Body fat scale readings: what is normal for lean mass, water and bone mass

Lean mass and body water by sex and age

The table at the top comes from the LEAD study (Ofenheimer and colleagues, Eur J Clin Nutr, 2020): 10,894 adults aged 18 to 81 from a population cohort in Austria, all measured on the same DXA model, the scanner that separates fat, lean tissue and bone in the image. It is the same cohort as our body fat table by age: that one covers fat, this one covers everything else. In kilos, lean mass barely changes with age (in women, from 38.7 to 39.0 kg between ages 18-29 and 70-81); what drops is its share of body weight, because fat goes up. LEAD did not measure water, but it can be estimated with a classic constant: water makes up about 73% of fat-free mass (Wang and colleagues, Am J Clin Nutr, 1999). Multiplying each lean-mass percentage by 0.73 gives this second table.

• Average lean mass: 39.7 kg in women and 56.3 kg in men, 61.7% and 70.7% of body weight
• Estimated water: around 45% of body weight in women and 52% in men; it falls with age because the lean share falls
• Total body water ranges from 40 to 60% of body weight depending on the person (Kuriyan, Indian J Med Res, 2018): the table sits within that range
• Every figure is by sex: only compare yours with your own sex and age

What the scale really measures (and what it calculates)

A bioimpedance scale sends an imperceptible current through your feet (or your feet and hands) and measures how hard it is for it to travel through your body. Water conducts well and fat conducts poorly. From that reading, plus your weight, height, age and sex, an equation works out the rest. The European Society for Clinical Nutrition and Metabolism guideline (ESPEN; Kyle and colleagues, Clin Nutr, 2004) puts it this way: bioimpedance can determine fat-free mass and total body water in people without fluid or electrolyte disturbances, as long as the equation suits the population, age and health condition. Those are the two things. Every other number on your scale comes from each manufacturer’s formulas on top of them.

• Body water and lean mass: what bioimpedance can estimate, with the right equation
• Body fat: weight minus lean mass; it inherits the error in lean mass
• Muscle: another formula on top of lean mass (see normal muscle mass percentage)
• Bone mass, visceral fat and metabolic age: each brand’s own formulas, with no shared published reference

Body water: why a low percentage usually means more fat, not too little water

Fat holds almost no water; lean mass is about 73% water (Wang, 1999). That is why the water percentage on your scale depends mostly on how much fat you have. A worked example: a 65 kg woman with 38% body fat has about 40.3 kg of lean mass, and 73% of that is about 29.4 litres of water, 45% of her weight. With 28% body fat at the same weight, she would have 46.8 kg of lean mass, 34.2 litres and 53%. She drank exactly the same: what changed is the fat. So a low water reading on the scale is almost never fixed by drinking more; if it rises as you lose fat, that is simply the maths at work.

Lean mass: what it includes and how much is normal

Lean mass and fat-free mass are used almost interchangeably: everything that is not fat. Muscle, yes, but also bone, organs, skin and water. That is why it is much bigger than the muscle figure, and why it cannot be compared with skeletal-muscle tables. To compare people of different heights, studies divide it by height squared, like BMI: that is the fat-free mass index. In LEAD, measured by DXA, the average was 14.7 kg/m² in women and 17.8 kg/m² in men. In 5,635 healthy adults in Switzerland measured with bioimpedance, the median at ages 18-34 was 15.4 kg/m² in women and 18.9 kg/m² in men (Schutz and colleagues, Int J Obes, 2002), and in men it barely changed with age. Two studies, two devices and two age groups, about one point apart: another reason to compare your figure only with yourself.

• Your index: lean mass in kg ÷ (height in metres)². With 40 kg and 1.65 m, 40 ÷ 2.72 = 14.7 kg/m²
• LEAD (DXA): 14.7 kg/m² on average in women and 17.8 kg/m² in men
• Schutz 2002 (bioimpedance): median of 15.4 and 18.9 kg/m² at ages 18 to 34
• No scale: the body fat calculator gives you your fat and lean mass in kilos from a tape measure and your weight

Bone mass: the number your scale cannot measure

It is the one people search for most (“normal bone mass for women”, “bone mass chart”) and the one that says least. The scale’s current travels through the water in your body, and what bioimpedance estimates, according to ESPEN, is fat-free mass and water, not bone. So the bone mass figure on your scale is not measured by the current: it comes from the manufacturer’s formula, and there is no published table to compare it with. To get a sense of scale, DXA does measure bone mineral: in 4,159 adults in Vietnam, it averaged 2.11 kg in men and 1.60 kg in women (Nguyen and colleagues, Eur J Clin Nutr, 2021). That is a different population and a different device: do not use it as your own chart.

Visceral fat and metabolic age: each brand’s own scale

The visceral fat “level” on your scale is not grams or centimetres: it is each manufacturer’s own scale, and we have not found a published reference that says which level is normal. Real visceral fat is measured with devices such as DXA (that is how LEAD measured it), and your waist reflects it better than any number on the scale: check it with the waist-to-height ratio calculator and read why in visceral fat: what it is and how to reduce it. “Metabolic age” is not a measurement either: it is a comparison each brand makes with its own formula. If what you want to know is how much you burn at rest, the BMR calculator uses a published equation.

How to weigh yourself so the numbers are useful

What moves the reading from one day to the next is mostly water and what you have eaten, not fat or muscle. In a study of 48 adults, eating during the day lowered impedance by 4.4%, raised bioimpedance water and fat-free mass by 2% (up to 4.5% in some people) and lowered estimated body fat by 1.4 to 2.4% (up to 10% in one case); and, unlike DXA, most bioimpedance figures did not fully return to baseline after another overnight fast (Tinsley and colleagues, Med Sci Sports Exerc, 2017). That is why a single reading says little and the trend says a lot.

1. 1

   Always at the same time

   In the morning, after using the toilet and before eating, drinking or training. That way you compare readings taken under the same conditions.

2. 2

   Clean, dry feet, same scale

   The current goes in through your feet: moisture, socks or switching devices all shift the number.

3. 3

   Look at the monthly trend

   One reading a week and the monthly average. If your lean mass holds while your weight goes down, you are on track.

4. 4

   Cross-check with a tape measure

   The body fat calculator gives you a second estimate of your fat and lean mass, and your waist does not depend on how much water you drank.

In short: on average, lean mass is 39.7 kg in women and 56.3 kg in men (DXA, 10,894 adults); water, at about 73% of that lean mass, comes to around 45% and 52% of body weight, and it falls as fat goes up. Bioimpedance estimates water and fat-free mass; bone mass, visceral fat and metabolic age are each brand’s formulas, and bone health is checked with a bone density scan. Measure the same way every time and watch the trend. And because fat and lean mass are gained or lost through what you eat every day, knowing what you really eat matters more than any decimal on the scale: with one photo of each plate, Renzy gives you the calories and protein without weighing anything.