What is Body Mass Index (BMI)? How to Calculate It

Quetelet Index, also known as Body Mass Index (BMI), is a measurement that is calculated using an individual’s mass (weight) and height. The BMI provides an accurate indicator of body fatness for most people, as it is proportional to the mass and does not depend on gender or ethnicity. However, it may not be as accurate in overweight individuals and athletes. 

The formula for calculating body mass index is BMI = kg/m2, where kg represents weight in kilograms and m2 represents the height in meters. Their BMI primarily measures people 18 to 65. BMI over 25.0 is considered overweight, while a healthy BMI is between 18.5 and 24.9.

Understanding BMI & Its Calculation Method

Body Mass Index, or BMI, refers to the amount of body fat a person has based on their height and weight between the ages of 18 and 65. BMI does not measure body fat directly, but it is a reliable indicator of body fatness. 

Having a high BMI can indicate having a high body fat percentage. A person with a high BMI may have too much fat compared to lean muscle. 

To calculate your BMI: divide your weight in pounds by your height in inches squared and then multiply by 703. 

The BMI calculator uses these formulas for BMI calculations:

BMI = weight ÷ height2

  • In US units: BMI = weight (lb) ÷ height2 (inches) * 703
  • In metric units: BMI = weight (kg) ÷ height2 (meters)

For example, a woman who weighs 160 pounds and is 5 feet 4 inches tall would divide 160 by 362, which equals 0.48. She would then multiply 0.48 times 703 and get 297.04 for her BMI. 

Another way to look at this calculation is using metric units where she would divide 120 by 1.63, which equals 68.91. She would then multiply 68.91 times 10,000 and get 6891 as her BMI in metric units. 

Both ways of calculating result in a number that provides information about whether a person’s weight is appropriate given his or her height or overweight/underweight. 

Why Is BMI Important?

A body mass index (BMI) measures body fat and is based on height and weight. It can be used by both men and women. BMI is an essential tool because it can help you determine whether you are at a healthy weight, overweight, or obese. 

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A healthy range for BMI is 18.5-24.9; people whose BMIs are less than 18.5 are considered underweight, while those with BMIs over 24.9 are considered overweight; those with BMIs over 30 are considered obese. If your BMI is 40 or more, then you have a condition called morbid obesity. 

How Can You Know If You Are Overweight or Obese with BMI?

A high BMI indicates high body fatness. BMI does not measure body fat directly. 

However, research has shown that BMI is moderately correlated with other measurements of body fat, such as skinfold thickness measurements, bioelectrical impedance measurements, underwater weighing, dual-energy x-ray absorptiometry (DXA) and other methods. 

In general, BMI is a good predictor of the amount of body fat for most people. People with increased muscle mass due to heavy lifting or exercise will have a higher BMI because muscle weighs more than fat. 

Women usually have more body fat than men, so they are at greater risk of having excess weight in their midsection. As we age, we lose muscle mass, making it easier for our bodies to store excess calories as body fat. 

Some ethnic groups also may be more prone to obesity because of inherited genetic differences. 

Certain medical conditions may also cause obesity by interfering with metabolism or limiting physical activity, including some types of cancer, sleep apnea, chronic obstructive pulmonary disease (COPD), cystic fibrosis, and endocrine disorders. 

The Problems with BMI

BMI is a technique calculated from a person’s weight and height. For most people, it provides a reliable indicator of body fatness and is used to screen for health risks associated with certain weight categories. However, BMI has its limitations. 

For example, it may overestimate body fat in athletes and others with a muscular build. Additionally, it doesn’t directly measure body fat, so it may not be as accurate in people who have an average weight but carry excess fat around their waist, which can lead to health problems. 

These inaccuracies make it difficult to know whether an individual with a high BMI is genuinely overweight or obese. 

A better way to determine if someone is overweight or obese would be by looking at other indicators such as waist circumference, sagittal abdominal diameter, upper arm muscle area, and skinfold thickness. 

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All these factors combined provide a more detailed analysis of the individual’s current level of health. These factors are also helpful when determining the additional weight one should gain during pregnancy or maintaining weight after pregnancy. 

One method of measuring this is through the percentage of body fat and visceral fat. Visceral fat is the type that builds up deep inside our abdomen and around our organs. 

Abdominal visceral fat poses a greater risk than subcutaneous adipose tissue because it secretes hormones and proteins that can cause inflammation throughout the body, leading to diabetes, heart disease, cancer, etc.

The 7 most common mistakes people make when using BMI

Here are the 7 most common mistakes people make about BMI calculation.

  • BMI isn’t an accurate measure of health for everyone.
  • BMI doesn’t take into account muscle mass.
  • BMI doesn’t take into account body fat percentage.
  • BMI doesn’t consider where your body fat is distributed.
  • BMI doesn’t take into account age or gender. 
  • BMI can’t tell you how much the weight you’re carrying around is composed of muscle, fat, bone, water, etc. 
  • It’s possible to have a high BMI and still be healthy due to factors not measured by the scale

Tips to Avoid Overweight and Obesity While Dieting

The first step to managing weight is understanding your body mass index (BMI). BMI is a calculation that uses height and weight to estimate how much body fat you have. 

To avoid becoming overweight or obese, aim for a healthy weight by following these tips.

  • Eat five servings of fruits and vegetables each day. 
  • Cut down on sugar-sweetened beverages like soda, sports drinks, and fruit juice. 
  • Eat more fish than red meat. 
  • Get at least 30 minutes of physical activity each day. – Limit time spent watching TV to less than two hours per day. 
  • Do not smoke. Smoking causes heart disease, lung cancer, and other serious health problems.
  • Drink alcohol in moderation. Alcohol can increase the risk of high blood pressure, stroke, breast cancer, and other cancers.
  • Maintain a healthy weight throughout life. As we age, our muscle tissues decline in size, but our bone tissues continue to grow, so it’s essential to maintain a healthy weight as we age. 
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For people with metabolic disorders such as diabetes, thyroid disease, kidney disease, or chronic pain, it’s essential to monitor their weight because excess pounds may affect those conditions. 

The Bottom Line About BMI

The bottom line is that BMI is a quick and easy way to estimate whether someone is at a healthy weight. However, it has its limitations and isn’t always accurate. 

If you’re concerned about your weight, talk to your doctor or a registered dietitian for more personalized advice. Remember that even if you’re not overweight according to the BMI standards, you might still be heavily based on your goal weight. 


Here are answers to some frequently asked questions about BMI. 

Why Should You Know BMI?

Maintaining a healthy weight is essential for overall health and can reduce your risk for developing some health conditions, such as diabetes, heart disease, and high blood pressure. 

Plus, carrying excess weight can lead to joint pain and other issues. Knowing your BMI can help determine if you’re at a healthy weight or need to make some changes.

What Are BMI Measurement options?

The metric and imperial systems are the most common ways to measure BMI. To calculate BMI using the metric system, divide your weight in kilograms by your height in meters squared. For the imperial system, divide your weight in pounds by your height in inches squared, then multiply that number by 703.

When Does BMI matter?

An individual’s BMI, or body mass index, is calculated based on their height and weight. A BMI of 25 to 29.9 is considered overweight, while a BMI of 30 or above is considered obese. BMIs below 18.5 are considered underweight.

Who Needs To Be Aware Of Their BMI?

Just about everyone should have an idea of what their BMI is? Knowing your BMI can be helpful if you’re trying to lose weight, gain weight, or maintain your current weight. 

Taking your BMI into account can give you an indication of whether or not you need to adjust your diet and exercise habits.