We always start from the assumption that the total body mass is roughly made up of the fat mass and the lean mass, but this is merely a mistranslation from English to Italian, which leaves us with an important concept, namely the fat-free-mass and not so much the lean mass, because it would be more accurate to refer to the net mass of fat mass, i.e. BM (body mass) = FM (fat mass) + FFM (fat-free mass). Actually, another component of this equation is the FFDM (fat-free dry mass), which refers to all the nutrients stored in the body, proteins and glycogen; water (TBW = total body water), taking into account that the body is mostly made up of water stored in the connective tissue; the bone and connective tissue, which can be divided into BM (bone mass) and all the rest. So our final equation then takes on a more appropriate, more precise connotation with the anatomical and cellular model:

body mass = FM + FFM + FFDM + TBW

There are various methods for calculating body composition, and I have provided a table showing the pros and cons of each:






Measurement of body weight and circumferences of certain body segments

Easy, repeatable; highly practical when carried out independently

Continuous variations according to the time of day; state of hydration; volume of organs in post-prandial state or for other reasons; operator\'s dexterity

Hydrostatic weighing

According to the Archimedes\' principle: a body is immersed in water to measure its density

FM estimation; relatively low cost; non-invasive.

Calculation of fat mass with constant density, actually subject to many variables; only applies to healthy individuals, not suitable for children, the elderly or those who are claustrophobic.

Dilution methods

Measurement of body fluids by ingestion or injection of a substance

Body fluids status measurement

Invasive and laborious system

Total K

Total body potassium measurement to determine the FFM

Error in FFM calculation of 3-5%; non-invasive

High cost; variability of individual K concentration; available in research centres


Neutron activation analysis of nuclear processes in tissues; spectrographic analysis of gamma rays

Probably the best method for high accuracy in the anatomical measurement of body sections

High cost; invasive method due to radiation


Measurement of subcutaneous folds using a calibre

Quick and easy

Wide variability of results due to numerous errors: uncalibrated plicometer; operator\'s dexterity; skin thickness not considered; variable fat distribution; reference point; menstrual cycle; race; time of day; hydration;


Bioelectric impedance analysis; detection of electrical conductivity in tissues

Rapid, non-invasive measurement of hydration status, with indirect estimation of FM and FFM

Estimation of FM and FFM depends on physiological state and hydration of subject; operator’s manual skills; apparatus; environmental factors (climate); measurement at least 4 hours after intake of food and drink, cannot be done during training or competitions, 10 minute wait-time with body in clinostatic position, not possible during luteal phase; side of body;


Bone density measurement and indirect estimation of fat and lean mass

Best method for measuring bone density; compartmentalisation of muscle sectors; fast; evolving technology

Low to medium cost (now); minimal radiation exposure


Computed tomography; X-ray source in specified body parts

Measures organ and tissue volumes; non-invasive; distinction between subcutaneous and visceral fat

High cost and the prerogative of specialist research laboratories


Nuclear magnetic resonance imaging, which detects the voltage decay after activation of hydrogen nuclei

Measures organ and tissue volumes; non-invasive; distinction between subcutaneous and visceral fat

High cost and the prerogative of specialist research laboratories


Closed sensing system, in which the subject is seated in a capsule and the volume and pressure is measured

Fast and practical

Constant FFM density coefficient; little or no availability

After taking a brief look at the various methods of measuring body composition, let\'s ask ourselves which of these is the most suitable, practical, low cost and reliable?

If the intention is to determine the amount of lean mass we have for the calculation of macronutrients, for most "athletic" subjects below 13-15% BF in the case of men and 21-23% in women, then in my opinion, this calculation can be carried out directly on the body weight (remember: BW, Body Weight), also because research has always performed calculations on body weight, and never on subjects with lower fat percentages. The only precise review on this matter was the recent one by Helms and Aragon (2018), but theirs was mostly a deduction to be applied to low-calorie diets for weight loss. In any case, the possibility to examine one\'s own percentage fat mass using DEXA, which is still the gold standard in body composition measurement and is now readily available at an affordable cost, is desirable if only to gain a more complete and undoubtedly more detailed picture. For reasons given in the table, I do not recommend relying on the BIA (bioimpedance) or plicometry values, partly because they are very rough, spannometric estimates. Although their value becomes more objective if repeated over time, they can never be defined in absolute terms.

A good empirical method is the measure the circumference of certain body segments, at the same time and under the same conditions, at least once a month. All you\'ll need is a tape measure, which you use to will measure your:

• Waist at the narrowest point (2 cm above the navel).

• Suprailiac (2 cm below the navel, in the lowest part of the transverse muscles of the abdomen).

• Chest (above the nipples).

• Thigh (in the middle, at the widest point that includes the adductor).

• Biceps (in the middle of the muscle, at rest and during contraction if you wish)

• Hips (on the iliac crest, just where the iliac spine bone protrudes, without including the buttocks).

For the sake of thoroughness, in addition to the circumferences, we can also use photographs, taking front, side and back views of the subject; these should always be taken in the same environment, with the same light (and therefore at the same time if the light is natural), and always shot from the same point.

Article edited by Edoardo Tacconi - Scientific Disclosure


L’alimentazione per l’esercizio fisico e lo sport (Michelangelo Giampietro; 2005 © Il Pensiero Scientifico Editore)

Alimentazione e Nutrizione umana (Mariani Costantini, Cannella, Tomassi; Terza edizione; 2016 © Il Pensiero Scientifico Editore)

300 Invictus (Edoardo Tacconi; 2019, Project Invictus ©)

Combest TM, Howard RS, Andrews AM. Comparison of Circumference Body Composition Measurements and Eight-Point Bioelectrical Impedance Analysis to Dual Energy X-Ray Absorptiometry to Measure Body Fat Percentage. Mil Med. 2017 Jul;182(7):e1908-e1912. doi: 10.7205/MILMED-D-16-00344. PubMed PMID: 28810989. 

V?trovská R, Vilikus Z, Klaschka J, Stránská Z, Sva?ina Š, Svobodová Š, Matoulek M. Does impedance measure a functional state of the body fat? Physiol Res. 2014;63 Suppl 2:S309-20. PubMed PMID: 24908237.

Bracco D, Thiébaud D, Chioléro RL, Landry M, Burckhardt P, Schutz Y. Segmental body composition assessed by bioelectrical impedance analysis and DEXA in humans. J Appl Physiol (1985). 1996 Dec;81(6):2580-7. PubMed PMID: 9018509.

Fang H, Berg E, Cheng X, Shen W. How to best assess abdominal obesity. Curr Opin Clin Nutr Metab Care. 2018 Sep;21(5):360-365. doi: 10.1097/MCO.0000000000000485. Review. PubMed PMID: 29916924; PubMed Central PMCID: PMC6299450.