Body Fat Percentage vs. Body Mass Index (BMI)

Body Fat Percentage vs. Body Mass Index (BMI)

Body Composition

Body weight gives us a very broad picture of diet and fitness progress. If we want more specific measures of how well our exercise routines and diets are working, we turn to other gauges, specifically body fat percentage and body mass index (BMI). These tools provide a more in depth analysis of the exercise we perform, amount of calories we eat and whether or not we are meeting our goals. Body fat percentage and BMI provide similar, but different pictures of our overall health and fitness & diet progress.

What is Body Fat Percentage?

Body fat percentage measures how much of total body weight is fat. If a 200 pound person is measured at 20% body fat, 20% of the 200 pounds is made up of fat. Since 20% of 200 is 40, this person is carrying 40 pounds of body fat.

Body fat is important for two main reasons: health and looks. Body fat percentage is a good indicator of overall health. People with high body fat are at an increased risk of heart disease, strokes, type 2 diabetes and certain forms of cancer. High body fat is also associated with an unsightly appearance. Lowering body fat improves overall health and physical appearance.

How to Measure Body Fat

Accurately measuring body fat is difficult and sometimes expensive. Typically, gyms and health care providers have specialized tools (skin folds, hydrostatic [underwater] weighing and Dual-energy X-ray absorptiometry [DEXA or DXA]) to provide relatively accurate body fat measurements.

There are easier and cheaper solutions to measure body fat at home, specifically bioelectrical impedance. This tool is available as an extra option on body weight scales or in a hand held unit. Though cheap, quick and easy to use, this method of body fat measurement is off by 3-5%.

When using any method to measure body fat (at home tools or otherwise), you should pay more attention to the trend of body fat over a long period of time rather than daily measurements. If the number goes down, you are moving in the right direction.

Pros and Cons of Body Fat

Getting a consistent reading of body fat over a long period of time is important because during weight loss, you want to lose as much fat as possible while preserving as much muscle mass as possible.

The problem with measuring body fat is it’s difficult to get an accurate assessment. When measuring body fat, the long term trend is more important than daily fluctuations. Going from 15% to 10% means you’re losing a significant amount of body fat but it doesn’t necessarily mean 15% and 10% are completely accurate.

Healthy and Unhealthy Body Fat Ranges

Essential fat refers to the amount of fat the body needs to function properly. Avoid going below this number. The ideal range for most people lies within the fitness category.

Classification Females Males
Essential Fat 10-13% 2-5%
Athletes 14-20% 6-13%
Fitness 21-24% 14-17%
Average 25-31% 18-24%
Obese 32% and up 25% and up

Source: ACE Fitness

What is Body Mass Index (BMI)?

Body fat is a very good indicator of overall health. People with a high body fat percentage are at a higher risk of developing health problems than those with a low body fat percentage. Unfortunately, it is difficult and time consuming to measure body fat.

Body Mass Index (BMI) was developed to indirectly measure body fat without the use of sophisticated, expensive and time consuming tools. BMI is a simple ratio between weight and height. See the BMI formula below.

When you plug values into the BMI equation, you’ll get a number typically between 18 and 30 (though it could be lower or higher). This number corresponds with a certain range (underweight, normal, overweight and obese) which provides a risk assessment. Those with higher BMIs are at an increased risk of developing the same health problems as those with high body fat.

How to Measure BMI

BMI is extremely easy to measure. All you need to know is body weight and height. With this easy to find information and a few minutes of basic math, you can calculate BMI.

BMI = weight in pounds / (height in inches^2) x 703 (or use the BMI Calculator)

Pros and Cons of BMI

BMI was developed as a quick and easy to way asses health risk. It is an indirect way to measure body fat and has a very strong correlation to it. Someone with a high BMI is very likely to also have a high body fat percentage. BMI attempts to predict body fat.

Since BMI indirectly estimates body fat, it doesn’t work for everyone. BMI only looks at total body weight and does not differentiate between muscle and fat. BMI overestimates body fat in athletes who have a low body fat percentage and a high amount of muscle mass. If you are an athlete, BMI might not give you an accurate health assessment.

The National Heart, Lung and Blood Institute recommends measuring waist circumference to ensure BMI provides an accurate assessment. Waist circumference should be under 40 inches for males and under 35 inches for females. From the NHLB website, “To correctly measure your waist, stand and place a tape measure around your middle, just above your hipbones. Measure your waist just after you breathe out.”

If you are still unsure whether BMI is accurate, the CDC recommends using other measurements associated with health risks such as measuring blood pressure, fasting glucose levels and physical inactivity. For the vast majority of the population (everyone except for athletes), BMI is a great indicator of overall health and ideal weight.

Healthy and Unhealthy BMI Ranges

Classification BMI
Underweight Below 18.5
Normal 18.5-24.9
Overweight 25.0-29.9
Obese 30.0 and Above

The Bottom Line

Body fat percentage and BMI are both good indicators of overall health and an individual’s risk of developing heart disease, type 2 diabetes, strokes and certain forms of cancer. Body fat is a much more accurate gauge of overall health but for those who lack the tools or time, BMI also paints a very clear picture.

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