IN-DEPTH ANALYSIS & METROLOGY
Understand adult BMI calculations, category boundaries, height-based weight formulas and why body composition needs more than height and weight.
01 1. The Geometry of Human Mass: Understanding Body Mass Index (BMI)
Body Mass Index is calculated as weight in kilograms divided by height in meters squared: BMI = kg / m². For example, 70 kg at 1.75 m gives 70 ÷ (1.75 × 1.75), or about 22.9. The displayed weight range above is calculated from BMI boundaries rather than from a measurement of body composition.
In customary units, BMI is approximately (pounds × 703) ÷ inches². For adults aged 20 or older, standard categories are below 18.5, 18.5 to less than 25, 25 to less than 30, and 30 or more. BMI is a screening measure, not a diagnosis or a personalized target weight. Children and teens need age-specific assessment.
02 2. The Limitation of Square-Law Scaling and the 'New BMI' Formula
Standard BMI uses height squared. Changing that exponent changes how the calculated index behaves across heights, which is why alternative formulas have been proposed. A mathematical change does not by itself establish better health predictions for an individual.
Nick Trefethen proposed an alternative formula: 1.3 × weight (kg) ÷ height (m)^2.5. It is included here as background, not as a replacement for the standard BMI calculation used by the tool above. This page does not validate that proposal or apply standard BMI category cutoffs to its results.
03 3. Ideal Body Weight (IBW) Formulas: Devine, Robinson & Miller
Height-based formulas sometimes called ideal body weight, including Devine, Robinson and Miller, are different calculations from a BMI-derived weight range. Their name does not mean that the result is the uniquely healthy weight for a person. The tool above calculates BMI and its associated range; it does not provide a medication or ventilation calculation.
For historical context, the Devine formula for heights over 5 feet is 50 kg + 2.3 kg per additional inch for men, or 45.5 kg + 2.3 kg per additional inch for women. At 5 feet 10 inches, the first formula gives 73 kg. This arithmetic example is not a prescription or a recommended weight target; clinical use requires the relevant professional method and context.
04 4. Beyond the Scale: Waist-to-Height Ratio (WHtR) and Somatotypes
BMI cannot distinguish muscle, fat and bone or show where fat is distributed. For example, knowing that someone is 6 feet tall and weighs 220 lb lets you calculate a BMI of about 29.8, but it does not reveal their body-fat percentage. Build descriptions such as ectomorph, mesomorph and endomorph are not measurements of health or a substitute for assessment.
Waist-to-height ratio is another measure: divide waist circumference by height using the same units. NICE recommends using it alongside BMI for adults with BMI below 35. The half-height message is a screening guide within that context, not a guarantee of low risk; the calculator above does not measure waist size or assess body-fat distribution.