BMI Calculator

Use Our BMI Calculator

Our Body Mass Index (BMI) Calculator helps you determine your BMI value and understand your corresponding weight category, and it even considers age. Use the “Metric Units” tab if you prefer the standard international units, or the “Other Units” tab to easily convert your measurements into either US customary or metric units. You’ll also notice that the calculator provides your Ponderal Index (PI) in addition to your BMI, both of which we’ll explain in more detail below.

Understanding BMI

BMI is a calculation that uses your height and weight to estimate your body leanness or body fat. It’s a widely used general indicator of whether you have a healthy body weight for your height. The BMI value obtained from the calculation helps categorize you as underweight, normal weight, overweight, or obese, depending on the specific range it falls within. It’s important to know that these BMI ranges can vary slightly based on factors like your region and age, and they are sometimes further divided into more specific subcategories, such as severely underweight or very severely obese. Being either overweight or underweight can have significant impacts on your health. While BMI isn’t a perfect measure of healthy body weight on its own, it serves as a valuable initial indicator of whether further assessment or lifestyle changes might be beneficial. Refer to the table below to see the different weight categories our calculator uses based on your BMI.

BMI Categories for Adults

This table shows the World Health Organization’s (WHO) recommended body weight categories based on BMI values for adults (men and women aged 20 and older):

BMI chart for adults

This is a graph of BMI categories based on the World Health Organization data. The dashed lines represent subdivisions within a major categorization.

BMI table for children and teens, age 2-20

The Centers for Disease Control and Prevention (CDC) recommends BMI categorization for children and teens between age 2 and 20.

BMI chart for children and teens, age 2-20

The Centers for Disease Control and Prevention (CDC) BMI-for-age percentiles growth charts.

Chart for boys
Chart for girls

Health Risks Associated with Being Overweight

According to the Centers for Disease Control and Prevention (CDC), being overweight increases your risk for several serious diseases and health conditions, including:

• High blood pressure
• Unhealthy cholesterol levels (higher LDL “bad” cholesterol, lower HDL “good” cholesterol, and high triglycerides)
• Type 2 diabetes
• Coronary heart disease
• Stroke
• Gallbladder disease
• Osteoarthritis (joint disease caused by cartilage breakdown)
• Sleep apnea and other breathing problems
• Certain types of cancer (endometrial, breast, colon, kidney, gallbladder, liver)
• Lower quality of life
• Mental health issues like clinical depression and anxiety
• Body pain and difficulty with physical functions
• Generally, a higher risk of death compared to individuals with a healthy BMI

As this list shows, being overweight can lead to numerous negative and potentially life-threatening health outcomes. It’s generally recommended to aim for a BMI below 25 kg/m², but it’s best to consult with your doctor to determine the ideal weight for your individual health and whether any lifestyle changes are needed.

Health Risks Associated with Being Underweight

Being underweight also carries its own set of health risks, including:

• Malnutrition, vitamin deficiencies, and anemia (reduced ability of the blood to carry oxygen)
• Osteoporosis (weakening of the bones, increasing fracture risk)
• Weakened immune system
• Growth and development problems, especially in children and adolescents
• Potential reproductive issues for women due to hormonal imbalances that can disrupt menstruation. Underweight women also have a higher risk of miscarriage in the first trimester.
• Increased risk of complications following surgery
• Generally, a higher risk of death compared to individuals with a healthy BMI

In some cases, being underweight can be a sign of an underlying medical condition, such as anorexia nervosa, which has its own serious risks. It’s important to consult your doctor if you believe you or someone you know is underweight, especially if the reason for the low weight isn’t clear.

Limitations of BMI

While BMI is a widely used and helpful tool for assessing healthy body weight, it’s important to understand its limitations. BMI provides only an estimate and doesn’t account for individual body composition. Because people have different body types and varying distributions of muscle, bone mass, and fat, BMI should be considered alongside other measurements rather than being the sole determinant of a person’s healthy weight.

In adults:

BMI is a measure of excess body weight, not specifically excess body fat. Factors like age, sex, ethnicity, muscle mass, body fat percentage, and activity level can all influence BMI. For example, an older, inactive person at a “healthy” BMI might still have a significant amount of unhealthy excess body fat, while a younger, muscular individual with the same BMI might be at a healthy weight due to higher muscle mass (muscle weighs more than fat). According to the CDC:

• Older adults tend to have more body fat than younger adults with the same BMI.
• Women tend to have more body fat than men with the same BMI.
• Muscular individuals and highly trained athletes may have higher BMIs due to their large muscle mass.

In children and adolescents:

The same limitations that apply to adults can also affect the accuracy of BMI in children and adolescents. Additionally, their height and stage of puberty can influence BMI and body fat levels. BMI is a better indicator of excess fat in obese children compared to overweight children, whose higher BMI could be due to increased fat or fat-free mass (which includes water, organs, and muscle). In thin children, differences in BMI can also be due to variations in fat-free mass.

Despite these limitations, BMI is a reasonably good indicator of body fat for 90-95% of the population and can be effectively used in conjunction with other assessments to help determine an individual’s healthy body weight.

BMI Formula

Here are the standard formulas used to calculate BMI in both the International System of Units (SI) and the US customary system (USC), using a person who is 5’10” (1.78 meters) and weighs 160 pounds (72.57 kilograms) as an example:

BMI Prime

BMI prime is a ratio that compares a person’s calculated BMI to the upper limit of what is considered a “normal” BMI by organizations like the WHO and the CDC. While this upper limit (referred to as BMIupper​) might vary slightly in some regions, such as parts of Asia, it is generally considered to be 25 kg/m².

The BMI prime formula is:

Because BMI prime is a ratio of two BMI values, it is a dimensionless number. Here’s how weight classifications are often determined based on BMI prime:

BMI prime provides a quick way to assess how much an individual’s BMI deviates from the upper end of the healthy range. It also allows for comparisons between different groups of people who might have slightly different upper BMI limits for what’s considered normal.

Ponderal Index (PI)

The Ponderal Index (PI) is another measure that assesses a person’s leanness or body build based on their height and weight, similar to BMI. The key difference in the formula is that PI uses the cube of the height instead of the square (as in BMI). While BMI can be a useful tool for studying large populations, it’s less reliable for determining leanness in individuals. Although PI has similar considerations, it tends to be a more reliable measure for very tall or very short individuals, whereas BMI can sometimes give uncharacteristically high or low body fat readings for those at the extreme ends of the height and weight spectrum. Here’s the formula for calculating PI using USC, again using our 5’10”, 160-pound individual as an example: