Assessments: Body Composition part 2 – Body Fat

In part 1, we covered ways to measure your weight (bathroom scales and BMI) and a measure of fat distribution (waist/hip ratio). Here in part 2, I’m going to cover ways to measure your body fat.  And this is the important thing.  A common goal of many people is to lose weight, when they should be trying to lose fat.  The following methods are ways to measure how much body fat you are carrying.  I would say that it is much more important to maintain a healthy fat level, than an arbitrary weight.

Skinfold Calipers

What is it:  These are tools that pinch various parts of your body (usually between 3-9 different places) and measures the thickness of the fat there.  The more body fat you have, the greater the measurement. You can either use the measurements directly (if the sum total of your measurements are reducing, you are losing body fat.  If they are increasing then you are gaining body fat), or they can be plugged into various complex equations.  These take your sex, height and age into account and give you an estimate of your body fat percentage (how much fat you have compared to everything else).

Skinfold Calipers

Your body fat percentage is compared to the following:


  • 3-5%  –  Essential Fat
  • 5-13%  – Sport performers
  • 12-18%  – Health/Fitness
  • 19-24%  – Potential Risk
  • >25%  – Obese


  • 11-14%  – Essential Fat
  • 12-22%  – Sport performers
  • 16-25%  – Health/Fitness
  • 26-31%  – Potential Risk
  • >32%  – Obese

Why are the womans percentages all higher than the mens?  Thats just biology for you.  Women have to be able to do things that men don’t have to worry about (like delivering babies).  So they are built in different ways. Pros:

  • These do directly measure how much body fat you have at various parts of your body.  This is a much more personalised assessment.
  • These can give an estimate of how much body fat you have in total.  So it does not matter how much you weigh, as long as your body fat percentage is under control.
  • Cheap – a robust entry level set of calipers can be bought for less than £20.
  • Quite simple to start using them, and when used correctly they are an accurate and reliable way to measure your body fat levels.
  • You get actual measurements from many parts of your body.  These include many “trouble spots” like belly, bingo wings, love handles etc.


  • Difficult to get consistent results.  Every person will pinch you in slightly different ways, so make sure you are assessed by the same person each time.
  • While simple to use, it does take a lot of experience to get consistent, reliable results.  It is often said you need to have done at least 100 assessments before you get consistant and reliable.
  • There are many equations out there that estimate your total body fat percentage.  Because of where the research was done (sports science universities), most of these equations relate to fit and healthy western white blokes.  But there are different equations for different sexes, ethnic origins, age-groups etc.  I’ve seen the same data plugged into 2 different equations and give results of 7% and 17%!  Thats a big difference!  So make sure you use the appropriate equation!
  • Because of how the equations work, a very small inaccuracy in the pinching can grow into a very big inaccuracy when estimating the total body fat percentage.
  • Due to the positions where the body fat is pinched, you need close physical contact with people who are not wearing many clothes.  High levels of professionalism and trust are required or you can get into serious trouble.
  • For practical reasons, it is very hard to assess large groups/populations of people in this way.
  • For practical reasons, it can be difficult to measure the extremely obese.

Summary:  This is the first method so far that actually gives an estimate of your body fat levels and can tell you if they are increasing or decreasing.  And I think thats probably the best way to use it.  While it is great fun to work out your body fat percentage and how much your flab weighs, it is very difficult to do this reliably. A better way is to assess the changes in your body fat levels.  You do this by comparing the sum total of your skinfold measurements and make sure it is going in the right direction.

BIA – Bioeletrical Impedance Analysis

Body Fat Scales

Body Fat Scales

What is it:  You might have seen these in many shops – Body Fat Scales.  These come in various shapes  and sizes but they work in the same way. An electrical charge is passed from one electrode to another (you don’t feel anything) and the devices estimates your body composition from the results.  How does it do this?  The device detects how much resistance the electrical current encountered through the body.  Muscle tissue (which contains relatively high amounts of water) presents little resistance to the electricity.  Fat however, contains relatively low amounts of water, so presents a higher resistance to the electricity. The device takes into account various factor about your (age, gender, height, weight) and using various equations gives an estimation of your body fat percentage. The 3 main shapes available are:

  • Handheld – this passes the charge up one arm, through the chest and down the other arm.  This only actually measures the top part of your body and the device uses various equations to”guess” the rest of your body.  As a rule, these are the least reliable.
  • Scales to stand on – this passes the charge up one leg and down the other.  This actually only measures the bottom half of your body and uses various equations to “guess” the rest of your body.
  • Freely connecting electrodes – these clip onto a hand a foot and passes the charge up the arm, through the torso and down the leg.  This measures from the tip of one arm, through the body to the tip of the foot…so it has a lot less “guessing” to do.  As a rule, these are the most reliable
Hand Held Monitor

Hand Held Monitor


  • These devices are coming down in price all the time.  I bought my scales for £20.
  • They are very portable.  Good ones can slip easily into a laptop case.
  • No training is required to use these.  Just enter the subjects physical data and it does the rest itself.
  • Non invasive and suitable for people to do practically anywhere.  People can keep their clothes on (only the points where the electrodes contact need be bare skin).
  • Under the right circumstances, they are highly consistent.
  • Fast.  It only takes a few seconds and you have a result.


  • A major factor effecting the results is the subjects hydration levels.  If they are well hydrated (drunk fluids within the past few hours, or not taken a piss recently), the machine will think there is more water containing muscle and underestimate fat levels.  If they are dehydrated (been exercising, sweating, just woken up, been drinking alcohol/caffeine in the last few hours or just taken a piss), then the machine will think there is more water scarce fat and overestimate fat levels.  According to my scales, by body fat varied by over 5% within 30 minutes…just because I had breakfast!
  • As with the skinfold calipers, there are various equations that can be used for various populations.  But with these devices you are usually restricted to the 1 or 2 preprogrammed into it.  If your subject does not of the appropriate population, then tough.  Different equations make a big difference!  I used the 2 preprogrammed equations in my scales and the results varied by 10%!

Summary:  This is a quicker, more convenient way to estimate body fat levels.  By taking the human element out of the measurement, it is easier to be consistent.  In other words, it is more automatic.  This is more than suitable for most people, as long as you bare in mind the many ways that throw off the results. But it can’t give you the fine detail the skinfold calipers can.  In other words, you don’t actually measure the body itself and you don’t get any information about different parts of the body. Make sure you know what kind of information you want and use the appropriate tool for the job.

Hydrostatic (underwater) weighing

Hydrostatic weighingDisclaimer!!  From this point forwards, I have no actual experience with these methods.  I’m just repeating to you what other, cleverer and richer people have said.  If you have actual experience with the following, please correct me if I’m wrong.

Hydrostatic Weighing

What is it:  With this method you are actually weighed twice.  Once on dry land (just put on the scales and weighed), then again while you are completely submerged under water after emptying your lungs of air.  Because we float slightly, there are 2 different measurements.  These 2 measurements are then put through various equations and the density of your body is worked out.  This is done using the archimedes principle (he who jumped out his bath shouting “eureka!”). As we all know, different materials (fat, muscle, bone etc) have different densities and so float to varying degrees.  By comparing your density to that of other known materials, your body fat percentage is worked out. Pros:

  • Very accurate and considered the “gold standard” when it comes to assessing body fat levels.  This means, all other ways of assessing body fat levels are compared and calibrated with this method.


  • Price and availability.  This method requires big, specialist equipment, usually found in sports science universities.  It requires specialist training to conduct it, so all this means you may have to travel very far to find one and it costs you loads.
  • As with the previous 2 methods, fat levels are still only estimated using equations derived from research on various populations.  So it requires using the correct equation or results can be wrong.
  • Because this measures density of your whole body, the air in your lungs are also included.  This means you are required to empty your lungs and then submerge your head under water.  This is neither easy or pleasant to do.  But the more air you have in your lungs, the less your average density so the more likely to have your fat levels overestimated.

Summary:  By all accounts, this is the best, most accurate way to measure your fat levels.  But the cost and practicalities make it very hard to justify for most people.  Do you really need that accuracy?  If you are professional athlete, maybe you do…but if your average Joe Bloggs, I doubt it.


What is it:  This is the only true and accurate way to determine your body fat levels…have a pathologist surgically cut every last gram off the body and put it all on a set of scales.




  • The only method to accurately measure the fat levels, as opposed to estimating it.


  • Expensive.  You will need a pathology lab and a qualified and experienced pathologist to do the procedure…
  • …Oh yes, and if you were not dead before this, you certainly will be dead after it.

Summary:  Not very practical, I think you will agree :).  I just put it in here for completeness. But on a slightly serious note, this is the only method that you should believe if it gives you a body fat percentage that includes a decimal point.  This method is accurate enough to do this.  With any other method, please ignore the decimal point…they simply do not have the resolution to be able to go that far.


As you can see from all the methods I’ve covered, there are pros and cons for each method.  There is no single “perfect” way to assess your body fat levels.  So which one(s) you choose depends on who you are and what your goals are. This is the bit I love!  We need to use our brains to work out which we should use. And that is all I’m going to say on the subject of ways to assess your body composition.  Let me know what you think! Don’t forget, you can subscribe to this blog via email or RSS using the button at the top right of the page, or you can print, email or share this article using the buttons below!

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