BMI Calculator

BMI categories — full WHO classification

The World Health Organization (WHO) uses an 8-tier classification for adults aged 20 and older. The simplified 4-category version (Underweight, Normal, Overweight, Obese) is most commonly used, but the full table below provides more granularity for clinical assessments.

Health risks of being overweight

A BMI of 25 or higher is associated with a significantly elevated risk for a range of chronic conditions. These risks increase progressively as BMI rises above 25, and are compounded by factors like age, physical inactivity, and family history.

• ·High blood pressure (hypertension) — excess body fat increases the workload on the heart and stiffens blood vessels
• ·Type 2 diabetes — excess fat, particularly around the abdomen, impairs insulin sensitivity
• ·Coronary artery disease and stroke — driven by elevated cholesterol, blood pressure, and chronic inflammation
• ·Sleep apnea — fatty tissue around the throat can obstruct the airway during sleep
• ·Osteoarthritis — extra weight accelerates joint cartilage breakdown, particularly in knees and hips
• ·Gallbladder disease — obesity increases cholesterol in bile, raising the risk of gallstones
• ·Certain cancers — including endometrial, breast, colon, kidney, and liver cancer
• ·Non-alcoholic fatty liver disease (NAFLD) — fat accumulates in liver cells and can progress to cirrhosis
• ·Depression and anxiety — the relationship between obesity and mental health is bidirectional and well-documented

Health risks of being underweight

A BMI below 18.5 can signal nutritional deficiency or underlying illness and carries its own serious health risks. Being underweight is associated with higher all-cause mortality in some populations, particularly older adults.

• ·Malnutrition and vitamin deficiencies — insufficient intake of key nutrients like iron, B12, and calcium
• ·Osteoporosis — low body weight is a major risk factor for reduced bone density and fracture risk
• ·Weakened immune function — the immune system requires adequate nutrition to operate effectively
• ·Anemia — iron deficiency is more common in underweight individuals, causing fatigue and reduced oxygen delivery
• ·Fertility issues — women with very low BMI may experience irregular or absent menstrual cycles
• ·Muscle wasting — the body breaks down muscle tissue for energy when calorie intake is insufficient
• ·Slower wound healing and higher surgical complications
• ·Growth and developmental problems in children and adolescents

Limitations of BMI

BMI is a useful population-level screening tool, but it has significant limitations when applied to individuals. Because it uses only height and weight, it cannot distinguish between fat mass and lean mass (muscle, bone, organs). This means BMI can misclassify people in both directions.

• ·Athletes and strength trainers often have BMIs in the "overweight" range despite very low body fat, because muscle is denser than fat
• ·Older adults tend to have more body fat at the same BMI compared to younger adults, due to age-related muscle loss (sarcopenia)
• ·Women naturally carry more body fat than men at an equivalent BMI — some guidelines use adjusted thresholds
• ·Ethnicity matters — people of Asian descent have higher metabolic risk at lower BMI values; many Asian health organizations set the overweight threshold at 23 instead of 25
• ·BMI cannot capture fat distribution — abdominal (visceral) fat is far more metabolically harmful than fat stored in the hips and thighs
• ·For people shorter than 5'0" (152 cm) or taller than 6'3" (190 cm), BMI becomes less reliable at the extremes

How BMI is calculated

BMI is defined as body mass in kilograms divided by the square of height in meters. In the metric system this is straightforward; in imperial units a conversion factor of 703 is applied to account for the unit difference.

Healthy BMI by age group

For adults 20 and older, the same BMI ranges apply regardless of age — but research shows that the health implications of a given BMI shift as we get older. For children and teenagers (ages 2–19), BMI is interpreted using age- and sex-specific growth chart percentiles rather than fixed cutoffs.

BMI vs. body fat percentage

Body fat percentage (BFP) is a more direct measure of body composition than BMI. While BMI is easy to calculate with just height and weight, BFP tells you the actual proportion of your body that is fat tissue. The two measures often agree, but diverge significantly in muscular or very sedentary individuals. Use our Body Fat Calculator (Navy method) to measure your BFP with just a tape measure.