Growth is primarily measured by height and weight. Adults should be weighed using a beam balance with non-detachable weights. Infants are usually weighed on a suspended scale. Height is typically measured with an anthropometer that consists of a vertical and horizontal bar. Infants and children under two years of age should be measured lying down for recumbent length. For people with severe spinal curvature or those unable to stand, knee height is used as a substitute measurement. Elbow breadth is used as a measure of frame size and it is independent of age or weight; this measure is of the distance between epicondyle of the humerus of the right arm. Head circumference is a useful measure in infants and young children as a measure of brain size.
Growth indices use two or more measurements such as weight and height, or weight and age. Weight-for-age is a useful index of acute malnutrition for children 6 months to 7 years old. It reflects weight or body mass in relation to other children of the same age and sex. Weight-for-age is also used to assess protein-energy malnutrition. Weight-for-height is a sensitive index of current nutritional status, reflecting weight in relation to other children of the same height and sex, and is independent of age for children one to ten years old. Height-for-age is an index of past or chronic nutritional status. Height-for-age reflects height or linear growth in relation to other children of the same age and sex.
Weight change is stable in healthy adults, usually no greater than 0.5 kg. Severe starvation leading to death can occur at a 30% change in weight. Three calculations of weight change can be made from the two measures of actual and usual weight: percent of usual weight, percent of weight loss, and rate of weight change.
Weight/height ratios are measures of weight corrected for height and are highly correlated with obesity. Quetelet's index, which is weight in kilograms divided by height in meters-squared, is commonly used as a measure of body mass and is also referred to as body mass index (BMI). BMI is correlated with percentage body fat and skinfold thickness and not correlated to height. A BMI of 18.5-24.9 is normal according to the standards of the World Health Organization. Patients with a BMI of under 18.5 are considered underweight; those with a BMI 25-29.9 are defined as overweight; 30-34.9 indicates mild obesity, 35-39.9 indicates moderate obesity, and 40 or greater indicates extreme obesity.