What BMI Is Considered Underweight? Health Risks of Low Body Mass Index — 2026

Body Mass Index (BMI) is a screening ratio of weight to height, not a direct measure of body fat. BMI = weight (kg) ÷ height (m)². In imperial units the formula is 703 × weight (lb) ÷ height (in)². The number is the same regardless of sex for adults.

The CDC applies the same four adult categories to men and women aged 20 and older. BMI is a starting point for population-level screening, not a diagnosis for any individual.

• Underweight: BMI below 18.5
• Healthy weight: BMI 18.5 to 24.9
• Overweight: BMI 25.0 to 29.9
• Obesity: BMI 30.0 and above (Class 1: 30–34.9, Class 2: 35–39.9, Class 3: 40+)

A BMI below 18.5 is classified as underweight by the CDC. Persistently low BMI can be associated with nutrient deficiencies, lower bone density, and a weaker immune system, though causes and meaning vary widely between people.

• Possible contributors: inadequate intake, high expenditure, illness, or genetics
• Health links reported in the literature: osteoporosis risk, anemia, fertility effects
• Action: a clinician can rule out underlying medical causes

Higher BMI is generally associated with higher risk of type 2 diabetes, high blood pressure, and cardiovascular disease, but the link is statistical, not deterministic. Two people with the same BMI can have very different risk profiles based on muscle, fat distribution, fitness, and metabolic markers.

Waist circumference and routine checks (blood pressure, blood sugar) add the context BMI alone cannot provide.

Standing height is the input for BMI and many other ratios, so a small error changes the result. Use a stadiometer when possible; at home, a flat wall and a hard-backed book work.

• Remove shoes and bulky clothing; stand with heels, buttocks, shoulders, and the back of the head against a flat wall.
• Keep the head level (Frankfort plane: the lower eye socket and the ear opening are horizontal) and look straight ahead.
• Take a normal breath in and stand as tall as you can; mark the wall at the top of the head with a pencil or book edge.
• Measure from the floor to the mark with a metal tape, to the nearest 0.1 cm (or 1/4 inch).
• Common errors: measuring in the evening (the spine compresses ~1–2 cm by day’s end), slouching, keeping shoes on, or compressing the hair.

BMI cannot distinguish muscle from fat, so trained athletes and very muscular people may read as "overweight" or "obese" despite low body fat. It also does not capture fat distribution, which matters for health risk.

BMI is not interpreted with adult cutoffs for children and teens (they use age- and sex-specific percentiles), and it is not a tracking tool during pregnancy. Waist circumference and body-fat percentage add useful context.