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Body Mass Index (BMI; in kg/m²) can be considered the simplest method for prediction of body fat, as it only requires the assessment of weight and height ^[2]. Values of body weight adjusted for height, in excess of 25 and 30, are considered to indicate overweight and obesity, respectively ^[3]. Following WHO recommendations for a healthy lifestyle, we will consider the followings scale ^[4]:

#	BMI (kg/m²)	Nutritional status
1	< 18.5	Underweight
2	18.5 - 24.9	Normal weight
3	25.0 - 29.9	Pre-obesity
4	30.0 - 34.9	Obesity class I
5	35.0 - 39.9	Obesity class II
5	> 40.0	Obesity class III

Body Fat Percentage (BFP):

Body fat is considering both the essential and the storage fat of your body. Researchs had determine categories of healthy body fat percentage for average men and women. If each individual is unique , these percentage could nonetheless be consider as a solid basis prior to a further dedicated analysis.

Body Fat Categorisation ^[13]

Description	BFP Men (%)	BFP Women (%)
Essential	2 - 5	10 -13
Athletes	6 - 13	14 - 20
Fitness	14 - 17	21 - 24
Average	18 - 24	25 - 31
Obese	> 25	> 32

The ideal BFP is however slightly more difficult to determine. It may depends on BMI, gender and also ethnical origins ^[9]. In their study, D. Gallagher and SB. Heymsfield obtianed the following table of ideal average BFP, which should be amended with considering higher BFP when age is getting higher at equivalent BMI.

Predicted Body Fat depending on BMI ^[9]

Gender	BMI (kg/m²)	20 - 39 years	40 - 59 years	60 - 79 years
Men
	< 18.5	8 - 13	11 - 13	13 - 14
	≥ 18.5	15 - 16	16 - 18	19 - 20
	≥ 25	20 - 23	22 - 24	24 - 25
	≥ 30	25 - 28	28 - 29	29 - 30
Women
	< 18.5	21 - 25	23 - 25	24 - 25
	≥ 18.5	27 - 28	28 - 29	30
	≥ 25	33 - 35	34 - 35	36
	≥ 30	39 - 40	40 - 41	41 - 42

Physical Activity Level (PAL):

PAL is defined as the ratio between the Total Energy Expenditure (TEE) and the Rest Energy Expenditure (REE), also referred as BMR especially for Katch & McArdle calculation ^[7].
TEE will consider all the energy spent for movement from skeletal muscles. The PAL determines how active you are in an average day. It includes all physical activities that will consume supplementary energy than the one used for your BMR. Researchers and usual recommendations will often consider four levels of physical activity (sedentary, low active, active, and very active) which correspond to different multiples of basal energy expenditure (1.0 to 1.4, 1.4 to 1.6, 1.6 to 1.9 and 1.9 to 2.5, respectively) ^[12].

Basic Metabolic Rate (BMR):

The BMR is estimating the amount of energy required by a body at rest in a temperate controlled environment. It provides an approximation of the energy consumed by the body when resting, without digestion or other activity: it is the basic, minimal energy you would use every single day ^[1].

Estimated Energy Requirements (EER):

The EER is estimating the amount of energy required for maintenance. It is the BMR multiplied by PAL. When you want to lose some weight you will need to have an avergare intake lower than your EER. On the contrary, you will need to have extra intake if you wish to gain weight 9for muscle building for example).

How Perfectio process data:

Without any information about your country, age, gender, weight or height, we use the average BMI of an adult, all gender considered, according to NCD Risk Factor Collaboration worldwide data.^[5]

Average BMI (kg/m²) ^[5]

NB1: For more accurate estimation, you would fill your informations in your profile.
NB2: According to these data from 2016, we can see that the average BMI is corresponding to Pre-Obesity.

Country	Men	Women	Combined
	26.528	26.528	26.123

Average Height (cm) ^[5]

Country	Men	Women	Combined
	172.608	160.680	166.064

Median Population Age (years) ^[8]

Country	Men	Women	Combined
	30.30	31.80	31.00

BMR estimation

BMR estimation was reviewed over recent years following new collected data collection. These equations are predicting BMR based upon Age, Mass, Height and Gender of the subject. However, recent research pointed out a more accurate prediction of BMR while considering the lean body mass (body mass free of body fat) only. In our calculations, we will privileged the Katch - McArdle equation to take the BFP into account.
Here are the 2 equations used at this day:

The Mifflin - St. Jeor equation ^[6]

This prediction model is based on the body mass alongside the height and age of the person. Gender is also playing a part through a specific constant for both male and fermale subjects.

BMR: P(kCal/day) ; Body mass: m(kg) ; Height: h(cm) ; Age: a(years) ; Gender specific constant: s(kCal/day) = +5 for males & -161 for females.

The Katch–McArdle formula ^[7]

This prediction model, recognised as more accurate in its estimations, is considering the body mass free of fat (also referred as Lean Body Mass). Instead of BMR, we will call this daily caloric use as Resting Daily Energy Expenditure (RDEE).

BMR: P(kCal/day) ; Body mass: m(kg) ; Body fat percentage: f (%) ; Lean body mass or Body mass free of fat: l (kg).

Where l is defined as follow:
Lean Body Mass equation

Body fat percentage

BMI related method ^[9]

Three equations were developed to estimate the percentage body fat of an indivual from its age, gender, BMI and ethnicity. The studies conducted by D. Gallagher and S. B. Heymsfield highlighted these strong dependencies.

1 - General equation:
Body Fat Percentage equation for general case

Body Fat percentage: f (%) ; BMI: (kg/cm²) ; Age: a(year) ; Gender: s = 1 for men, 0 for women

2 - Asian men equation:
Body Fat Percentage equation for Asian men case

Body Fat percentage: f (%) ; BMI: (kg/cm²) ; Age: a(year)

3 - Asian women equation:
Body Fat Percentage equation for Asian women case

Body Fat percentage: f (%) ; BMI: (kg/cm²) ; Age: a(year)

U.S. Navy Method ^[10]

This prediction model was developed at the Naval Health Research Center by Hodgdon and Beckett in 1984. It involves a dependency between height of subject and some circumferences of specific parts of the subject's body: waist and neck, plus the hips for women.

1 - US Navy equation for men:
Body Fat Percentage US Navy equation for men

Body Fat percentage: f (%) ; Waist: w(cm) ; Neck: n(cm) ; Height: h(cm)

2 - US Navy equation for women:
Body Fat Percentage US Navy equation for women

Body Fat percentage: f (%) ; Waist: w(cm) ; Neck: n(cm) ; Height: h(cm) ; Hip: hip(cm)

BMI related method ^[11]

This prediction model from P. Deurenberg is the one which is widely used accross the web and nutrition calculators online. Even if easier to calculate, we will prefere others equations that would provide a better accuracy ono their prediction due to the consideration of more parameters.

1 - Body Fat equation for men:
Deurenberg Body Fat Percentage equation for men

Body Fat percentage: f (%) ; BMI: (kg/m²) ; Age: a(year)

2 - Body Fat equation for women:
Deurenberg Body Fat Percentage equation for men

Body Fat percentage: f (%) ; BMI: (kg/m²) ; Age: a(year)

Physical Activity Level

Average Physical ACtivity Levels ^[14]

PAL for sustainable life is ranging between 1.1 to a maximum of 2.5.

The average PAL will increase from 1.4 at age 1 year to a peak at 1.7 - 1.8 at reproductive age. It will declines again to 1.4 at age 90 years.

[1] - McNab BK. On the utility of uniformity in the definition of basal rate of metabolism. Physiol Zool. 1997 Nov-Dec;70(6):718-20. doi: 10.1086/515881. PMID: 9361146.

[2] - Nevill AM, Metsios GS, Jackson AS, Wang J, Thornton J, Gallagher D. Can we use the Jackson and Pollock equations to predict body density/fat of obese individuals in the 21st century? Int J Body Compos Res. 2008 Sep 2;6(3):114-121. PMID: 20582331; PMCID: PMC2891061.

[3] - Gallagher D, Heymsfield SB, Heo M, Jebb SA, Murgatroyd PR, Sakamoto Y. Healthy percentage body fat ranges: an approach for developing guidelines based on body mass index. Am J Clin Nutr. 2000 Sep;72(3):694-701. doi: 10.1093/ajcn/72.3.694. PMID: 10966886.

[4] - WHO. A healthy lifestyle - WHO recommendations. WHO website. 2010 May.

[5] - NCD Risk Factor Collaboration (NCD-RisC). Worldwide trends in body-mass index, underweight, overweight, and obesity from 1975 to 2016: a pooled analysis of 2416 population-based measurement studies in 128·9 million children, adolescents, and adults. Lancet. 2017 Dec 16;390(10113):2627-2642. doi: 10.1016/S0140-6736(17)32129-3. Epub 2017 Oct 10. PMID: 29029897; PMCID: PMC5735219.

[6] - Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO (1990). "A new predictive equation for resting energy expenditure in healthy individuals". The American Journal of Clinical Nutrition. 51 (2): 241–247. doi:10.1093/ajcn/51.2.241. PMID 2305711.

[7] - McArdle, William (2006). Essentials of exercise physiology (3rd ed.). Lippincott Williams & Wilkins. p. 266. ISBN 9780781749916.

[8] - "Median age – The World Factbook". Central Intelligence Agency. Retrieved March 22, 2021.

[9] - Gallagher D, Heymsfield SB, Heo M, Jebb SA, Murgatroyd PR, Sakamoto Y. Healthy percentage body fat ranges: an approach for developing guidelines based on body mass index. Am J Clin Nutr. 2000 Sep;72(3):694-701. doi: 10.1093/ajcn/72.3.694. PMID: 10966886.

[10] - Hodgdon, J. A., & Beckett, M. B. (1984). Prediction of body fat content in military personnel from body circumferences and height. Medicine & Science in Sports & Exercise, 16(2), 198.

[11] - Deurenberg P, Weststrate JA, Seidell JC. Body mass index as a measure of body fatness: age- and sex-specific prediction formulas. Br J Nutr. 1991 Mar;65(2):105-14. doi: 10.1079/bjn19910073. PMID: 2043597.

[12] - Barr, S. I., Murphy, S. P., Agurs-Collins, T. D., & Poos, M. I. (2003). Planning diets for individuals using the dietary reference intakes. Nutrition reviews, 61(10), 352-360.

[13] - The American Council on Exercise - Body Fat Categorization. ACE website

[14] - Westerterp KR. Physical activity and physical activity induced energy expenditure in humans: measurement, determinants, and effects. Front Physiol. 2013 Apr 26;4:90. doi: 10.3389/fphys.2013.00090. PMID: 23637685; PMCID: PMC3636460.

[15] - Ohkawara K, Ishikawa-Takata K, Park JH, Tabata I, Tanaka S. How much locomotive activity is needed for an active physical activity level: analysis of total step counts. BMC Res Notes. 2011 Nov 24;4:512. doi: 10.1186/1756-0500-4-512. PMID: 22114990; PMCID: PMC3245484.

TORFIE

Amor Mundum Fecit

Body Fat Categorisation ^[13]