BMI Calculator

BMI (Body Mass Index) is calculated by dividing weight (in kilograms) by the square of height (in meters). It is a simple and universally applied method for assessing the weight status of adults internationally. The steps to calculate BMI are as follows:

Measure your weight (in kilograms).
Measure your height (in meters).
Square the value of your height (height times height).
Divide the weight by the result of step 3, the square of height.

The formula is expressed as: $[ \text{BMI} = \frac{\text{weight (kg)}}{\text{height (m)}^2} ]$

For example, if a person weighs 70 kilograms and is 1.75 meters tall, then their BMI is calculated as follows:

$[ \text{BMI} = \frac{70}{1.75^2} = \frac{70}{3.0625} = 22.86 ]$

Based on the BMI result, one can assess a person's weight status, such as normal weight, overweight, or obesity. However, it is important to note that BMI is only a rough indicator and does not fully reflect a person's health condition, as it does not take into account factors like muscle mass, bone density, and overall body fat percentage.

BMI table for adults

This table is universally applicable to individuals who are 20 years old or older, without any differentiation based on gender.

BMI	Weight Category
Below 16	Severe Thinness
16 - 17	Moderate Thinness
17 - 18.5	Mild Thinness
18.5 - 24.9	Normal weight
25 - 29.9	Overweight
30 - 34.9	Obesity Class I
35 - 39.9	Obesity Class II
40 and above	Obesity Class III

What the BMI categories mean:

Severe Thinness: Signals seriously low body weight that is medically dangerous
Moderate Thinness: Signals seriously low body weight that is medically dangerous
Mild Thinness: May be due to naturally high metabolism or lots of physical activity
Normal weight: Healthy body weight for given height
Overweight: At risk of developing obesity-related health issues
Obesity: High risk of cardiovascular diseases, diabetes, other complications

Health Hazards of Excess Weight

Carrying excess weight can heighten the likelihood of various grave health issues. The Centers for Disease Control and Prevention (CDC) provides the following list of potential health threats linked to overweight:

Elevated blood pressure
Augmented LDL cholesterol levels, often tagged as "bad cholesterol," diminished HDL cholesterol, which can be beneficial in controlled amounts, and elevated triglyceride levels
Type II diabetes
Coronary artery disease
Cerebrovascular accidents (strokes)
Gallstone disease
Osteoarthritis, a degenerative joint condition due to cartilage erosion
Sleep apnea and respiratory difficulties
Specific types of cancer such as those of the endometrium, breast, colon, kidney, gallbladder, and liver
Impaired quality of life
Psychological disorders, including clinical depression and anxiety
Bodily discomforts and hindered physical functions
Overall, a higher mortality rate compared to individuals with a normal BMI

The list highlights the considerable, sometimes deadly, consequences of excess weight. It is advisable to maintain a BMI below 25 kg/m^2, though individuals should seek medical advice to determine the need for lifestyle modifications to improve health.

Health Dangers of Insufficient Weight

Being underweight is not without its dangers, which include:

Nutritional deficiencies, vitamin scarcity, and anemia (a reduced ability to carry oxygen in the blood)
Osteoporosis, leading to fragile bones and a heightened fracture risk
Weakened immune response
Developmental problems in youths and adolescents
Fertility issues in women due to hormonal imbalances affecting menstrual cycles, with an elevated risk of miscarriage in early pregnancy
Surgical complications
Overall, a higher mortality rate compared to individuals with a normal BMI

Sometimes, being underweight might indicate an underlying health issue, such as anorexia nervosa, which carries additional risks. Medical consultation is recommended for underweight individuals, especially if the cause of low body weight is unclear.

Shortcomings of the BMI Metric

BMI serves as a commonly embraced gauge of body weight healthiness, yet it is not without its constraints. It provides a rough estimation, overlooking the nuances of body composition. Given the diverse array of human physiques and the unique distributions of muscle, bone density, and adipose tissue, BMI ought to be integrated with additional assessments and not relied upon as the exclusive barometer for evaluating healthy body weight.

For Adults:

The precision of BMI is compromised because it measures surplus weight rather than excess fat. Its reliability is affected by a range of factors, including but not limited to age, gender, ethnicity, muscle mass, adipose content, and activity levels. Consider an elderly individual at a 'healthy' weight but sedentary: they may carry unhealthy fat levels despite not being overweight. Conversely, a younger individual with the same BMI but more muscle could be deemed healthy. In the case of athletes, particularly bodybuilders, BMI might categorize them as overweight due to dense muscle, despite a healthy body composition. The CDC acknowledges that:

Older adults generally carry more adipose tissue than younger adults with an equivalent BMI.
Women are prone to higher fat percentages than men at the same BMI.
Athletic or muscular persons may exhibit elevated BMIs because of substantial muscle mass.

For Children and Adolescents:

The limitations of BMI for adults also apply to younger demographics. In children, growth and pubertal stages can affect both BMI and body fat percentages. While BMI is a more reliable indicator of excess adipose tissue in obese youths, it may reflect increased fat or lean mass in those who are merely overweight. In leaner children, BMI variations may arise from differences in lean mass.

Despite these limitations, BMI is indicative of body fat in approximately 90-95% of the population and can be efficiently paired with other metrics to gauge an individual’s healthy body weight.