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About

The Quetelet Index, commonly known as BMI, is the standard heuristic for classifying weight relative to height. While it does not distinguish between muscle and fat, it serves as a critical preliminary screening tool for public health. This calculator goes beyond the simple number by mapping the result to a visual "Risk Gauge." It helps identify potential associations with cardiovascular diseases, type 2 diabetes, and hypertension. The tool adheres strictly to World Health Organization (WHO) cutoffs.

BMI obesity health risk analysis weight

Formulas

The Body Mass Index is calculated as mass divided by height squared.

BMI = mass kg(height m)2

If using Imperial units (lbs, inches), a conversion factor of 703 is applied:

BMI = 703 × mass lbsheight in2

Reference Data

ClassificationBMI Range (kg/m2)Health Risk Profile
Underweight< 18.5Risk of nutritional deficiency, osteoporosis, and immune dysfunction.
Normal Range18.5 - 24.9Lowest statistical risk of weight-related chronic diseases.
Overweight25.0 - 29.9Moderate increased risk of cardiovascular issues and fatigue.
Obese Class I30.0 - 34.9High risk of hypertension, diabetes, and sleep apnea.
Obese Class II35.0 - 39.9Very high risk. Medical intervention often recommended.
Obese Class III 40.0Extremely high risk (Morbid Obesity). Significant impact on life expectancy.

Frequently Asked Questions

No. BMI does not distinguish between lean muscle mass and fat mass. Bodybuilders often fall into the "Obese" category despite having low body fat. In these cases, BIA or calipers are preferred.
Yes. Older adults often lose muscle and gain bone fragility. A slightly higher BMI (25-27) in the elderly can sometimes be protective against fragility fractures, unlike in younger adults.
The WHO defines 18.5 as the lower threshold where the body may start catabolizing its own tissue for energy, leading to immune system weakness and hormonal imbalances.