How is BMI calculated?

BMI is your weight in kilograms divided by the square of your height in metres (kg/m²). In imperial units the formula is 703 multiplied by weight in pounds divided by height in inches squared. The result is a single number that places you in a category.

What is a healthy BMI range?

The WHO defines a normal weight as a BMI of 18.5 to 24.9 kg/m². Below 18.5 is underweight and 25 or above is overweight, with obesity starting at 30. These cut-offs are screening thresholds for the general adult population.

Why might BMI be misleading?

BMI cannot tell muscle from fat, so very muscular people can score as overweight despite low body fat. It is also not validated for children, pregnant people, or the very elderly, and the WHO suggests lower thresholds for many Asian populations.

What are the obesity classes?

Obesity is split into class I (BMI 30 to 34.9), class II (35 to 39.9), and class III, sometimes called severe or morbid obesity, at 40 and above. Higher classes correlate with greater risk of cardiovascular and metabolic disease.

Does this work for children?

No. Adult BMI cut-offs do not apply to children and teenagers, whose BMI must be interpreted against age and sex percentile charts. Use a paediatric BMI-for-age tool and consult a healthcare professional for anyone under 18.

BMI Classification Reference

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Body Mass Index is the most widely used screening number for weight relative to height. This tool calculates your BMI from metric or imperial measurements and maps it onto the World Health Organization’s adult classification, from severe underweight through to obese class III, with the exact numeric cut-offs.

How it works

BMI is defined as weight in kilograms divided by the square of height in metres:

BMI = weight_kg / (height_m * height_m)

For imperial measurements the equivalent formula multiplies pounds by 703 and divides by height in inches squared, which reproduces the same metric result. The tool then compares the value against the WHO bands — underweight below 18.5, normal 18.5 to 24.9, overweight 25 to 29.9, and obese class I, II and III at 30, 35 and 40 — and highlights the matching row.

Notes and limitations

BMI is a population screening tool, not a body-composition measurement. It treats muscle and fat the same, so athletes and very muscular people may be misclassified as overweight. It is not validated for children, pregnancy, or the frail elderly, and the WHO recommends lower action points of 23 and 27.5 for many Asian populations because cardiometabolic risk appears at a lower BMI. Treat the result as a starting point for a conversation with a clinician, not a diagnosis.