Body fat: what it is, the types, and how it's measured

Not just “how much extra you weigh”

When people talk about body fat, the most common image is extra weight to shed. Biologically, though, body fat — the technical name is adipose tissue — is tissue in its own right, made of specialised cells that behave a lot like cells in any other organ: they communicate with each other, respond to hormonal signals, and carry out specific jobs, not just storing energy.

According to Cleveland Clinic, adipose tissue insulates the body from cold, cushions the organs, helps regulate hunger and satiety, plays a role in glucose and cholesterol metabolism, contributes to insulin sensitivity, produces heat in some of its forms, has a role in immunity, and is involved in the metabolism of certain hormones, including sex hormones. It’s a long list, and it’s why specialists avoid treating body fat as a simple excess to subtract.

Practical example: an athlete deep into muscle hypertrophy training can be heavy, and even land in the “overweight” BMI category on paper, while carrying a low body fat percentage. It’s a textbook case of BMI’s limits, which we’ll get to shortly.

White fat and brown fat

Adipose tissue isn’t all the same. White fat is the most abundant type and sits under the skin, around the organs and in bone marrow: it’s the reserve depot most people picture. Brown fat is rarer and changes with age: newborns carry a fair amount of it, the share drops as a person grows up, and in adults what’s left concentrates around the upper back. Its job also differs from white fat’s: it produces heat, rather than simply storing energy.

Where it collects: subcutaneous and visceral

Beyond type, where fat is stored matters just as much. Medical sources distinguish two main zones, with very different health implications.

Type	Where it sits	Typical share	Associated risk
Subcutaneous	Just under the skin	About 90% of the total	Few known health problems
Visceral	Around the abdominal organs (pancreas, liver, intestines)	About 10% of the total	Linked to higher blood pressure, blood sugar and cholesterol, and a greater cardiovascular risk

Visceral fat is the one specialists watch most closely: it isn’t visible to the eye as a “belly”, but sits deep, wrapped around the organs. According to Harvard Health, having a lot of visceral fat can push up several markers tied to heart disease, including blood pressure, blood sugar and cholesterol. Subcutaneous fat, by contrast, the same source notes, causes few known health problems, even though it belongs to the same family of tissue.

How it’s measured, in practice

There’s no single way to estimate body fat, and the most common methods vary widely in precision.

Method	What it measures	Practicality	Precision
BMI (weight/height²)	Just a weight-to-height ratio, not fat	High, needs only a ruler	Low as a fat proxy: doesn’t distinguish fat mass from muscle
Waist circumference	Abdominal fat, a proxy for visceral fat	High, just a tape measure	Indicative, not quantitative
Waist-to-hip / waist-to-height ratio	Distribution of abdominal fat	High	Indicative
Bioimpedance / skinfold calipers	Estimates total body fat percentage	Medium, needs dedicated tools	Practical but less precise: the literature suggests it tends to underestimate fat compared with DEXA
DEXA	Detailed body composition (fat, muscle, bone)	Low, requires clinical equipment	Reference standard in scientific research

BMI doesn’t measure body fat

BMI (body mass index) is the most widely used calculation, but it’s worth being clear about what it actually is: a ratio of weight to height squared, nothing more. The U.S. National Heart, Lung, and Blood Institute (NIH) says so explicitly: BMI doesn’t account for muscle mass, bone density or body composition, and is “just one piece of the puzzle” to weigh alongside other factors. The official NIH/WHO adult thresholds are underweight below 18.5, normal weight 18.5-24.9, overweight 25.0-29.9, and obesity 30.0 and above — thresholds that remain useful at the population level, but need to be read with that limit in mind.

Waist circumference and ratios

A more direct way to get a sense of visceral fat is to measure waist circumference with a flexible tape, just above the hip bones. According to Cleveland Clinic, the risk of excess visceral fat rises above 102 cm (40 inches) in men and above 89 cm (35 inches) in women. The men’s threshold is independently confirmed by Harvard Health too. There’s also the waist-to-hip ratio (above 0.90 in men and 0.85 in women indicates abdominal obesity) and the waist-to-height ratio, which should stay under 0.5.

DEXA: the research standard

DEXA (dual-energy X-ray absorptiometry) is the method scientific studies use as a reference to estimate how much fat, muscle and bone make up a person’s body. It’s accurate, but requires clinical equipment: that’s why, day to day, people more often rely on bioimpedance scales or skinfold calipers, tools that are practical but less precise, and that tend to underestimate body fat compared with DEXA.

The reference percentages: what the numbers actually say

The most solid source on “expected” body fat percentages is a study published in 2000 in the American Journal of Clinical Nutrition by Gallagher and colleagues, which measured 1,626 adults — women and men, of different ethnic backgrounds — using DEXA and, at two of three centres, a 4-compartment model considered even more accurate. The goal was to link NIH and WHO BMI thresholds to expected body fat percentages, broken down by sex and age.

Women	BMI < 18.5	BMI ≥ 25	BMI ≥ 30
20-39 years	21%	33%	39%
40-59 years	23%	34%	40%
60-79 years	24%	36%	42%

Men	BMI < 18.5	BMI ≥ 25	BMI ≥ 30
20-39 years	8%	20%	25%
40-59 years	11%	22%	28%
60-79 years	13%	25%	30%

Two things stand out in the table. First: at the same BMI, women consistently show higher fat percentages than men. Second: at the same BMI, the percentage rises with age — the authors state it plainly, once BMI is controlled for, older age is associated with a higher body fat percentage.

The same authors caution, though, against treating these as a clinical verdict: the study, they explain, was mainly about working out a statistical method for deriving these thresholds, rather than pinning down final, universal body fat numbers. These are figures tied to BMI thresholds, useful as a general reference, not a limit that applies to any one person.

Practical example: a 45-year-old man with a BMI of 26 falls into the NIH’s “overweight” category. According to the table, the expected body fat percentage for a man his age in that BMI range is around 22%. The number describes a statistical average, not whether that particular person should be concerned — which is why that judgement stays with a healthcare professional.

How much visceral fat matters worldwide

This isn’t just an individual issue. According to the World Health Organization, in 2022 1 in 8 people worldwide were living with obesity, and 2.5 billion adults were overweight, of whom 890 million had obesity — 43% and 16% of the world’s adult population, respectively. Among children and adolescents aged 5-19, also in 2022, over 390 million were overweight and 160 million were living with obesity. Numbers that explain why research into measuring and interpreting body fat — not just weight — remains an active field, spanning the muscular system and adipose tissue alike.

What this article doesn’t say

The percentages and thresholds covered here are statistical tools, built to study whole populations or offer a first point of reference, not to replace a medical assessment. None of the thresholds mentioned settle what’s “right” for any one person: age, sex, medical history, medication and other conditions all change the picture. For any personal assessment — whether a given body fat percentage or BMI should be considered a concern in your specific case — the right reference remains a doctor, a dietitian or another qualified healthcare professional.