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How to Calculate Body Fat Percentage and Why

A common question asked by newcomers to weight loss is “why do I need to calculate Body Fat Percentage?” (those who have even heard of Body Fat Percentage, that is.)

Well, as you’re about to see, it’s actually very important to the success of any weight loss program. In fact, Body Fat Percentage is THE most vitally important measurement to monitor if weight loss is your goal.

Here’s why . . .

Why You MUST Calculate Body Fat Percentage

One of the major reasons that ineffective weight loss programs and fad diets fail to work is that they cause your body to lose muscle as well as body fat. This is particularly so when you also neglect to do any appropriate exercise.

Research has shown that . . .

it’s possible for as little as half of the weight lost by someone on a poor weight loss program to be from fat loss.

“So what?” you ask?

Well, the number one rule of weight loss is that

you should ALWAYS focus on losing body fat, and ONLY body fat.

It’s important to do your very best to preserve muscle as much as possible.

When your body loses muscle, your Basal Metabolic Rate, or BMR, (otherwise simply referred to as your metabolism) slows down. This means your body starts burning fewer and fewer calories each day, so that it loses less and less weight each day.

This often continues until your weight loss stalls altogether. This is what’s widely known as Dieter’s Plateau, or Weight Loss Plateau.

The end result of all this is that you quickly gain back all of the weight you lost, and often more, since your metabolism has been damaged. And as if that weren’t bad enough, you generally gain back more fat than you lost, and less lean muscle than you lost.

A double whammy!

So then, how do you prevent this disaster from sabotaging your weight loss program?

Simple . . . you calculate Body Fat Percentage regularly (weekly is probably best). That will tell you exactly how much body fat you’re losing (or gaining) each week, and how much lean mass (most, if not all, of which will be muscle mass) you’re losing (or gaining).

You can then take the appropriate steps to adjust your program, if and when it’s necessary.

Why the Simple Bathroom Scales Are Virtually Useless

Let’s look at a few simple questions relating the weight loss.

Let’s assume you recently started a new weight loss program, and after 4 weeks you found that you’d lost 4kg of body weight. Most women would be pretty happy with their progress at that point.

However, how would you know . . .

  • how much of the 4kg is lost body fat, and how much is lost muscle?
  • whether or not you were headed straight toward Weight Loss Plateau?
  • how long your current rate of weight loss would continue?

Now let’s assume that you’d lost no weight at all after the first 2 weeks of your new program. Most women would be very disappointed with that result.

But how would you know . . .

  • whether or not your weight loss program was really working at all?
  • whether you’d actually lost say 1kg of body fat, and gained 1kg of muscle?
  • when you’d start to see a decrease in your bodyweight?

The answers to all these questions are the same . . . “you simply wouldn’t”.

This is the limitation of simple bathroom scales that makes them virtually useless on their own for monitoring your weight loss progress.

Measuring body weight alone can’t allow you to detect changes in your body fat – therefore the simple bathroom scales on their own are powerless to tell you what’s really going on in your body.

This is why women’s obsession with weighing themselves is totally unfounded.

How to Calculate Body Fat Percentage

OK then, so if the old familiar bathroom scales are out, what should you be using instead to monitor your weight loss?

Well, they’re not necessarily out – it’s just that they need a little help to be of any use to you.

The answer is to calculate body weight AND calculate Body Fat Percentage.

Now, there are a variety of ways in which you can measure or calculate Body Fat Percentage. So let’s take a look at some of the most common ones.

Height/Weight Tables

Maybe you’ve seen or even used these before. You look up your height and your weight on the table (as the name suggests!), find where the two readings cross, and presto . . . instant Body Fat Percentage!

If that sounds too easy to be true, you’re right.

Unfortunately these tables don’t really have any way to actually calculate Body Fat Percentage. It’s more of a guess than anything.

They make some gross assumptions about your physique, which means that if your muscle mass, bone mass, or some other component of your body is anything other than average, your result can be way off the mark.

So at the end of the day, not a consideration. Let’s move on.

Girth Measurements

The word girth is just a fancy term for circumference, or the measurement around some part of your body. So for example, you can take girth measurements around your waist, your hips, your thighs, your upper arms, and so on.

Although girth measurements are simple enough to do – you only need a plain tape measure – the process needed to calculate Body Fat Percentage from the results can get very messy. You need to plug them into complicated equations and tables, and . . .

Realistically? Not a great option.

And besides, once again they make assumptions about your physique so they can be quite unreliable, even at the best of times.

Next . . .

Skinfold Measurements

Unlike the above 2 options which basically just try to guess your Body Fat Percentage, taking skinfold measurements actually allows you to directly calculate Body Fat Percentage with a good degree of accuracy and repeatability.

This method involves measuring the thickness of pinches of skin at various sites around your body, using a device called skinfold calipers. You then plug the results into equations or tables to come up with a Body Fat Percentage value.

There are both advantages and disadvantages to skinfold measurements though. On the plus side . . .

  • They’re very accurate and reliable when done correctly.
  • It’s a suitable method to use in the privacy of your own home.
  • The skinfold calipers are light and portable.
  • The skinfold calipers can be reasonably inexpensive.

The disadvantages of skinfold measurements, however, are:

  • An experienced user of skinfold calipers is needed to get accurate results.
  • It’s not always easy to find the right spots on your body to pinch.
  • It’s hard to test yourself – you really need someone else measuring you.
  • It’s best top have the same person testing you each time for repeatability.
  • The better-quality skinfold calipers can be reasonably expensive.

Skinfold testing is used extensively by trainers and fitness professionals to calculate Body Fat Percentage because of its accuracy and reliability. Bear in mind though, that they’re very experienced in conducting this test.

Personally, I don’t really like it. I find it fiddly and time-consuming, and in my experience most overweight people find it embarrassing and invasive to have someone measuring pinches of their skin.

The Accu-Measure

While we’re on the topic of skinfold measurement, I should also tell you about a very cheap, simple-to-use skinfold caliper that’s available, called the Accu-Measure Personal Body Fat Tester. You can see it in the picture here.

To calculate Body Fat Percentage with this gadget you take just one skinfold measurement, and use it to look up the corresponding Body Fat Percentage for your age on a simple table. Very quick, very easy. And the way it’s designed makes it very easy to get repeatable results.

The main problem with the Accu-Measure is that it calculates your Body Fat Percentage in quite wide steps of about 2%. This makes it unsuitable for tracking changes in your Body Fat Percentage on a weekly or even monthly basis, since it just won’t move that fast. It should be quite OK for just taking measurements every now and then however.

Because the Accu-Measure only involves taking one skinfold reading, its accuracy would have to be questionable. Still, if your goal is simply to roughly calculate Body Fat Percentage changes over long periods of time it might be worth a try.

Bioelectrical Impedance Analysis

Bioelectrical Impedance Analysis, or BIA for short, is a technique that involves passing a very small electric current through your body via various electrodes attached to your skin. Don’t be alarmed though, it’s perfectly safe and totally painless.

The test basically measures how easily the current travels through the water that’s in your body’s muscle and fat tissue. The more muscle you have, the more water your body holds (muscle contains more water than fat does). And the more water in your body, the easier it is for the current to pass through it. The more fat, the more resistance to the current. Simple!

So once this electrical resistance is measured, you can then easily calculate Body Fat Percentage based upon your age, sex, height and weight.

BIA is one of the most convenient, exact and accessible methods available to calculate Body Fat Percentage. It does, however, require specialized equipment and a trained operator.

Well, up until recently, that is – since personal Body Fat Scales are now so affordable and freely available.

Body Fat Scales

Body Fat Scales look very much like your garden variety bathroom scales, except for the fact that they have several metal pads on the top, which act as electrodes. This is because they calculate Body Fat Percentage based on the BIA principle.

This device is without a doubt the quickest and most convenient method for tracking changes in your Body Fat Percentage from week to week as you’re losing weight.

Once you’ve programmed in your age, sex and height, you simply turn them on, step on, and read your Body Fat Percentage, body weight and your body’s water content – all in seconds.

Body Fat Scales aren’t necessarily as accurate as skinfold testing, professional BIA testing, or some of the even more accurate testing methods available. But their main advantages are that they’re affordable, quick and easy enough to use every day (even every hour if you want!) in your own home, and they read out your Body Fat Percentage in steps of 0.1%.

This makes them perfect for monitoring your progress each week while you’re losing weight. They’re great for detecting small changes in your Body Fat Percentage, which is exactly what you need.

Hydrostatic Weighing

Hydrostatic Weighing is a scientific term that simply means “to weigh underwater”. Yep, this means that to undergo this test, you need to be totally submerged in water (hopefully for a just a short time) and weighed.

Using your hydrostatic weight and your normal, in-air body weight, your total body density is calculated. And from that, a very accurate value for your Body Fat Percentage is calculated. Accuracy is normally within 2-3%, provided that you’re able to successfully expel all the air from your lungs for the test.

Hydrostatic Weighing is in fact considered one of the gold standard methods to calculate Body Fat Percentage because of its accuracy.

This test is really only used by individuals who need to know their Body Fat Percentage very accurately. As you can appreciate, it’s a pretty extreme test and it does have some downsides. It can be expensive and logistically involved, and it can also be quite hard to find places that have a Hydrostatic Weighing tank.

Bod Pod

A Bod Pod is an enclosed testing apparatus that resembles a giant egg. It’s basically designed to measure your body’s volume, or the amount of space you occupy, whilst sitting inside. The Bod Pod is made up of two separate chambers – one in which you sit and a reference chamber. A diaphragm between the chambers oscillates during the test and the pressure effect it causes is used to measure the volume of air in the test chamber. When this is subtracted from the volume of the unoccupied test chamber, the result is your body volume.

The Bod Pod test is very similar to Hydrostatic Weighing in terms of how it calculates your body density. The only difference is that it uses your body volume and body weight (yes, they need to weigh you as well as part of the test), whereas in Hydrostatic Weighing uses your body weight in air and your body weight in water.

It’s been shown that Bod Pod testing can achieve a similar level of accuracy as Hydrostatic Weighing in its ability to calculate body fat percentage, as long as your wear a tight-fitting swimsuit and cap for the test. And it has an added advantage – you get to keep dry!

Dual Energy X-Ray Absorptiometry

Wow, quite a mouthful huh! Fortunately for us, it’s also referred to simply as DXA (formerly DEXA).

A DXA scan is simply an X-ray type scan (as the name suggests) that very quickly and very accurately measures your body density. And as for Hydrostatic Weighing, that’s then used to calculate Body Fat Percentage.

This process is fast challenging the gold standard status of Hydrostatic Weighing because of its high level of accuracy.

The only major drawback to DXA is the fact that it requires very specialized equipment and a trained operator (as does all medical X-ray equipment). That means that while it’s not overly expensive to have done (around about $60-$100 here in Australia), it’s not a test you would do on a regular basis.

On the plus side however, DXA scans are far less prone to operator error or variation between operators.

And the Winner is?

OK then, so with all these options available to calculate Body Fat Percentage, which is the best way forward?

Well, what I recommend to women and certainly what I do myself is to use a good quality set of Body Fat Scales to calculate your Body Fat Percentage each week. The goal here is to detect changes in your Body Fat Percentage with each reading, and Body Fat Scales are perfect for this. The exact Body Fat Percentage value isn’t so important.

Once you get to your goal physique, or at least close to it, I suggest you have a DXA scan done, just for your own reference, so you know exactly what your real Body Fat Percentage is.

If you do go for this option, it’s a good idea to take a measurement on your Body Fat Scales as soon as you get home from the test – before eating, drinking or visiting the bathroom, and preferably without having sweated. This will tell you how much your scales vary from reality – something good to know for future reference.

Bear in mind though that this variation in reading won’t necessarily be true for another person using your Body Fat Scales (the inaccuracy in the scales won’t be fixed for everyone). Nor will it be true for you if you were to lose or gain a substantial amount of weight from that point.

One final point to consider is that you may choose to also do a DXA scan before starting on your weight loss program. If you calculate your Body Fat Percentage before getting underway, you’ll know EXACTLY how much body fat you need to lose. This isn’t absolutely essential however, so it’s really up to you whether you do this or not.

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Fabian Colussi is a women's Bikini and Figure competition coach for natural athletes, certified personal trainer and gym instructor, and women's fitness consultant. He also has a background in martial arts, is an NLP Master Practitioner, and has a certification in Hypnotherapy. Fabian is a co-owner and co-founder of Million Dollar Baby Fitness.

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  1. Natalia Giovanini
    4 January, 2012

    Thanks for the post. Really looking forward to read more. Much obliged!

  2. Evelyn Bryd
    15 January, 2012

    You ROCK! I found exactly the information I was looking for! thanks!


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