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Body Mass Index (BMI) measures body fat based on height and weight, which applies to adult men and women. A BMI calculator calculates BMI value and weight status while considering age. It is a general indicator of whether a person has a healthy body weight for height. Using this, you can get accurate results about whether the person is underweight, normal, overweight, or obese.

BMI levels

BMI is calculated by a number representing a person's fat or obesity level. Based on the Centres for Disease Control and Prevention, a BMI of 30 or above indicates obesity. The range of BMI is shown below based on the person’s weight:

BMI table for adults (kg/m2)

BMI < 16 = severe thinness

BMI 16 – 17 = Moderate thinness

BMI 17 – 18.5 = mid thinness

BMI 18.5 - 25 = Normal

BMI 25 – 30 overweight

BMI 30 – 35 = Obese Class I

BMI 35 – 40 = Obese Class II

BMI > 40 = Obese Class III

BMI table for children and teens (%)

BMI categorization for children and teens between the age of 2 and 20,

BMI < 5% = underweight

BMI 5% - 85% = healthy weight

BMI 85% - 95% = at risk of overweight

BMI > 95% = overweight

Uses of BMI

BMI can be used for the assessment of overweight and obesity. For calculation, it required height and weight, which is inexpensive and used in most clinics. Health providers use BMI to diagnose weight types and a screening tool for health conditions.

Why does BMI matter?

Generally, a person should maintain a BMI below 25 kg/m2. If BMI results from being an overweight higher risk of developing a range of serious diseases and health conditions.

The risk of being overweight includes
  • Arthritis
  • Diabetes
  • Liver disease
  • Cancer (breast, colon, and prostate),
  • High blood pressure,
  • High cholesterol,
  • Sleep apnea,
  • Type II diabetes
  • Coronary heart disease
  • Stroke
  • Osteoarthritis
  • Gallbladder disease
  • Low quality of life
  • Mental illness
  • Body pains
  • Increased risk of mortality

Risk of being underweight include:
  • Malnutrition
  • Anemia
  • Weakened immune system
  • Osteoporosis
  • Infertility
  • Potential complications at risk of surgery
  • Women have a disrupted menstrual cycle and a higher chance of miscarriage in the first trimester.
  • Growth and development issues

What is a healthy BMI?

The range of a healthy BMI is considered to be 18.5 to 24.9. Factors like genetics, activity level, smoking, drinking alcohol, and mental health conditions affect your overall health and cause you to develop certain medical conditions.

Limitations of BMI!

BMI cannot show whether a person's weight is healthy. It is the combination with other indicators that provide a complete picture. BMI has its limitations. It is an estimation that needs to take body composition into account. Due to various body types and distribution of muscle, mass, and fat, BMI is considered with other measurements rather than being the sole method for determining a person's healthy body weight.

Age and sex

BMI incorporates weight and height for adults ages 20 years and older, but it does not take age or sex into account. Women have more body weight than a man with the same BMI. Older people tend to have more body fat than younger people with an equal BMI. BMI may not give the necessary detail to determine whether a person's weight is healthy.

Body composition

BMI does not reflect the location of the body. For example, if a person has fat around the waist and surroundings, the abdominal organs may be at risk of health problems, and those with fat in other areas. It is based on the study of scientific reports and a 5-year investigation of 1964 people.

BMI and health

If a person having a high BMI has high fat, that falls in the obesity category. According to BMI, it is overweight but has healthy levels of fat. For muscular persons like athletes and bodybuilders, height and weight measurements may not indicate health because they have more muscle weight than fat. They have a BMI in a high range.

BMI is not accurate in some stages!

BMI cannot indicate the state of a person's health at some stages like

  • Pregnancy and breastfeeding
  • Childhood and adolescence because a person is still growing.

The calculations of BMI are different for children and teens.

Accurate BMI

According to the National Heart, Lung and Blood Institue, an assessment of weight and health risks should incorporate three measures:

  • BMI
  • Waist circumference
  • Risk factors and condition of obesity

Waist circumference

If a person has fat around the waist rather than the hips, they have a risk of heart disease and type 2 diabetes. This risk may cause if they have a waist size greater than 35 inches for non-pregnant women and greater than 40 inches for men.

Risk factors for obesity conditions!

The following issues increase the risk of developing heart disease:

  • Hypertension
  • High level of low-density lipoprotein
  • Low level of high-density lipoprotein
  • High level of triglycerides
  • Physical inactivity

A person can lose weight,

  • If they have a BMI of 30 or greater Or,
  • If they have a BMI of 25-29.9 plus two or more risk factors

The formula for BMI calculation!

The formula for calculating BMI in the International System of Units (SI) and the US customary system are:

USC Units:

BMI = 703 x mass (lbs) / height 2 (in)

SI, Metric Units:

BMI = mass (kg) / height2 (m)

How to calculate BMI?

The following steps are useful for calculating your BMI:

  • Multiply your weight in pounds by 703
  • Divide the answer by height in inches
  • Divide the answer by height in inches again.


A person weighs 180 lbs and is 5 feet and 5 inches tall (65 inches total). Calculate their BMI.

By applying the above three steps, you can get the final result:

  • 180 x 703 – 126, 540
  • 126, 540 / 65 = 1,946.769
  • 1,946.769 / 65 = 29.95

The final result of BMI would be 29.9

Ponderal Index!

The Ponderal index is also called the Corpulence index. It was first used to measure a body composition proposed by Rohrer, a Swiss physician. It is the improvement over BMI.

Levels of Ponderal Index

Ponderal Index 8 – 11 = underweight

Ponderal Index 11 – 15 = normal weight

Ponderal Index 15 – 17 = overweight

Ponderal Index > 17 = obese

For adults, the value of PI between 11 and 15 is considered a normal weight.

A value of between 2.2 and 3 is considered normal for infants and children.

Limitations of Ponderal Index

    • Ponderal Index is an indicator, and it is not an absolute measurement
    • It does not account for bodybuilders with more muscle
    • It does not differentiate between body types with different fat dispositions.

The formula of the Ponderal Index

USC Units:

PI = height (in) / 3mass lbs

SI, Metric Units:

PI = mass (kg) / height3 (m)


BMI is not an accurate measure of health. It is useful for important conditions that become likely when a person is overweight. BMI is one of the quick tools for many healthcare providers to assess risk for certain health conditions. It's always good to check your BMI and recognize its limitations.


Q: What is BMI?

A: Body Mass Index (BMI) is the body mass divided by the square of the body height.

Q: What is the unit of BMI?

A: The unit of BMI is kg/m2

Q: What is a healthy BMI?

A: The optimum range of healthy BMI is 18.5 to 24.9.

Q: Does the BMI scale work for athletic people?

A: BMI is not useful for bodybuilders and athletes as they build high muscle mass.

Q: What is a BMI calculator?

A: It is an online or offline device that measures your body mass index.