BMI vs. Body Fat – Slim Trim Shape

Measuring your BMI and body fat are two ways to measure your fitness level and current risk for certain health conditions. Both the BMI and body fat measurements can tell you a lot about the condition of your body. Because using the BMI or the body fat percentage have their strengths and weaknesses, using both ways of measuring your body together will give you a more accurate picture of your health.

How Should I Take My Measurements?

Our bodies fluctuate throughout the day. Taking measurements after we wake up and urinate will exclude weight from our food and fluid intake. Removing layers of clothing which may have varying widths and weights will help make your measurements accurate.

If using a weighing scale, try to keep the scale in the same spot everyday. Choose a spot that has a hard surface for more accurate measurements. Place your scale on the tile, not the carpet.

If using a measuring tape, use one that is soft but not elastic. While measuring, make sure that the tape wraps around your body part but does not constrict it.

It is normal for our weight to fluctuate throughout the day for many reasons, and measurements may vary day to day as well. Salt and carbohydrate intake can cause water retention. Food that still sits in our bodies may cause extra weight. Exercise can cause a loss of water weight. Menstruation can cause changes in weight. Medication and alcohol may cause changes in weight as well.

What is BMI?

Body Mass Index (BMI) is a measure of body fat based on your height, which determines the effect of your weight on your health. Your BMI measurement can tell you your risk for certain diseases. An increase in BMI can increase your risk in developing heart diseases, diabetes, other weight-related diseases such as strokes and cancers, and premature death.

The BMI is calculated as follows:

BMI = (weight in kilograms)(heights in meters)2

The results can be interpreted as follows:

  • Underweight: BMI of less than 18.5
  • Normal Weight: BMI of 18.6 to 24.9
  • Overweight: BMI of 25.0 to 29.9
  • Obese: BMI of 30 or more

The BMI is not a good tool for distinguishing muscle from fat. A bodybuilder with a lot of muscles may have the same high BMI as a sedentary person with a high amount of body fat. The body mass index can be interpreted as a recommendation for what an appropriate weight range is for your height. It is not a hard standard.

What is Body Fat?

Unlike BMI, body fat percentage can distinguish muscle from fat. Body fat percentage produces a more accurate number. Body fat percentage calculates the percent of fat in the body. It is important to know how much fat your body has. Too much fat can affect how your organs function. Generally, the body fat percentage measures the weight of your fat divided by your total weight.

There are many ways to measure body fat:

  • Calipers – A Caliper measures the distance between two ends. It is used to measure the thickness of your fat in various parts of your body such as your arms, chest, thighs, and waist.
  • Dexa Scan – Dual energy X-ray absorptiometry is a body fat percentage test where an x-ray of your body is taken in a medical facility.
  • Formulas – There are a handful of formulas you can use to measure your body fat percentage.
  • Underwater Density Test – The Underwater Density Test is a method of measuring your body fat where you submerge your body in water.
  • Weighing Scales – Certain weighing scales can measure body fat percentages.

Here are the standard ranges of body fat percentages for adults:


  • Essential Fat: 10 to 12 percent
  • Athletes: 14 to 20 percent
  • Fitness: 21 to 24 percent
  • Acceptable: 25 to 31 percent
  • Obese: 32 percent and above


  • Essential Fat: 2 to 4 percent
  • Athletes: 6 to 13 percent
  • Fitness: 14 to 17 percent
  • Acceptable: 18 to 25 percent
  • Obese: 26 percent and above

Essential fat is the minimum percentage of fat that the body requires to function properly without any impediments. Body fat percentages below the recommended essential fat range can be detrimental to one’s health and can lead to organ failure, as the body needs fat to survive. Women have a higher range of essential fat than men for reproductive purposes.

Comparing and Contrasting BMI and Body Fat: When Should These Be Used?

BMI vs. body fat: The BMI and body fat measurements both have their pros and cons. Each measurement has their own purpose and interpretation.


The BMI measurement will not be capable of telling you how much fat and muscle you have. However, measuring your body fat will tell you how much fat you have.

Ease of Use

The BMI is good for getting a cheap, quick, and easy estimate of your risk for weight-related diseases. Because there are many ways of measuring body fat, the results are not all equal. While the most accurate methods of finding out how much body fat you have are not very accessible, the most common methods of finding out body fat percentage such as the skin fold measurement and the Bioelectrical Impedance (BEI) analysis have different levels of accuracies and rates of measurement errors.

BMI and Body Fat

Relying on BMI measurements alone will give you an incomplete picture of your health status. While the BMI and body fat measurements may not be correlated all the time, taking both measurements will help better inform you of where you are in terms of your progress and health risks.

The accuracy of both the BMI and body fat measurements depend on several factors. Errors in measurement can occur all the time. For instance, different tools may give you different measurements, and different people using the same tool may give you different measurements. When relying on these numbers, remember that they may not be accurate all the time. When using the BMI and body fat percentage to measure yourself, try to have your doctor or healthcare provider take these measurements. Remember that since our bodies fluctuate, it is better to rely on general trends of both BMI and body fat percentage measurements than to obsess over daily fluctuations.