Difference Between BMI and Body Composition

BMI and body composition are two methods of measuring a person's physical health and fitness level. This helps design proper diet and exercise plans suited to the individual.

By: Nina Teicholz, Published on: 2023-12-29, Last Updated: 03-09-24

Reviewed by: Emily Morse

Table of Contents


Body mass index (BMI) and body composition are two methods of measuring a person's physical health and fitness level. Both provide useful but different types of information.

BMI is a simple calculation using a person's height and weight. It provides a rough estimate of body fatness. On the other hand, body composition is a more detailed measurement of the different components that make up a person's body - fat mass, lean mass, bone mass, and water. Assessing body composition provides more specific information related to health risks compared to BMI.

The main purpose of using BMI is that it is fast and easy to calculate and screen for weight categories that may lead to health issues. However, BMI has limitations in distinguishing between fat and muscle and determining the exact body fat percentage.

Measuring body composition more accurately diagnoses high body fat, low muscle mass, and improper fat distribution. This helps design proper diet and exercise plans suited to the individual. It also tracks changes in fat and lean mass during weight-loss programs where you need to track calories. Information from body composition analyses can determine health risks like obesity, nutrient deficiencies, osteoporosis, etc.

What is BMI and How is it Calculated?

BMI stands for Body Mass Index. It is a simple calculation using a person's weight and height. BMI is used as a screening tool to indicate if a person is underweight, overweight, obese, or within a healthy weight range.

To calculate BMI, a person’s body weight in kilograms is divided by the square of their height in meters (kg/m2). BMI categories are:

  • Below 18.5 is considered: Underweight
  • Between 18.5 and 24.9: Healthy weight
  • Between 25 and 29.9: Overweight
  • 30 and above: Obesity

BMI provides a rough estimate of body fat and the health risks associated with different weight categories. It is easy to measure and calculate at home or in clinics. Due to this, BMI is useful for initial assessments of body weight.

However, BMI has limitations in determining body composition -

  • It does not differentiate between fat, muscle or bone mass
  • People with more muscle may fall into overweight or obese categories
  • It does not consider fat distribution patterns
  • Age, gender and ethnicity are not accounted for

Therefore, people with the same BMI can have very different levels of body fat and muscle composition. Relying solely on BMI can incorrectly classify a level of body fatness.

Though easy to use, BMI should be supplemented with an evaluation of body composition rather than the only method for health assessment.

Components of Body Composition

Body composition refers to the percentage of fat, muscle, bone, and water that make up a person's body weight. Assessing body composition gives more specific information about health and fitness than looking at weight or BMI alone.

The main components that are measured for body composition are:

Fat Mass - This refers to essential body fat plus storage body fat. High amounts of excess body fat can increase the risk of conditions like heart disease.

Lean Mass - This includes muscle, bone, vital organs, and water. Higher lean mass indicates more muscle strength and better fitness.

Water - Water percentage can impact many functions. Too little water leads to dehydration, whereas too much water or fluid retention also causes problems.

Bone Mass - Bone density screening detects osteoporosis and the risk of fractures. Higher bone mass supports strength.

Some common methods used to analyze body composition are skinfold measurements, bioelectrical impedance scales, dual energy x-ray scans, and water immersion tests. Which is discussed in a previous blog, "The 7 Methods To Measure Body Composition".

Tracking detailed body composition over time provides better insight compared to only tracking body weight. It shows the effectiveness of diet, nutrition, and exercise programs more accurately.

Key Differences Between BMI and Body Composition

BMI and body composition both aim to evaluate a person's health and fitness. However, there are important distinctions between what the two measures indicate:



Body Composition

What it measures

Total body weight and height

Proportion of fat, muscle, bone, and water

How it measures

Formula using weight and height

Skinfold calipers, DEXA scans, and bioimpedance analysis

 Differentiates fat, muscle, and bone



Consider fat distribution.


Yes (visceral vs. subcutaneous)

Accuracy for athletes?

Less accurate, can classify muscular individuals overweight

More accurate, reflects muscle mass

Provides detailed information


Fat percentage, location, lean mass, water, and bone density

Helpful for

General health assessment, identifying weight categories

Customized diet and training plans, specific health risk assessment


In conclusion, both BMI and body composition measurements provide useful but different health and fitness information.

BMI is an easy, low-cost method that quickly estimates general weight categories and body fatness levels. Due to convenience and cost, BMI serves well as an initial screening tool for weight problems within large populations. It gives a rough indicator to direct people towards more comprehensive assessments if needed.

However, BMI has major limitations when used as the only measure or for tracking individual progress over time. It does not distinguish between fat, muscle, or water weight, and people with lean, muscular builds can get wrongly classified. The calculation also does not account for fat distribution patterns and associated health risks.

In most cases, a combination of initial BMI screening and periodic body composition assessment provides the best health evaluation for both populations and individuals. This balanced approach makes tracking weight goals efficient yet more effective.

Frequently Asked Questions

Why is BMI inaccurate for some people?

BMI doesn't account for muscle mass, bone density, or body fat distribution. Athletes and individuals with naturally high bone density may have a high BMI, even if they have low body fat.

What is the main difference between BMI and body composition?

The main difference is that BMI only uses weight and height to estimate body fat, while body composition actually measures the percentage of fat, muscle, water, and bone in your body.

Can you manipulate BMI or body composition readings?

BMI can sometimes be manipulated by extreme muscle building or rapid pre-test dehydration, leading to false readings. But with the right protocols, both BMI and body composition tests are quite accurate.

Can you have a high BMI but still be healthy?

Yes, people with a lot of muscle mass can have BMIs in the overweight or even obese range but still have healthy, low-body fat. BMI cannot distinguish lean mass from fat mass.

Is it possible to have a normal BMI but high body fat?

Yes, it's very possible to have a BMI in the normal or even "healthy" weight range while still having excess levels of body fat. Muscle mass can offset body fat in the BMI calculation.

At what point is excess body fat considered obese?

For men, having more than 25% body fat is considered obese, for women, it's over 32%. These levels of fat mass substantially raise the risk of various health issues. Bringing body fat% down is key.

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