Measuring Body Fat
by Michael Lombardi, NIST - October 2010
People are sensitive about their weight. I learned that lesson the
hard way in the third or fourth grade, when our regular teacher was
sick. The substitute teacher seemed nice, but she was much older
and bigger than our regular teacher, who was a very small woman
who had probably just left college. At recess, the boys in my class
were discussing the “differences” between the two teachers, and
one of them dared me to ask the substitute teacher how much she
weighed. I knew she weighed a lot and must have known there was
danger involved, otherwise it wouldn’t have been a “dare,” but I was
too young and naive to realize what a rude and insensitive question that was. After all, the only women I had ever talked to were
my mom and my grandmothers; I didn’t have a sister, and for the
most part freedom of speech was practiced in my Italian-American
household. Thus, I ran over amiably with a foolish smile on my
face to address the teacher. About one second after I popped the
question, the teacher popped me with a right hook that would have
made Joe Frazier proud. When I finally stopped crying, it was clear
to me that asking people how much they weighed was hazardous
to your health.
To this day, it seems to me that weight is a dangerous topic of
discussion, perhaps more dangerous than religion or politics. So
against my better judgment, and with great fear and trepidation,
I’ve decided to tackle a very controversial topic – the metrology of
body fat. This could turn out badly, but I’ve always been a sucker
for a dare.
Body fat is measured as a percentage of a person’s total weight.
The idea is to find out how much of the body is fat and how much is
lean body mass, which basically consists of muscle, bone, and tissue.
My substitute teacher didn’t appear to believe it, but fat isn’t necessarily bad. In fact, some body fat is essential for a person to stay alive,
or to maintain reproductive functions. Woman require an essential
amount of body fat ranging from about 10 to 13 %, whereas men
only require about 1 to 3 %. However, not even the most athletic
people have body fat percentages that are that low, meaning that
almost everyone is carrying more fat than they absolutely need to
survive. Today, the average woman has body fat ranging from about
25 to 31 %, and the average man has body fat ranging from about
18 to 24 %. A woman or man who is considered to be in top physical condition would have a body fat percentage about 10 percentage
points lower than the average, typically under 20 % for a women or
under about 13 % for a man.
Excess weight can cause a lot of health issues, and thus body fat
measurements are an important medical topic. A great deal of research and effort has gone into the measurement of body fat percentage, and the amount of available literature exceeds that of many
other areas of metrology. Any type of body fat measurement will
provide a general idea of a person’s body composition, and more importantly, will inform a person who is trying to reduce their body
fat of their progress. Even so, all methods are just approximations,
with fairly large uncertainties. It seems that the four most popular measurement methods for determining body fat percentage are
hydrodensitometry weighing (underwater weighing), anthropometry-skinfold (calipers), bioelectrical impedance analysis (BIA), and
dual energy X-Ray absorptiometry (DXA). Let’s take a brief look at
each of these methods.
Hydrodensitrometry, or underwater weighing, is based on the fact
that fat floats and lean tissue sinks. That fact is easy to believe, as
many of us will note that floating is easier now than it was when we
were kids. The equipment available for this measurement ranges in
sophistication from the standard stainless steel tank with a chair or
cot mounted on an underwater scale, to a chair and scale suspended
from a diving board over a swimming pool.
The person under test (PUT) is first weighed outside of the water.
They are then placed in a chair and lowered into the water until
they are completely immersed, with all of the air exhaled from their
lungs, and weighed again. To get an accurate measurement, the
process is typically repeated at least three times. A lower immersed
weight indicates a higher body fat percentage. For most PUTs, this
method has a fairly low uncertainty, typically 2 or 3 %, and for years
was considered the best way to measure body fat percentage.
Underwater weighing has several disadvantages, however. For
one thing, the equipment is highly specialized and not always available, and not everybody is OK with the idea of total submersion.
The method also assumes that the densities of fat mass and fat-free
mass are constant for all PUTs. Not true. For example, athletes tend
to have denser bones and muscles than non-athletes, which may
cause their body fat percentage to be underestimated. In contrast,
the body fat of elderly patients suffering from osteoporosis may be
overestimated. For an accurate measurement, it’s also important to
know if the PUT has expelled all of the air out of their lungs, and
for the water to be completely still, without any wind or movement.
Anthropometry-skinfold measurements are more convenient.
They are done with hand held calipers that exert a standard pressure. Measurements are made by grasping the skin and underlying
tissue, shaking it to exclude any muscle, and pinching it between
the jaws of the caliper. The test is performed at three to seven body
locations on the PUT where fat normally accumulates, such as the
back of the arm, waistline and hip. Skinfold measurements assume
36 Metrologist : October 2010 www.ncsli.org
that the fat found under the sampled body
locations accurately represents the proportion of fat found throughout the rest of the
body. Each body location is typically measured from two to three times to reduce the
uncertainty. A simple, less accurate variation of this test is known as “pinching the
love handles.” It is performed daily by nonmetrologists in front of hundreds of thousands of bathroom windows, worldwide.
Most practitioners declare themselves to be
out of tolerance.
After the skinfold measurements are recorded, a calculation is used to derive a body
fat percentage based on the sum of the measurements. Different prediction equations
are needed for children and specific ethnic
groups, and believe it or not, over 3,500
equations have been validated! The underwater weighing method was used as the reference for many of the prediction equations.
Skinfold measurements have several advantages: they are easy to do, inexpensive,
and the equipment is portable. The quality
of the calipers affects the measurement uncertainty, they should be accurately calibrated and have a constant specified pressure.
Inexpensive models sold for home use are
usually less accurate than those used by professionals. The final uncertainty is typically
about 2 to 4 %, and depending on the skill
of the technician performing the measurement, can be about as reliable as underwater
Bioelectrical Impedance Analysis (BIA)
measures body fat percentage by sending
a low-level, safe, electrical
current through the body.
The current travels at a different rate through the various body tissues. It passes
easily through muscle tissue, which contains a large
amount of fluid, but it travels more slowly as it passes
through fat tissue. The resistance encountered when
the current hits the fat tissue is called bioelectrical impedance.
With the exception of the informal
“pinching the love handles” method, the
BIA method is the most common type of
body fat measurement performed at home.
Commercially-available devices that perform this measurement first appeared in
the mid-1980s. Today, so many have been
built and sold that they can sometimes be
purchased for less than $10. All you have to
do is enter your gender, height, weight, and
age, and these devices estimate your body fat
percentage. BIA analyzers are even built-in
to many digital scales.
The BIA measurements are simple, painless, and fast, but may not be especially accurate. Accuracy is often compromised with
extremely lean or obese people, and to get a
good reading you have to pay special attention to the level of fluids in the body. For example, measurements should not be made if
you have had a meal in the last two hours; or
if you have just exercised vigorously or taken
a bath or shower.
The most accurate method of measuring
body fat is normally dual energy X-Ray absorptiometry, or DXA. These measurements
can normally only be done in a medical
clinic, since they involve a whole-body scanner that aims two X-ray beams with differing energy levels at the patient’s bones. The
scan lasts for about 20 minutes. When soft
tissue absorption is subtracted out, the bone
mineral density can be determined from
the absorption of each x-ray beam by bone.
This technique is typically used to diagnose
and follow osteoporosis, but the absorption
readings are so accurate that the DXA has
probably now surpassed underwater weighing as the new “gold standard” for body fat
measurements, with lower uncertainties for
most PUTs. Since it involves a whole body
scan, DXA also can tell us where our fat is
located, on the off chance that we don’t own
Since I am too cheap or too scared or too
much in denial (take your pick) to get my
own body fat measured, allow me to close
with another pointless anecdote. A friend of
mine grew up in a modest house in the Denver suburbs that his dad originally bought
for about $10,000. A few decades later, the
average house in Denver had become much
larger, and my friend’s house was considered
even more modest than it had been originally. But now it was worth a lot more. While I
was visiting my friend’s dad a few years ago,
he told me that “I always dreamed of living
in a $100,000 house. I just never thought it
would be this one.”
Such it is with weight. As a scrawny 130
lb teenager, I dreamed of being a big, strong
200 lb man. A few decades later, I finally obtained that 200 lb body. I just never thought
it would be this one.