BMI, Body Composition and Muscle: What the Numbers Mean

Overview

Body mass index (BMI) is a number worked out from weight and height. It is quick and widely used, and it has real limits. This page explains what BMI shows, what it misses, what other measures add, and why keeping muscle matters. It does not tell you what your weight "should" be.

What BMI is

NIDDK says overweight and obesity are defined by BMI, with these ranges for adults aged 20 and older: 18.5 to 24.9 is healthy weight, 25 to 29.9 is overweight, 30 or more is obesity, and 40 or more is severe obesity. [1] WHO uses a BMI of 25 or more for overweight and 30 or more for obesity, and notes that other measures, such as waist circumference, can also be used. [2]

What BMI misses

• It cannot tell muscle from fat. NHS says the BMI calculator cannot tell the difference between muscle and fat, so a person with a lot of muscle may be placed in the overweight or obese category without much body fat. [3]
• Cut-offs differ by background. NHS says people from South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds are at risk of health problems at a slightly lower BMI, so lower cut-offs are used. [3]
• It does not show where fat is carried. NHLBI says extra fat in the stomach area is a bigger risk factor for heart disease than extra fat in other parts of the body. [4]
• It is not a diagnosis of health. A Lancet Diabetes & Endocrinology Commission of 58 experts said BMI-based measures can both underestimate and overestimate body fat and give inadequate information about health for an individual. [5] It recommended using BMI as a measure of health risk at a population level or for screening, not as an individual measure of health. [5]

Other measures

Why muscle matters during weight change

When people lose weight, some of what is lost is lean mass. A 2026 meta-analysis of 20 trials found lean mass made up 25% to 39% of total weight lost with semaglutide, tirzepatide or liraglutide, and about 26% with lifestyle programs; lifestyle plus resistance training had the lowest share, about 17.5%. [7] Another meta-analysis of 34 trials found that adding exercise to calorie restriction prevented nearly half (45.7%) of the loss of fat-free mass. [8] Evidence label: established that exercise helps preserve lean mass during calorie reduction; see Physical activity, strength and keeping muscle. Muscle loss with age is covered at Sarcopenia and muscle loss with age.

Weight-inclusive note

BMI categories can feel like labels. An international consensus statement says weight stigma can cause physical and psychological harm and that affected people are less likely to receive adequate care. [9] NIDDK notes that physical activity has important health benefits that are independent of weight loss itself. [6]