Body fat part 3: Why the scale can steer you wrong

June 22nd, 2008  |  Published in Why eat (or not)  |  6 Comments

Body fat is generally measured and expressed as a percentage. So, if we have a 200 lb. person with 20% body fat, then we know that 40 lbs. of that person is body fat, and the rest is the good stuff: bones, organs, and most importantly muscle. The non-fat-stuff is commonly called lean body mass, or LBM. Our aforementioned person thus has 160 lbs. of LBM.

When calculating fat loss it is best to combine numeric weight loss with the percentage of body fat. This is done because numeric weight (i.e. the weight on the scale) does not give us the full picture of how much of a person is fat and how much is LBM. If we take two people who are 200 lbs., and one of them is our 20% body fat person, and the second is someone with 10% body fat, clearly the second person is in better shape. However, judging this by the scale alone would be misleading.

Many athletic people are “overweight” by the standards of scale weight, but still fit and lean, because muscle is much denser than fat. This is why the conventionally used body mass index (BMI) is a poor tool of assessment for athletic people. I have known several people who got in trouble from doctors, military bosses, etc. for being overweight, when the assessor could clearly observe that they were muscular and lean. Frankly, I’ll be happy when they junk that stupid BMI thing in favour of body composition tests, and I’m not just saying that because I’m pushing “overweight” myself.

(On the other hand, many folks delude themselves about how “fit” they are. Yeah, muscle is denser than fat, but be honest with yourself. Unless you’re someone who hits the gym 5 days a week, there’s a good chance the BMI applies to you. Sorry.)

To give you an idea of how body fat assessment can be used to establish changes in body composition, let’s say that we have a 200 lb. person who begins a fitness program at 30% body fat. That means she has 60 lbs. of body fat and 140 lbs. of LBM. Let’s then say that she gets to 160 lbs. and 20% body fat. Now she has 32 lbs. of body fat, and 128 lbs. of LBM. Some LBM has been lost in the process, but it’s only 12 lbs. worth, whereas 28 lbs. of body fat has been lost.

Let’s say in a second example that our 200 lb. person hasn’t been training or eating right, and manages to get down to 160 lbs. through a combo of chain smoking, black coffee, and long hours of low intensity, endurance-based cardio. However, because of her poor training and eating habits, she’s only made it down to 25% body fat. This means she’s lost 20 lbs. of fat and 20 lbs. of muscle.

Person 1: 160 lbs, 20% body fat. 32 lbs fat, 128 lbs LBM.

Person 2: 160 lbs, 25% body fat. 40 lbs fat, 120 lbs LBM.

Same finishing weight, big difference in results. The second person will probably look and feel worse, will not be as lean, and most importantly, will not have the all-important LBM that keeps the metabolic fires stoked. The second person will likely put that 40 lbs. right back on in the long run. The first and second people are the same numeric weight, but their body composition will be significantly different.

Having argued in favour of body fat assessment, I should caution you that the tools of body fat assessment vary wildly in their accuracy and ease of use. In general, it’s a sad truth that the easier the body fat measurement, the more inaccurate it’s likely to be. Body fat calculations are based on population norms, which at the time many of the calculations were developed, meant white male college students (they were easy to get hold of for university lab research, which, by the way, is another reason to critically read scientific studies that use this as a normative population). Athletes, people of nonwhite backgrounds, older folks, basically anyone outside of that “norm” can get an inaccurate reading. There are apocryphal stories, for example, of black athletes getting negative body fat percentage readings (this means, perhaps, that they actually give body fat to the people around them?). I’ve heard people claim to be 4% body fat because their tape measurement said so, and I usually tell them that if they’re 4%, then:

    a) they should see horizontal striations (ridges) on the muscles in their ass;
    b) they should be covered in visible veins (not just a few);
    c) they should be able to see the lymph nodes in their groin;
    d) unless they’re a competing bodybuilder about to go on stage, their family is probably booking their funeral.

Here’s a rundown of ways to measure body fat, from easiest and most inaccurate to hardest and most precise.

Tape measurements of body circumference (e.g. waist, hips) combined with height/weight measurements are unbelievably inaccurate. Just for fun, I tested a few of the online tape measurement calculators, and got results ranging from 12% to 28% body fat. Tape measurements are handy for knowing overall size losses or gains, but are largely useless for knowing body fat.

Bioelectrical impedance devices, such as the Tanita body fat scale, determine body fat by sending little electrical pulses through the body. Not bad for average people, usually quite inaccurate for athletic people. Measurements will also vary significantly based on hydration levels. Still a margin of error in the range of +/- 5%, and there’s a big difference between 10% and 15%.

Skinfold calipers take a pinch of skin at various sites on the body. This is more accurate if it’s done by someone experienced, but there is still a margin of error of around 2-3%. If you want to assess your own body fat, this is probably the best way to do it as long as you remember that you need practice, and the margin of error remains significant.

Hydrostatic, or underwater weighing, requires the person to be submerged in a tank of water and to expel all the oxygen from their lungs as they are measured. This is quite accurate but hard to obtain unless you live near a friendly university research lab.

DEXA, or dual energy x-ray absorptiometry, is likewise accurate but also involves a complex procedure with special equipment. However, if you’re going for a DEXA bone density scan, see if you can sneak in a body fat assessment while you’re there.

Autopsy is the most accurate and effective means of body fat assessment. And, guess what, it’s a little inconvenient for most of us.

I actually didn’t mention the one method that’s both easy and potentially very accurate: visual inspection by a trained eye. People who’ve been in the bodybuilding biz long enough, and who are sufficiently observant, can tell someone’s body fat just by looking at them. I can take a pretty decent guess, but no doubt there are bodybuilding veterans who have the incisive visual accuracy of autopsy. You don’t need to be a bodybuilding pro to do this yourself, though. Sometimes the best tool for assessing your body fat is a full length mirror and an overhead light. Look at yourself from all angles. Observe any visible muscle definition: the “v” of the deltoids at the top of the arm, the lumps of the ab muscles, the ridge under the calf. Observe also where you deposit your fat: breasts, belly, upper arms, waist, hips, thighs, lower back. If you like, take pictures every month or few months, and use them for a visual comparison of body fat gains or losses. Familiarizing yourself with your individual body fat patterns will help you see changes in your body composition. This exercise is meant to be simple observation only, not judgement.

body fat norms

Normal and ideal ranges for body fat vary with gender. On average, women have a higher body fat than men. At one extreme, male bodybuilders before a contest can drop their body fat to around 4-5%, while women can drop to around 6-7%, commonly with the assistance of drugs. Female fitness competitors are usually in the range of 10-14%, depending on the aesthetic of whatever it is they’re posing for (e.g. swimsuit, onstage, fitness shoots, etc.). These very low body fat percentages are generally maintained for only a short period, normally before a photo shoot or contest. Very low body fat percentages are extremely difficult to maintain for most people, since the body has metabolic and hormonal mechanisms in place to prevent what it perceives as a shortage of available resources.

For general health and fitness, for men, somewhere between 10-15% is a good range to shoot for, though the North American average is undoubtedly higher. Men who want to see a six-pack of abs usually have to be under 10-11% to do so, since that’s normally where they store fat. For women, 20-25% is the approximate ideal for general health. Athletic women may keep their body fat as low as the mid-teens with no ill effects, since energy balance (calories in versus calories out) is the prime determinant of health in this case. While low body fat is correlated with problems common to elite female athletes, such as disordered or absent menstruation and loss of bone density, it is not a particular body fat percentage per se which is responsible. Rather, since low body fat is often correlated with a negative energy balance (in other words, taking in fewer calories than one burns), it can appear as if body fat levels alone are responsible. A lean woman who is careful to adequately meet her nutritional and caloric needs, and not overtrain, should see no detrimental effects from lower than average body fat.

For men, anything over 20% approaches obese territory, while for women this percentage is closer to 30% and over. Folks who have been overfat for a long time, particularly if they were overfat as children, will find it more difficult to drop to the lower end of the body fat range. One piece of good news, however, at least for pear shaped people, is that gynoid fat deposition is associated with fewer adverse health effects than android fat deposition. So, women with a gynoid fat pattern can carry a bit more fat with fewer consequences than the android folks (sorry, but that’s biology for ya).

I usually hover in the range of 15-18%, depending on the way I’m training, and my training goals. Once I start getting to around 15%, people start to tell me that my face is looking fuglier than normal. (Heehee.) For gaining strength and mass, it appears that a slightly higher body fat percentage is ideal. As always, an ideal body fat percentage for you will depend on many individual things: gender, age, overall health and medical conditions (including supplementation with hormones), starting body fat levels, and training goals.


  1. Elizabeth says:

    May 4th, 2009at 3:32 pm(#)

    Good to know about the electric method — a friend of mine went to the doctor about a sudden unexplained weight loss, and they tested her at 5% bodyfat. Now, she’s a slender person, but she’s neither a bodybuilder nor a concentration camp survivor, so I figured that had to be bullshit. Even 10% sounds way too low for the way she looks, though she has a batch of food allergies which may make it hard for her to put on fat.

    I knew I’d be able to find the answer here, though. Thanks!

  2. Vicki says:

    January 21st, 2010at 9:12 pm(#)

    What a great post! It’s really good to read about strength-training and fitness for women that doesn’t end with “so you can fit into those skinny jeans!” ;) Not that I mind skinny jeans.

    May I humbly suggest that you change the word “gender” in the first sentence under “body fat norms” to sex–I’m pretty certain that that is what you mean (in most common usage, sex=biologically male or female, gender=male- or female-identified).

  3. Lisa says:

    March 3rd, 2010at 7:52 am(#)

    I have been working with a personal trainer who measured my body fat through the skin fold caliper method. I currently weigh 280 lbs, am 45, female, 5’8 and have always had a large body frame. They measured me at 36% body fat and set an ideal weight at about 200 lbs. and 20% body fat.

    When I was in my early 30’s I was a runner, fit and weighed 165 lbs (size 8 then vs. size 20 now), so if I compare those numbers with my goal numbers set by my trainer, there’s a 40 lb difference. This seems inaccurate to me.

    So my question is around how our bodies change over time as we age.

  4. Mistress Krista says:

    March 3rd, 2010at 10:28 am(#)

    Lisa: First, bear in mind that at the farther ranges of %, the formulae often break down. Thus, measuring people with much less or much more fat than average can result in imprecise results. This is especially true when using skinfold calipers for higher bodyfat levels, as it’s tough to get a proper skinfold at many sites like the thighs. However, you can still use skinfolds to track general trends in body composition, by tracking the overall change in total millimetres measured (in other words, the sum of all the skinfolds). You won’t get an accurate % necessarily but total mm are still useful.

    Over time our bodies store more bodyfat in the abdominal cavity. This is the #1 reason why age is important in calculating bodyfat %. Additionally, it may not be as desirable for older women to be as lean as younger women, as leanness (technically, the hormonal environment associated with leanness, and what it takes to maintain it) can affect bone density.

    Nevertheless, the exact % doesn’t really matter as much as how you feel and perform. The number attached to the % is just a way of expressing a physical state where you have lots of good lean mass, and enough — but not too much — fat mass to support your overall health and athletic performance. We can’t really know what an ideal weight nor % is for a given person; we can only speculate and make informed guesses. We do want people to retain as much lean mass as possible; good nutrition and training just helps keep that valuable lean mass on you (and if we’re really lucky, helps you build a little more lean mass) as fat goes away.

    Throughout your fitness journey and life, your goals will probably change. Use an iterative process where you use evidence of health indicators and athletic performance to evaluate your progress and goals as you go along. If you get to 200 lbs and feel great at whatever bodyfat % that is, then awesome. If you feel good but want to keep on truckin’ with fat loss, then that’s cool too.

  5. Mike says:

    September 13th, 2010at 3:28 pm(#)

    Love this posting on scales. I found it while looking for reasons why after a long airplane ride, the fat measuring scale indicates a LOWER fat % than before the ride. I dont hav a statistical sample yet so it could be a fluke, but 3 data points (2 for me and one for my wife) – as it is counterintuitive for me i am wondering if anyone has a scientific perspective on this. Given that a) the fat scales work with an impedance measurement, b) water conducts electricity better than fat and c) plane journeys dehydrate you, i would expect the fat readings to go up (dehydration=less conductivity=higher reading)…

  6. Your Authentic Self | Nancy Johnson Chavez says:

    July 17th, 2011at 5:27 am(#)

    […] Body fat part 3: Why the scale can steer you wrong :: […]

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