BMI categories group adults by the relationship between weight and height. Health organizations use the same basic cutoffs worldwide, though interpretation can differ by age, muscle mass, and ethnicity.
Standard adult BMI categories
- Underweight: below 18.5
- Normal weight: 18.5 to 24.9
- Overweight: 25.0 to 29.9
- Obesity: 30.0 and above
Obesity is sometimes split into Class I (30–34.9), Class II (35–39.9), and Class III (40+). These subgroups help clinicians discuss risk—they are not labels for public use.
What each category suggests
Underweight may reflect low intake, high activity, illness, or natural build. Persistent underweight can coincide with nutrient gaps or bone loss in some people.
Normal weight falls inside the range linked to lower average health risk in population studies. Individual fitness and lab markers still vary.
Overweight indicates excess weight for height by this formula. Many adults in this band are metabolically healthy; others benefit from gradual habit changes.
Obesity correlates with higher average risk for conditions such as type 2 diabetes and cardiovascular disease in large datasets. Personal risk depends on many factors beyond BMI.
Calculate your number
Use our on-site tool—no external links: BMI Calculator. For background on cutoffs and age, read Healthy BMI Ranges Explained.
Limits of categories
Athletes, older adults, and some ethnic groups may be misclassified. See How Accurate Is BMI? and BMI vs Body Fat Percentage.