BMI vs Body Fat: What’s the Difference and Which is More Important?

There are many measures available to budding athletes and their trainers, showing everything from heart health to lung capacity to maximal load capabilities. Biometrics are some of the more commonly measured aspects of gym goers’ lives, with few so focused upon as body composition- the ratio, generally speaking, of lean body mass (muscle tissue, bones, internal organs and so on) to fat.

Whether you’re looking to lose, gain or maintain your current weight, knowing what your current weight actually means is important.

With this in mind, we’re going to dive into two of the most common ways of measuring body composition health: BMI and body fat analysis. We’ll look at the relative merits and downsides of each, what each can be used for, how reliable each is as a measure of the state of your health, and which, if either, is better.


The body mass index (BMI) is a measure that uses your height and weight to work out if you are a healthy size. It is one of the quickest, easiest facets of your own personal biometrics that it’s possible to find. There is no specialist knowledge or equipment needed to work it out.

Your BMI is found by dividing your weight in kilograms by your height in metres squared. The resulting number will generally fall between about 15-40. It will show whether you are underweight, a healthy weight, overweight or obese.

BMI ranges

For the majority of adults, a healthy BMI range will be between 18.5 to 24.9. Though not always the case, this is generally so in individuals with a normal musculature and build.

BMI ranges are:

  • Below 18.5: underweight 
  • Between 18.5 and 24.9: healthy weight
  • Between 25 and 29.9: overweight
  • Between 30 and 39.9: obese
  • Over 40: severely obese

So, for instance, let’s take a 25-year-old woman with normal musculature who weighs 62 kg at 1.65 metres. Her BMI would be:

62/ (1.65×1.65) = 22

She is a healthy weight. If she were to then put on 10 kg in body fat, her new BMI would be:

72/ (1.65×1.65) = 26.44

She is now overweight, though only just. She would need to lose a couple of kilograms to fall back to a healthy weight. On the other hand, if she were to lose 25 kilograms, her new BMI would be:

47/ (1.65×1.65) = 17.26

She is now underweight, though again only just. She would need to put on a couple of kilograms to know that she was a healthy weight once more.

As we can see from this, BMI measurements allow for quite a broad range of weights. The woman in our example could afford to yo-yo over a range of 25 kilograms whilst only just slipping outside of a healthy weight range. She will therefore know that she has quite a lot of wriggle room with regards her weight: she can afford to lose or gain weight whilst still knowing she is healthy. Then, when she gains or loses too much, she can easily see that she has become unhealthy and can adjust her diet and lifestyle.

This is the main value of using BMI as a measure of health. In giving an absolute reading that can be applied across the board in this way, BMI measurements supply very useful data from which healthcare and fitness professionals can take their cues. It’s both quite forgiving and very easy to use, it gives a hard number that shows you quite clearly where you are with regards your weight, and it makes manipulating your weight to keep yourself as healthy as possible very simple.

Accuracy of BMI measurements

These show that there are clearly some great benefits to the BMI formula.

It takes the natural variations in peoples’ body types and lifestyles into account, allowing a healthy weight range according to their height.

With this in mind, however, there are several other factors that need to be taken into account when deciding how overweight individuals actually are. It isn’t as cut-and-dried as the above scenario might have you believe. We purposefully used an untrained individual with a normal level of musculature in our example.

For many people, a BMI reading may not prove an accurate testament to their body fat levels.

Muscle is far denser than fat. Very muscular people, such as bodybuilders and certain athletes, may be classed as obese by their BMI measurements even though they have healthy body fat levels. The extra weight they are carrying in the form of muscle skews the measurements.

Let’s take our 25-year-old woman from the above example. Let’s say she begins at 62 kg which, as we have seen, gives her a BMI of 22. This is perfectly healthy. Now let’s say she begins an intense bodybuilding program, maintaining a caloric surplus containing plenty of protein for hypertrophy, with the goal of putting on a lot of lean mass.

Within a year, she has put on 10 kg of muscle, without gaining any fat. She is now 72 kg which, according to her new BMI of 26.44, means she is technically overweight. Yet her body fat levels haven’t changed. If anything, she is far healthier and fitter than she was before. She does not need to lose any body fat- nor, probably, should she- to be healthy.

People with above average levels of muscle mass should therefore be cautious when working out their BMI. It will no doubt be a fair amount higher than their body fat levels might suggest. If you tend to lift weights and engage in any lifestyle factors that mean you have greater musculature than average, your BMI most likely is not a good way to measure the health of your body composition.

So, should we use BMI?

Yes, in short. BMI is a fantastic rule of thumb.

Most people are not overly muscular athletes. BMI will therefore be perfect for the majority of the population. Untrained individuals, endurance athletes and gym-goers not focussed on hypertrophy will all benefit from knowing where they place on the scale.

Doctors will benefit from being able to tell their patients, objectively, how healthy their weight is. Healthcare providers can tell their patients who need to put weight on that they are underweight, objectively signalling to those with eating disorders such as anorexia or bulimia that their bodies are undernourished. Similarly, they can tell their patients who are overweight or obese that they need to lose weight, with objective data to cite.

If you are an overly muscled athlete, the chances are you probably have access to either kit or personnel who can use different, more complex, more industry specific means of body fat testing-which we will go into now.

Lean Body Mass Calculation

Lean Body Mass Calculation

It can be very useful to know your lean body mass. It’s a great indicator of overall health, adding some much-needed detail to more conventional measures of weight like BMI. Like BMI, it can help you to see how healthy you are at your current weight and, thus, will influence what your health and fitness goals should be.

It can also help you keep those goals on track and healthy. Knowing your lean body mass can help those looking to lose weight maintain their weight loss purely from fat. If you can keep your lean body mass stable whilst bringing your overall weight down, you will know that you’re only losing fat- not muscle or bone density, as is often the case.

On the other hand, if you’re looking to put on weight in the form of muscle, you will want to build your lean mass at the same rate as your overall weight. This will make sure that you are not putting on any extra bodyfat as you watch the scales go upwards.

But what is lean body mass and how do you work it out?

Lean Body Mass

For this, we will usually need to know your body fat percentage (though we will run through some easy-to-use, accessible yet inaccurate, methods below in which this isn’t the case).

Lean body mass is your overall weight minus your weight from bodyfat. Basically, if you subtract the weight that comes from fat (your body fat percentage) from your total weight, you will have your lean body mass.

Calculating Lean Body Mass

There are two main ways in which we can calculate your lean body mass. There is a trade-off, here, between accessibility and accuracy. As with BMI, the easy-to-use calculations that we can all do are often inaccurate. The most accurate methods are often beyond many of us, unless we have access to specialised equipment.

The two methods are a simple calculation based on weight and height. The second involves several ways in which to get the aforementioned body fat measurement.

1. Using your weight and height

Though crude (indeed, arguably cruder than using BMI, because it purports to be more accurate), the following formulae are incredibly accessible for most people. Use the equations below, with all weights in kg and all heights in cm.

  • For men: Lean body mass = (0.32810 × W) + (0.33929 × H) − 29.5336
  • For women: Lean body mass = (0.29569 × W) + (0.41813 × H) − 43.2933

For an easier version of this, you can always look up an online calculator for lean body mass that will work it out for you based on the same measurements.

This is not accurate, but may work well as a best guess or good rule-of-thumb as you try to decide your health and fitness goals.

2. Using your body fat percentage

Body fat testing is the best way to find out how healthy your body composition is and what your lean-to-fat body mass ratio is.

Find out your body fat percentage using one of the techniques listed below. Divide this number by 100 to make it a decimal. So, if somebody’s body fat percentage is 25, we are looking at 25/100 = 0.25. They then need to multiply this by their total weight.

Going with the above, we can take an 80kg man with 25% body fat. We use 25/100 = 0.25 to find the decimal. We multiply this by 80, giving us:

80 x 0.25 (the same as 80/4, in effect) = 20.

His body fat accounts for 20 kg, his lean weight for 60. This is quite overweight: most clinicians and trainers would advise him to bring his body fat down to more like 15% or so. His aim would be to remain at 60kg of lean mass whilst bringing his total mass down.

This is all well and good if we can find out our body fat percentages. However, it’s not so easy. There is always a trade-off between accessibility and accuracy/ efficacy. The more accurate a method, generally the less accessible it will be to most people.

Methods of finding out your body fat percentage include:

Bioelectrical impedance scale

Your local gym may offer this facility. A bioelectrical scale is a set of scales with electrodes that you stand on, and/or grip in your hands, as they send a gentle electrical current through your body. As muscle and fat conduct electricity differently, they will tell you everything you need to know. They are safe, though not always accurate (other factors, like hydration, can sometimes skew readings).

Skin fold assessment

Use skin fold callipers on a range of points on your body. You will pinch skin and measure the thickness, then simply put these numbers into a conversion table or calculator for lean body mass.

Many personal trainers at your local gym will be well-practiced in this technique. It’s not the most accurate method but is easy and relatively reliable and consistent.

Hydrostatic weighing

This is where we get very accurate and very inaccessible. Hydrostatic weighing involves a comparison between your weight on land to your weight while completely submerged in water. A technician can then calculate your body fat percentage.

As before, it’s very reliable: however, it involves large amounts of specialist kit, will not be available at your local gym, and can cost a fair amount of money.

Using the most accurate method available to you for finding your body fat percentage will be the best way of finding your lean weight and gauging how healthy your current body composition is. It will be the most accurate way of measuring your progress as you try to gain or lose weight, ensuring that you can guarantee maintaining your lean body mass whilst losing body fat. For trained, muscular individuals, it will be far more reliable as a yardstick than simple BMI measurements.

However, there is a third way. It is open to the same vagaries as BMI testing- it is accessible yet crude- and will not give you the same kind of reliable data as a body fat calculation. However, if you know that you have regular musculature and want to know how much you should ideally weigh given your height and gender, it may well be another good tool to have in your arsenal.

Ideal Body Weight Calculation

Ideal Body Weight Calculation

Ideal Body Weight (IBW) calculation was originally designed for use in a medical, pharmaceutical setting, with applications of the technique mostly relating to dosage estimates for common drugs.

Indeed, these days IBW is most commonly used for both working out drug dosage, with metabolism of many drugs aligning more closely with IBW than with total body weight, and for weight classification in many sports.

IBW is far from a perfect measurement. In fact, the further somebody is from what might be considered a normal body shape, the less accurate it becomes. Again, as with BMI measurements, we are generally talking about highly muscled individuals. As with BMI, IBW doesn’t distinguish between lean body weight and body fat, so that somebody with low body fat levels and a high lean body mass can appear overweight when figuring out their IBW.

You should always consider IBW in this light, as you should BMI. The simple fact of being over or under your healthy IBW range needn’t mean that anything is amiss with your weight, especially if you have above average muscle mass. It is a very inexact science for some athletes.

So, why are we including it here? We’ve already discovered that body fat calculations are far more reliable, especially for muscular individuals. The answer is the same as with BMI: IBW can be a great yardstick, easy to use and accessible to everybody. It is good for those just starting out at the gym to work out where they want their weight to be. Where BMI tells you where you are now, IBW can be used to form a rough idea of where your fitness journey needs to take you.

If you are over it, you need to lose weight. Broadly speaking. There is no measure or formula that can tell somebody what their exact, healthiest weight is or should be. But, if you’re writing out your goals, knowing a rough idea can be incredibly valuable.

If you are under your IBW, you need to gain weight.

Formulas for Finding Your IBW

There are several formulae that are in use, or have been in use at various times, for estimating your ideal body weight. All of the main ones are listed below: these can all be used to find a rough approximation of your IBW, taking your height and weight into account.

Of all those below, the B. J. Devine formula is currently the most widely used formula for the measurement of IBW.

The formulae are:

B. J. Devine Formula (1974)

Male: 50.0 kg + 2.3 kg per inch over 5 feet
Female: 45.5 kg + 2.3 kg per inch over 5 feet

Invented for medicinal dosage purposes.

G. J. Hamwi Formula (1964)

Male: 48.0 kg + 2.7 kg per inch over 5 feet
Female: 45.5 kg + 2.2 kg per inch over 5 feet

Similar to the Devine Formula, it was originally intended as a basis for medicinal dosages based on weight and height. Over time, the formula became a universal determinant of IBW.

J. D. Robinson Formula (1983)

Male: 52 kg + 1.9 kg per inch over 5 feet
Female: 49 kg + 1.7 kg per inch over 5 feet

Modification of the Devine Formula.

D. R. Miller Formula (1983)

Male: 56.2 kg + 1.41 kg per inch over 5 feet
Female: 53.1 kg + 1.36 kg per inch over 5 feet

Modification of the Devine Formula.

Using these body weight to height conversion formulae will help you to see where your healthy bodyweight should be. Use it instead of, or in conjunction with, a BMI calculator. Between them, you will be able to gauge the gap between your current weight levels and where you need your health and fitness journey to take you.

If, however, you have access to the right facilities and/or are a heavily muscled athlete, use body fat measurements. They will give you data that tell a far more realistic story.

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Mike Julom, ACE CPT

Mike is an ACE Certified PT and a CrossFit Level 1 Trainer. He is an avid lover of all sports. Basketball, tennis, athletics, volleyball, soccer, squash, golf, table tennis, even darts, you name it! He's a very active CrossFit athlete and has been WOD'ing for over 7 years. With such an intense fitness regime, Mike has learned to take care of his body physically, nutritionally, and spiritually. Mike founded ThisIsWhyImFit as a way to share his vast knowledge of exercises, diets, and general fitness advice.

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