Meaning of Body Mass Index (BMI)

How is BMI calculated?

The formula is weight (kg) ÷ height (m²). Since it can be calculated with only height and weight, it is widely used in health screenings and population studies. For measurement, it is best to use accurate height and weight, excluding shoes and thick clothing.

BMI is not a test for diagnosing a disease on its own, but a screening indicator to identify individuals who require further evaluation.

What are the standards for Korean adults?

According to the criteria of the Korea Disease Control and Prevention Agency and the Korean Society for the Study of Obesity, a BMI below 18.5 is classified as underweight, 18.5~22.9 as normal, 23~24.9 as pre-obesity, 25~29.9 as stage 1 obesity, 30~34.9 as stage 2 obesity, and 35 or higher as stage 3 obesity.

Individual treatment is not determined by the category name alone; comorbid conditions and waist circumferenceweight change history is considered together.

What does BMI miss?

Even with the same BMI body fat percentageand muscle mass, the location of fat accumulation may differ. An athlete with high muscle mass may have a high BMI but low body fat, while an elderly person may have low muscle mass and high body fat even with a normal BMI.

Separate interpretation is also required in situations where body composition differs from general assumptions, such as edema, pregnancy, or amputation.

Why is waist circumference measured together?

Fat accumulated in the abdomen is related to metabolic risk, so it complements the distribution of fat relative to weight. For Korean adults, 90cm for men and 85cm for women are used as the criteria for abdominal obesity.

Even if the BMI is within the normal range, if the waist circumference is large blood pressureand blood sugar, a risk assessment including blood lipids may be necessary.

How is it used in weight management?

BMI is used as one indicator to simply check the starting state and long-term changes. The goal is not just to hit a specific BMI number, but to improve waist circumference, metabolic indicators, physical strength, and daily functioning.

If there is unintended rapid weight change, edema, or muscle loss, do not judge weight loss success by BMI changes alone; evaluate the cause.

Why do BMI standards differ by country and age?

Even with the same BMI, body fat distribution and diabetes·cardiovascular disease risk can vary by population, so Asian and domestic guidelines may use different ranges than Western standards. If you see different normal or obesity criteria in search results, you should first check which country's and which guideline's adult standards they are.

During the growth period, BMI percentiles from age- and gender-specific growth charts are used instead of adult numbers. BMI limitations become more significant when evaluating weight during pregnancy, athletes with very high muscle mass, edema, or the nutritional status of the elderly.

How should height and weight be measured?

Height is measured without shoes, with heels and the body straight; weight is measured on the same scale on a firm, flat floor. Since weight can vary depending on the time of day, clothing, meals, and bowel movements, similar conditions should be maintained for long-term comparison.

An adult's height can also vary depending on posture, measurement method, and spinal changes. If a height value measured long ago is used as a fixed value, the BMI calculation may change, so it should be re-verified if necessary.

Over what period should BMI changes be monitored?

Even if BMI fluctuates over a few days due to water and intestinal contents, body fat does not change by the same amount. You can reduce short-term fluctuations by weighing yourself several times under the same conditions to see the weekly average and comparing waist circumference on a monthly basis.

During the weight loss process, record the starting BMI along with the percentage change in weight, waist circumference, blood pressure, blood glucose, and exercise performance. Crossing a specific BMI threshold is not the sole criterion for success.

When should a low BMI be a cause for concern?

A low BMI may be a physical characteristic, but if it is accompanied by recent unintended weight loss, loss of appetite, and muscle weakness, nutritional deficiency or disease must be checked. Even within a normal BMI, a body composition with low muscle and high abdominal fat can exist.

In particular, for the elderly and those with chronic diseases, muscle mass, function, and the ability to eat are evaluated before attempting further weight loss to reach a lower number. BMI is an indicator that initiates an evaluation, not a standalone basis for diagnosis or prescription.

Why body conditions differ even with the same BMI

BMI is a screening indicator calculated by dividing weight by the square of height, so it does not distinguish between fat, muscle, bone, and water. People with high muscle mass may have a high BMI, while elderly individuals or those with sarcopenia may have high body fat and metabolic risks even if they are within the normal range. For individual evaluations, waist circumference, body composition, blood pressure, and test results must be reviewed together.

Since the patterns of metabolic risk at the same BMI vary by population and ethnicity, application guidelines are checked when interpreting the standards. Small decimal changes may be less than the margin of error for scales and water retention, so their significance is not exaggerated.

How to use BMI for change goals

Rather than targeting only a BMI category, we track the rate of change from initial weight, waist circumference, and physical function. Even if a category boundary is not crossed, improvements in blood pressure, blood glucose, sleep, and mobility can be meaningful changes.

Conversely, even if the BMI is normal, we do not aim to lower it further if there is unintended weight loss, loss of muscle strength, or nutritional deficiency. BMI is not a diagnostic chart that replaces individual goals, but a starting point to identify people who require further evaluation.

How to view BMI for edema, pregnancy, and athletes

BMI cannot distinguish the components of body weight. Because the fetus and amniotic fluid during pregnancy, increased body fluids from edema and ascites, and high muscle mass from strength training are all included in the same weight figure, it is difficult to apply the fat classification of general adults directly.

Use indicators appropriate for the situation. For pregnancy, we look at pre-pregnancy BMI and weight change by gestational week; for edema, we look at symptoms and the underlying cause; for athletes, we look at waist circumference, body composition, and performance. In cases of sudden weight gain, evaluating fluid changes and symptoms takes priority over the BMI category.

What can be missed by using only BMI for the elderly

As people age, height may decrease, muscle mass may decline, and fat distribution may change; thus, muscle strength and nutritional status can worsen even if BMI remains constant. Sarcopenic obesity can exist within a normal or high BMI, and a low BMI combined with unintended weight loss serves as a clue to nutritional risk.

We check the recent weight loss rate, calf and waist circumference, grip strength, walking speed, and food intake together. Prioritizing functions such as climbing stairs and standing up, as well as protein and total energy intake and disease management, may be more important than the goal of further lowering weight.