BMI Calculator

Body Mass Index (BMI) is a simple measure that compares a person's weight to their height. It is widely used as a general screening tool to indicate whether someone has a healthy body weight for their height. BMI was devised in the 1830s by the Belgian statistician Adolphe Quetelet and is still one of the most common weight-status measures used in public health today.

BMI is calculated by dividing body weight in kilograms by the square of height in meters. The result is then matched against ranges — such as underweight, normal, overweight, or obese — to give a rough sense of whether a person is at a healthy weight. BMI does not measure body fat directly, but it correlates reasonably well with body fat for most people, making it a useful first step in identifying whether further assessment may be needed.

The table below shows the World Health Organization's (WHO) recommended weight classification based on BMI for adults aged 20 and older. The same ranges apply to both men and women.

The BMI scale below shows how the adult weight categories sit relative to one another, based on the WHO classification. The colored segments represent the major BMI categories from severe thinness on the left through obesity on the right.

The Centers for Disease Control and Prevention (CDC) recommends that BMI for children and teens be interpreted using BMI-for-age percentiles, which account for the natural changes in body composition that happen as children grow. A percentile compares a child's BMI with the distribution of BMIs among children of the same age and sex. Adult BMI ranges should not be used for children or teens.

BMI-for-age percentiles are plotted on sex-specific growth charts produced by the CDC in the United States and by the WHO internationally. Because children grow at different rates, a single BMI number is not compared against adult categories. Instead it is located on the growth chart for the child's age and sex, and the percentile is tracked over time to see whether the child is following a healthy growth curve.

For example, a boy whose BMI is at the 75th percentile is heavier than 75% of boys of the same age. Health providers generally regard the 5th through 85th percentiles as a healthy range, values between the 85th and 95th as at risk of overweight, and values above the 95th as overweight for that age and sex.

Carrying excess weight raises the risk of a number of serious health conditions. According to the Centers for Disease Control and Prevention (CDC), being overweight is associated with:

• High blood pressure
• Higher LDL ("bad") cholesterol, lower HDL ("good") cholesterol, and high triglycerides
• Type 2 diabetes
• Coronary heart disease and stroke
• Gallbladder disease
• Osteoarthritis, a joint disease caused by the breakdown of joint cartilage
• Sleep apnea and other breathing problems
• Certain cancers, including endometrial, breast, colon, kidney, gallbladder, and liver cancers
• Mental health conditions such as clinical depression and anxiety
• Body pain and difficulty with everyday physical functions
• A generally increased risk of early death compared with people at a healthy weight

For most people, keeping a BMI below 25 kg/m² is a reasonable goal, but anyone concerned about their weight should talk with a healthcare professional about whether lifestyle changes are appropriate for them.

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

• Malnutrition, vitamin deficiencies, and anemia
• Osteoporosis, which weakens bones and increases the risk of fractures
• A weakened immune system
• Growth and development problems, particularly in children and teenagers
• Hormone-related reproductive issues in women, including irregular menstrual cycles and a higher chance of miscarriage in the first trimester
• More complications during and after surgery
• A generally increased risk of early death compared with people at a healthy weight

In some cases, being underweight can be a sign of an underlying condition, such as an eating disorder. If you or someone you know is underweight for no obvious reason, it is worth speaking with a doctor.

Although BMI is widely used and useful as a screening indicator, it has important limitations. BMI is only an estimate and cannot account for body composition — the relative amounts of muscle, bone, and fat in a person's body. For this reason, BMI should be considered alongside other measurements rather than used on its own to judge health.

In adults: BMI measures excess weight rather than excess fat, so it can be misleading for some people. Older adults tend to have more body fat than younger adults with the same BMI. Women tend to carry more body fat than men at the same BMI. Muscular individuals and highly trained athletes may have a high BMI because of muscle mass rather than excess fat. As the CDC notes, a sedentary older person and an athletic younger person can share the same BMI yet have very different levels of body fat.

In children and adolescents: the same limits apply, and height and the stage of sexual maturation also influence how BMI relates to body fat. BMI is a better indicator of excess fat in children with obesity than in children who are merely overweight, and in thin children a low BMI can reflect either low fat or low fat-free mass.

That said, BMI is a fair indicator of body fat for roughly 90–95% of the population and, used together with other measures, is an effective and inexpensive way to screen for weight-related health risk.

BMI is calculated from weight and height using these standard equations:

BMI prime is the ratio of a person's measured BMI to the upper limit of the "normal" BMI range, which is 25 kg/m² according to organizations such as the WHO and the CDC.

BMI prime = BMI ÷ 25

Because it is a ratio of two BMI values, BMI prime is a dimensionless number. A BMI prime below 0.74 is considered underweight, 0.74 to 1 is normal, above 1 is overweight, and above 1.2 is obese. The table below shows weight classifications with their corresponding BMI and BMI prime values.

The Ponderal Index (PI) is similar to BMI in that it measures a person's leanness based on height and weight, but it cubes the height instead of squaring it. While BMI works well when looking at large populations, it is less reliable for individuals, especially very tall or very short people. The Ponderal Index tends to be more reliable for people at the extremes of the height and weight spectrum, where BMI can read uncharacteristically high or low.

PI = mass (kg) ÷ height³ (m)

In US customary units this is equivalent to computing the same ratio after converting weight to kilograms and height to meters. This calculator reports the Ponderal Index alongside BMI so you can see both measures for the same inputs.