BMI Calculator
Body mass index (BMI) is a widely used screening tool that estimates body fatness by relating a person's weight to their height. It is calculated by dividing weight in kilograms by the square of height in metres (kg/m2) or, in imperial units, by dividing weight in pounds by the square of height in inches and multiplying by 703. The Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) classify adult BMI into four main categories: underweight (below 18.5), normal weight (18.5 to 24.9), overweight (25.0 to 29.9), and obese (30.0 and above). The obese range is further subdivided into Class I (30.0 to 34.9), Class II (35.0 to 39.9), and Class III (40.0 and above, sometimes called severe obesity). BMI is a population-level screening measure, not a direct measure of body fat percentage. It does not account for muscle mass, bone density, age, ethnicity, or body composition, so an athletic person with high muscle mass may have a high BMI without excess fat, and an older adult may have a normal BMI while carrying excess fat. Use this calculator alongside a healthcare provider's assessment, not as a standalone diagnostic tool. This calculator supports both metric and imperial units and shows your BMI value, weight-status category, and a visual indicator of where you fall on the WHO scale.
Your BMI is --, which falls in the -- range.
How BMI is calculated
BMI is a simple ratio of weight to height squared. It is the same formula worldwide and applies to adults 20 and older. The metric formula is more straightforward; the imperial formula includes a scaling factor to account for unit conversion.
Metric: BMI = weight (kg) / (height (m))^2
Imperial: BMI = (weight (lb) * 703) / (height (in))^2
Worked example
Weight 70 kg, height 175 cm (1.75 m):
- Height in metres = 175 / 100 = 1.75 m
- Height squared = 1.75^2 = 3.0625 m^2
- BMI = 70 / 3.0625 = 22.9
- Category: Normal weight (18.5 to 24.9)
WHO and CDC BMI categories
| Category | BMI Range | Health Risk |
|---|---|---|
| Underweight | Below 18.5 | Increased risk of nutritional deficiency, bone loss, infections |
| Normal weight | 18.5 to 24.9 | Lower overall health risk for most adults |
| Overweight | 25.0 to 29.9 | Increased risk of hypertension, type 2 diabetes, heart disease |
| Obese Class I | 30.0 to 34.9 | Higher risk of metabolic and cardiovascular disease |
| Obese Class II | 35.0 to 39.9 | Substantially increased disease risk |
| Obese Class III | 40.0 and above | Very high risk; medical intervention often recommended |
Important limitations of BMI
BMI is a screening tool, not a diagnostic measure. It does not distinguish between muscle and fat, nor does it account for bone density, age, sex, ethnic variation, or overall fitness level. Athletes, for example, may have high BMI due to muscle mass but low body fat. Conversely, sedentary individuals may have normal BMI despite carrying excess body fat.
For a complete health assessment, consult a qualified healthcare provider. They can evaluate BMI alongside other measures such as waist circumference, physical fitness, blood pressure, cholesterol levels, and personal medical history.
BMI calculator: frequently asked questions
What is BMI and why does it matter?
Body Mass Index (BMI) is a screening tool that estimates body composition based on height and weight. The CDC and WHO use it to identify population health trends and categories for health risk. A BMI outside the normal range may signal a need to consult a healthcare provider about diet, exercise, or underlying conditions.
Is BMI accurate for everyone?
BMI is a screening tool, not a diagnostic measure. It does not account for muscle mass, bone density, age, sex, ethnicity, or fitness level. Athletes and very muscular individuals may have high BMI despite low body fat. Conversely, someone may have normal BMI but still carry excess body fat. Always consult a healthcare provider for a full health assessment.
How do WHO and CDC BMI categories differ?
The CDC and WHO use identical BMI categories: Underweight (below 18.5), Normal weight (18.5 to 24.9), Overweight (25.0 to 29.9), Obese Class I (30.0 to 34.9), Obese Class II (35.0 to 39.9), and Obese Class III (40.0 and above). Some countries have adjusted thresholds for specific populations; check your local health authority for regional guidance.
What is a healthy BMI range?
The WHO and CDC define a healthy BMI as 18.5 to 24.9. This range is associated with lower overall health risk for most adults. However, the optimal BMI for an individual depends on age, sex, ethnicity, muscle mass, and medical history. A healthcare provider can advise on a target range tailored to your circumstances.
Can BMI be misleading?
Yes. BMI can be misleading for individuals with very high muscle mass (athletes), very low muscle mass (elderly, sedentary), or unusual height proportions. Body composition analysis (via DEXA scan or bioelectrical impedance) is more precise but less practical for population screening. For individual health assessment, BMI is a starting point, not a diagnosis.
Does BMI change with age?
BMI calculation does not change with age, but the CDC and WHO acknowledge that body composition changes over the lifespan. An older adult may have the same BMI as a younger adult but with different muscle and fat distribution. Age-specific guidance is available from geriatric specialists and organizations like the American Geriatrics Society.
Official sources
- CDC BMI for adults: BMI Categories.
- WHO BMI classification: Global Health Observatory.
Reviewed by the CalculatorHub team, edited by James Graham, 13 June 2026. See our methodology. General information, not medical advice.