Three measurements, three numbers – and none of them says the same thing. The scales report two kilos more, the caliper a lower value than last month, and the body fat scale in the bathroom shows something different every morning. That is not a measurement error: each method captures something different and has its own limits. This article works through the common procedures, names the reference values from the update Akademie course chapter “Diagnostik und Assessment” and puts into context what each number is good for.
What anthropometry actually measures
The term body measurement – technically anthropometry – covers all the methods used to record body dimensions such as height, body weight or waist circumference, together with their classification and interpretation. These data are recorded systematically and then documented in the nutritional assessment. The aim is more modest than many expect: the measurements give you an overview of the current state. They are not a diagnostic instrument. Diagnosing malnutrition, obesity or a metabolic disorder belongs in medical hands – your job is to collect the data cleanly and to know its limits.
One point up front: a change in body weight is not the same as a change in the fat percentage. The compartment model shows why. Our weight consists of fat mass and lean mass; the latter comprises the body cell mass BCM (skeletal muscle plus organ cells) and the extracellular mass ECM (bone, connective tissue, extracellular fluid). Bone mass varies little; muscle mass and water content, by contrast, are easy to change and largely determine what appears on the scales.
Height, weight and BMI: quick, but crude
You measure height with a tape measure or a measuring station, preferably in the morning, standing upright and without shoes. For body weight, three rules apply: always the same scales, always under the same conditions (ideally in the morning and fasted) and with as little clothing as possible. Typical causes of fluctuation are water retention – through hormones or through diet – as well as constipation or diarrhoea.
The body mass index follows from these two values. It describes the relationship between height and body weight and offers a guide value for classification as underweight, normal weight or overweight.
The WHO classification
- Underweight: below 18.5
- Normal weight: 18.5 to 24.9
- Overweight: 25 to 29.9
- Obesity class 1: 30 to 34.9
- Obesity class 2: 35 to 39.9
- Obesity class 3: 40 and above
Obesity is defined as an increase in body fat beyond the normal level; the basis for calculating the classification is the BMI.
Three influencing factors that distort the BMI
- Age: As age increases, the BMI shifts upwards. For older people a BMI between 24 and 29 is in fact healthier.
- Height: Very short people often have a BMI rated too high, very tall people one rated too low. At 170 cm its informative value is best.
- Body composition: The BMI assumes an average body composition. Where muscle mass is increased, it loses its informative value.
That sets out the field of application clearly: weight and BMI classify the current state quickly, but say nothing about body composition. In athletes in particular, or in very short or very tall people, the BMI can fall outside the norm even though body composition is healthy. The weight history is also worth recording: current values, the weight during the teenage years, the maximum and minimum weight so far, and the desired weight.
Waist circumference and waist-to-hip ratio
Among the circumference measurements, waist circumference and the waist-to-hip ratio (WHR) count as the best established method, because they allow visceral obesity to be assessed – a good marker for the risk of cardiovascular disease.
You measure the waist circumference unclothed, in the morning before breakfast, midway between the costal arch and the iliac crest – in most people just above the navel. Relax the abdominal muscles, breathe in, breathe out in a relaxed way, place the tape measure in a straight line and only then take the measurement. You measure the hip circumference standing straight with the feet hip-width apart, at the widest point of the buttocks.
The WHO reference values for an increased risk of cardiovascular disease:
- Waist circumference, increased risk: men over 94 cm, women over 80 cm
- Waist circumference, substantially increased risk: men over 102 cm, women over 88 cm
- Waist-to-hip ratio, substantially increased risk: men from 0.9, women from 0.85
Waist circumference on its own already shows better than the BMI whether body composition carries a health risk – this visceral fat is also the subject of the article Losing belly fat. The data need nothing more than a tape measure and show more reliably than the scales, over time, whether a reduction in fat has taken place.
Circumference measurements and skinfold thickness
The upper arm circumference allows a good estimate of muscle mass and is measured with the arm hanging loosely, halfway between the acromion and the tip of the elbow. The thigh circumference is recorded at its widest point, midway between the kneecap and the groin. Both are worth measuring on the left and on the right.
You measure the skinfold thickness with a caliper, classically on the back of the unclothed upper arm at the same height as for the upper arm circumference. You take the skinfold over the triceps between finger and thumb, pull it away parallel to the long axis, clamp it in the caliper, read the value within a few seconds and repeat the measurement three times for the mean.
This is where the grey area begins. Medical reference values exist only for the triceps skinfold thickness, and then in order to detect possible malnutrition – so of little use for work as an Ernährungscoach. For extrapolating to the total body fat percentage, various calculators are in circulation which, depending on the provider, use different measuring sites and different formulas; how accurate they are is unclear. What remains informative is the trend – provided the same person always measures under the same conditions. With the caliper in particular, individual measuring techniques lead to large differences.
Bioelectrical impedance analysis: more accurate, but not direct
Bioelectrical impedance analysis (BIA) makes use of the electrical conductivity of the body. Electrodes are attached at defined points on the wrist and ankle and send an alternating current through the body. The resistance that the body opposes to the current is called impedance and consists of two values: the resistance from body fluid not bound in cells and the reactance from the resistance of the cells.
Decisive for the interpretation: BIA does not state directly how much body fat and muscle mass are present. It determines these values indirectly, by assuming an average hydration of the fat-free mass of 73 per cent – an assumption that does not hold for every person. The only value determined directly is the phase angle, normal range 5.5 to 7.5 degrees; a high value correlates with a good nutritional and health status, a low one can stem from muscle loss or overhydration. For the body cell mass, 41 to 45 per cent of total body weight applies in men and 30 to 33 per cent in women, for the fat mass 10 to 15 per cent in men and 20 to 25 per cent in women.
For the measurement: lie down quietly for at least five minutes beforehand, spread the arms and legs slightly, disinfect the skin sites, apply the electrodes according to the device instructions (usually 5 cm apart), no sport and no nicotine on the day of measurement. And there are clear limits: in people with a pacemaker, a defibrillator or other implanted electronic devices, no BIA is carried out, for safety reasons. Water retention, dehydration, anomalies of body shape and an extremely low or high BMI reduce the informative value; with a fever the result is invalid, because conductivity depends on body temperature. You are unlikely to have a BIA device of your own – buying one is expensive. But if a client has the measurement taken at an appropriate institution, you should be able to read the values.
Body fat scales: the reality check
So-called body fat or diagnostic scales measure not only the weight but also the fat and muscle percentage, and in some cases the water percentage as well. The disadvantage inherent in their design: in most models the electrodes sit only in the scales, so they record only the fat percentage of the lower body. A test by Kassensturz from 2016 examined seven such scales; four failed. The weight could be determined fairly accurately, but the measurement of the fat percentage was very inaccurate: the best scales were about 20 per cent off the reference value. The devices determined the water percentage relatively well – which is of less relevance to most clients.
What follows from this is not a ban but a classification. If someone already owns such scales or wants to buy some, you explain that the measurements are often inaccurate and recommend, if the budget allows, a model with additional hand sensors, because that records the whole body. Even inaccurate scales can show over time whether the measures are taking effect – as long as measurement always takes place under the same conditions.
Why three measurements produce three numbers
The answer lies in the methods themselves. The BMI presupposes an average body composition. The waist circumference describes the distribution of fat, not the amount of fat. The caliper measures a skinfold and extrapolates via a formula whose accuracy is unclear. BIA measures electrical resistance and derives the values from it via assumptions. And the bathroom scales measure, strictly speaking, only the lower half of the body.
From this follows a simple working rule: never compare numbers from different methods, only the same method with itself over time – and combine. Anyone who wants to build muscle and reduce fat measures the upper arm and thigh circumference alongside the waist circumference. That makes changes visible which are not apparent on the scales – which also helps when the weight stays put, as described in the article What to do when the scales stop moving?.
Course tip from update Akademie: How to build a complete nutritional assessment, carry out the measurements systematically and classify the results correctly is what you learn in the Ernährungscoach course – the chapter “Diagnostik und Assessment” works through exactly the methods in this article, with reference values, measurement protocols and limits. Anyone who only wants to go deeper into this one topic will also find it as the Ernährungsworkshop Diagnostik und Assessment.
Frequently asked questions about measuring body fat and body dimensions
Is the BMI still up to date?
The BMI remains a quick guide value for classification as underweight, normal weight and overweight and is the basis for calculating the weight classification of obesity. It says nothing about body composition. Where muscle mass is increased, and in very short or very tall people, it loses informative value; it fits best at a height of around 170 cm.
At what waist circumference does it become critical?
For an increased risk of cardiovascular disease, the WHO names a waist circumference over 94 cm in men and over 80 cm in women. Values over 102 cm in men and over 88 cm in women count as substantially increased. For the waist-to-hip ratio, the threshold for a substantially increased risk is 0.9 in men and 0.85 in women. These values are risk markers and do not replace a medical assessment.
How accurate are body fat scales really?
In the Kassensturz test of 2016, four out of seven scales failed. The weight was recorded well, the fat percentage by contrast very inaccurately: even the best devices were around 20 per cent off the reference value. For monitoring the trend under constant conditions they remain usable.
Who should not have a BIA measurement?
In people with a pacemaker, a defibrillator or other implanted electronic devices, no BIA is carried out, for safety reasons. With a fever the result is invalid, because conductivity depends on body temperature. Water retention, dehydration, anomalies of body shape and an extremely low or high BMI reduce the informative value.
Conclusion
None of the common methods delivers the one true number. Height, weight and BMI allow a rough assessment and are worth having above all at the start of a consultation. Waist circumference and waist-to-hip ratio are the most informative of the simple methods, because they depict the distribution of fat and with it a health risk. Caliper, BIA and body fat scales come closer to body composition, but work with assumptions and formulas of limited accuracy. Anyone who knows this, communicates it openly and follows the same measuring route over time obtains usable information.
