BMI and Waist Circumference

Body mass index is weight in kilograms divided by the square of height in metres. Waist circumference is the girth at the level between the lowest rib and the iliac crest. Together they classify weight status and estimate central adiposity, which tracks with cardiometabolic risk more closely than BMI alone.

Both are quick, inexpensive and reproducible. Neither measures body fat directly.

Usage. They are used in routine assessment of adults, in cardiovascular and diabetes risk estimation, and in monitoring change in weight over time.

Preparation and technique. Weight is measured on calibrated scales, with the patient in light clothing and without shoes. Height is measured standing, with the head in the horizontal plane. Waist circumference is measured at the end of a normal expiration, with a non-stretchable tape held horizontally and the abdomen relaxed.

Adult BMI categories (kg/m²). Under 18.5 is underweight, 18.5 to 24.9 is the usual range, 25.0 to 29.9 is overweight, and 30.0 or more is obesity (class 1 is 30.0 to 34.9, class 2 is 35.0 to 39.9, class 3 is 40.0 or more).

Waist circumference. Increased risk is generally taken as over 94 cm in men and over 80 cm in women, and high risk as over 102 cm in men and over 88 cm in women. Lower thresholds apply in South Asian, East Asian and some other populations, commonly about 90 cm in men and 80 cm in women.

Waist-to-height ratio. A ratio of 0.5 or more indicates increased central adiposity at any height.

Interpretation.

• A raised BMI with a raised waist circumference carries the highest risk.
• A normal BMI with a raised waist circumference suggests central adiposity and carries increased risk.
• A BMI under 16 kg/m² indicates severe thinness and needs urgent nutritional assessment.

Causes of a misleading BMI. High muscle mass (athletes), oedema, ascites, sarcopenic obesity in older adults, and ethnicity specific differences in body fat at a given BMI.

Critical values and findings needing immediate action. A BMI under 16 kg/m², or rapid unintentional weight loss, requires urgent nutritional and medical assessment.

Limitations and artefacts. BMI does not distinguish fat from lean mass or show fat distribution. Waist measurement is affected by recent meals, posture and the site chosen. Height falls with age, so BMI is overestimated in older adults unless height is re-measured.

Follow up tests. Fasting plasma glucose, HbA1c, lipid profile, liver function tests and blood pressure measurement in raised risk patients, and bioelectrical impedance analysis or DEXA when body composition matters.

Special groups. Children use age- and sex specific centiles or z-scores, not adult categories. In pregnancy, BMI is recorded at booking, and waist circumference is not used. Older adults are assessed with attention to muscle loss.