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This is the World Health Organization's (WHO) recommended body weight classification based on BMI values for adults. It is used for both men and women, age 20 or older:

The Centers for Disease Control and Prevention (CDC) recommends BMI categorization for children and teens between age 2 and 20 using age- and sex-specific percentile charts, because body composition varies greatly as children grow:

The same factors that limit BMI for adults apply to children — height, level of sexual maturation, and body composition all influence BMI in growing individuals. BMI is a better indicator of excess body fat for obese children than for overweight children, whose elevated BMI may be driven by increased muscle or bone mass rather than fat.

Below are the equations used for calculating BMI in both the International System of Units (SI) and the US customary system (USC). Example uses a 5'10", 160-pound individual:

The WeightGPT BMI calculator runs both formulas instantly so you never need to do the math manually. Just enter your height and weight in your preferred units and get your result immediately.

Being overweight increases the risk of a number of serious diseases and health conditions. Below is a list of said risks, according to the Centers for Disease Control and Prevention (CDC):

• High blood pressure (hypertension)
• Higher levels of LDL cholesterol ("bad cholesterol"), lower HDL cholesterol ("good cholesterol"), and high triglycerides
• Type II diabetes
• Coronary heart disease
• Stroke
• Gallbladder disease
• Osteoarthritis — joint disease caused by breakdown of joint cartilage
• Sleep apnea and breathing problems
• Certain cancers (endometrial, breast, colon, kidney, gallbladder, liver)
• Low quality of life
• Mental illnesses such as clinical depression, anxiety, and others
• Body pains and difficulty with certain physical functions
• Generally increased risk of mortality compared to those with a healthy BMI

As can be seen from the list above, there are numerous negative, in some cases fatal, outcomes that may result from being overweight. Generally, a person should try to maintain a BMI below 25 kg/m², but ideally should consult their doctor to determine whether any lifestyle changes are needed to be healthier.

Being underweight has its own set of associated health risks:

• Malnutrition, vitamin deficiencies, anemia (lowered ability to carry oxygen in blood vessels)
• Osteoporosis — bone weakness that increases the risk of fractures
• Decrease in immune function — higher susceptibility to illness
• Growth and development issues, particularly in children and teenagers
• Reproductive issues for women due to hormonal imbalances that can disrupt the menstrual cycle; higher chance of miscarriage in the first trimester
• Potential complications as a result of surgery
• Generally increased risk of mortality compared to those with a healthy BMI

In some cases, being underweight can be a sign of underlying conditions such as anorexia nervosa, which has its own serious risks. Consult your doctor if you think you or someone you know is underweight, particularly if the reason is not obvious.

Although BMI is a widely used and useful indicator of healthy body weight, it does have important limitations. BMI is only an estimate that cannot take body composition into account. Due to the wide variety of body types and distribution of muscle, bone mass, and fat, BMI should be considered along with other measurements rather than being used as the sole method for determining a person's healthy body weight.

In adults:

• BMI measures excess body weight, not excess body fat specifically
• BMI is influenced by age, sex, ethnicity, muscle mass, body fat distribution, and activity level
• Older adults tend to have more body fat than younger adults at the same BMI
• Women tend to have more body fat than men for an equivalent BMI
• Muscular individuals and highly trained athletes may have high BMIs due to large muscle mass, not excess fat

In children and adolescents:

• Height and level of sexual maturation can influence BMI and body fat
• BMI is a better indicator for obese children than overweight children
• Differences in BMI among thin children can be due to fat-free mass variations

That being said, BMI is fairly indicative of body fat for 90–95% of the population, and can effectively be used along with other measures to help determine a healthy body weight. For a more complete picture, use our Body Fat Percentage Calculator alongside BMI.

BMI Prime is the ratio of a person's measured BMI to the upper limit of BMI that is considered "normal" by the WHO and CDC. This upper limit is 25 kg/m².

Since BMI Prime is a ratio of two BMI values, it is a dimensionless number. A person with a BMI Prime less than 0.74 is classified as underweight; 0.74 to 1.0 is normal; greater than 1.0 is overweight; and greater than 1.2 is obese.

BMI Prime makes it easy to see at a glance how far above or below the normal range a person falls. For example, a BMI Prime of 0.92 means you are 8% below the upper limit of normal BMI. A BMI Prime of 1.15 means you are 15% above it. It also allows for meaningful comparisons between groups of people who may have different upper BMI limits (e.g., in Asian populations where the overweight threshold is lower).

The Ponderal Index (PI) is similar to BMI in that it measures the leanness or corpulence of a person based on their height and weight. The main difference is that PI cubes height instead of squaring it:

While BMI can be useful for large populations, it is not always reliable for individuals at the extreme ends of the height spectrum. The Ponderal Index is more reliable for very tall or very short individuals, as BMI tends to record uncharacteristically high or low body fat levels for those people. The normal Ponderal Index range is approximately 11 to 15 kg/m³.