Calculator Tools • Health
BMI Calculator
Enter weight and height, toggle metric/imperial units, review WHO categories, and export a PDF report with contextual insights and limitations.
Calculator Tools
BMI Calculator
Instant BMI with medical-grade context.
BMI introduction
Body Mass Index estimates tissue mass using weight divided by height squared. Because the formula is simple, clinicians can quickly triage whether additional lab work, imaging, or nutrition interventions are required. BMI correlates well with body fat at the population level, but it does not distinguish muscle from fat or describe where fat is stored.
Use BMI as a conversation starter, not a diagnosis. Pair it with waist-to-height ratio, DEXA, or lab work for precise risk scoring—especially in athletes, children, older adults, or diverse ethnic groups.
WHO BMI table for adults
| Classification | BMI range (kg/m²) |
|---|---|
| Severe Thinness | < 16 |
| Moderate Thinness | 16 – 16.9 |
| Mild Thinness | 17 – 18.4 |
| Normal | 18.5 – 24.9 |
| Overweight | 25 – 29.9 |
| Obese Class I | 30 – 34.9 |
| Obese Class II | 35 – 39.9 |
| Obese Class III | ≥ 40 |
Ranges above apply to adults 20+ years. Athletes with high lean mass may score “overweight” despite low body fat.
Risks of chronic overweight
- Elevated blood pressure, cholesterol, and triglycerides
- Higher likelihood of type 2 diabetes and insulin resistance
- Joint stress leading to osteoarthritis or accelerated wear
- Sleep apnea, breathing issues, and reduced cardiorespiratory fitness
- Increased risk for certain cancers, gallbladder disease, and fatty liver
Risks of chronic underweight
- Compromised immune response and slower recovery from illness
- Hormonal disruption affecting fertility, bone density, and mood
- Greater injury risk due to reduced lean mass and lower strength reserves
- Micronutrient deficiencies and challenges maintaining core temperature
Limitations & formula
BMI is a population-level screening metric. It does not distinguish muscle from fat mass, nor does it directly measure distribution (visceral vs. subcutaneous) or account for age, sex, or ethnicity specific nuances.
Metric formula
BMI = weight_kg ÷ (height_m²)
Imperial formula
BMI = (703 × weight_lb) ÷ (height_in²)
BMI Prime expresses BMI as a fraction of the upper “normal” limit (25 kg/m²). Values above 1 indicate you are above the normal boundary, while values under 0.74 may suggest undernutrition.
Children & teens (ages 2–20)
Pediatric BMI is compared against CDC percentile charts that adjust for age and sex. A percentile indicates the percentage of peers who fall below that value. Always interpret childhood BMI with a pediatrician since growth spurts, puberty, and genetics have a large influence.
| Category | Percentile range |
|---|---|
| Underweight | < 5th percentile |
| Healthy weight | 5th – < 85th percentile |
| At risk of overweight | 85th – < 95th percentile |
| Overweight | ≥ 95th percentile |
Tip: capture height, weight, and date at each pediatric checkup to monitor percentile trends rather than single readings.