
- What is a healthy BMI range?
- For most adults, BMI 18.5-24.9 is associated with lowest health risks. However, optimal BMI varies by individual factors including age, muscle mass, bone structure, and overall health status. Some research suggests BMI 20-25 may be optimal for longevity in most populations.
- How is BMI interpreted for children and teens?
- For ages 2 to 19, BMI must be interpreted using BMI-for-age percentiles and requires both age and sex. Categories are: Underweight (<5th), Healthy weight (5th to <85th), Overweight (85th to <95th), and Obesity (≥95th percentile). Severe obesity is often defined as ≥120% of the 95th percentile or BMI ≥35.