What is body composition? Fat, fat-free mass, and why weight alone does not tell you

What is body composition?

Body composition describes what makes up your weight. Which? says the body is made up of two types of mass: body fat and fat-free mass.⁴ Fat-free mass is everything that is not fat. The BHF lists what a bathroom scale is really weighing: your bones, muscle, blood, organs and fat, all together.¹

That is the point of the idea. A scale gives one number. It cannot tell how much of your weight is fat and how much is bone or muscle, nor where the fat is located.¹ Body composition adds the missing information.

Fat and fat-free mass in everyday terms

A simple way to picture it is four things: fat, muscle, bone and water. That is a teaching simplification, not a scientific model. Researchers describe the body at several levels at once, from atoms and molecules to cells, tissues and organs and the whole body.⁶⁷ A 2025 review of the field says investigators can now quantify every major component at those levels.⁶ The author declares that he serves on the Medical Advisory Board of Tanita, a body composition scale maker, and of three other companies, which is worth knowing when reading its enthusiasm.⁶

Fat is not simply bad. Which? explains that some “essential fat” is needed to stay healthy, because it helps protect internal organs, stores fuel for energy and regulates important hormones.⁴ It also says women typically need a higher body fat percentage than men and that there is no universally agreed ideal.⁴ The BHF says where the fat sits matters too: belly fat is linked to higher risks of heart and circulatory diseases and type 2 diabetes.¹

Why can two people at the same weight have different body compositions?

Because weight is the sum of the parts, and the parts can differ. The numbers here are invented to show the arithmetic. Take two adults who each weigh 12 st 7 lb. A stone is 14 lb, so that is 12 x 14 + 7 = 175 lb, and one pound is 0.45359237 kg, so each weighs 79.4 kg. One has 20% body fat and the other 30%.

Both adults would stand on a scale and read 12 st 7 lb. Only the split shows the difference.

It also explains why a number on the scale can stay put while your body changes. The BHF notes that your weight may not fall even while you lose fat, because you may also be gaining muscle as you make healthy changes.¹ There is a small energy effect too. Wang and colleagues re-evaluated tissue rates and report resting use of about 13 kcal per kilogram per day for skeletal muscle and 4.5 for fat tissue,¹⁰ so 10 kg of muscle would use around 130 kcal a day at rest against 45 for 10 kg of fat. The gap is 85 kcal a day, which is real but modest. The BHF puts it simply: the more muscle you have, the more calories you burn at rest.¹⁷

How is body composition measured?

The BHF describes DEXA, underwater weighing and BOD POD air displacement as specialist methods, and skinfold callipers and smart scales as things you can use yourself.¹ The table below adds what the other sources report. Costs are the BHF’s, from a page updated on 21 May 2024.

How bio-impedance works, in a sentence

The BHF explains that a smart scale sends tiny electrical impulses through the body and measures how quickly they return, since current flows more easily through water-rich tissue such as muscle and blood than through fat or bone.¹ A European review (ESPEN) says the method can determine fat-free mass and total body water in people without significant fluid or electrolyte abnormalities when appropriate equations are used, and that it suffers from a lack of standardised method and quality control.⁸ In other words, the number depends on the equation inside the device.

That is why a scale’s figure is an estimate. The detail is in the guides to smart scale accuracy and body fat percentage.

How does body composition relate to BMI and waist measures?

BMI and waist-to-height ratio are the measures the NHS and NICE use, and neither says anything direct about composition. The NHS says the BMI calculator cannot tell muscle from fat, so a very muscular person may be classed as overweight or obese without much body fat.³ The BHF says BMI is inaccurate for pregnant people and for people with a lot of muscle, and that excess fat around the waist can raise cardiovascular risk even with a healthy BMI.²

NICE guideline NG246 says to interpret BMI with caution in adults with high muscle mass, and in people aged 65 and over, because a slightly higher BMI may be protective when older.⁵ It also says that in adults with a BMI below 35, waist-to-height ratio should be measured as well as BMI, and that bioimpedance should not be used as a substitute for BMI as a measure of general adiposity.⁵

Research agrees that BMI blurs composition. In a study of 13,601 US adults, BMI failed to discriminate between body fat percentage and lean mass in both sexes in the intermediate range of 25 to 29.9.¹¹ So the national pairing is BMI plus a tape measure. A body composition scale is an extra, not a replacement. See the guides to BMI and healthy weight and waist measurement.

Muscle, strength and age

The NHS recommends adults do strengthening activities that work all the major muscle groups on at least 2 days a week, alongside 150 minutes of moderate or 75 minutes of vigorous activity.¹⁶ To get benefit from strength exercises, you should do them to the point where you need a short rest before repeating.¹⁶

For older adults, the revised European consensus on sarcopenia (EWGSOP2) describes it as a muscle disease rooted in adverse muscle changes that build up across a lifetime, common among older adults but also occurring earlier in life.⁹ No percentage of muscle loss by decade was found in the sources read, so this guide does not give one, and it does not suggest that muscle loss is inevitable.

Reading a home scale: what to follow and what to ignore

Saga found smart scales very accurate for weight and significantly less so for body composition, and concluded that trends over time are more reliable than any one-off reading. A body fat percentage falling over three months, for example, would probably reflect a real change.¹³ The scale is therefore best used as a guide.¹³

• Follow the weight trend, weighing once a week. An NHS weight-loss guide advises this because weight fluctuates from day to day.¹⁸
• Follow your waist-to-height ratio with a tape measure, as NICE does for adults with a BMI under 35.⁵
• Follow body fat and fat-free mass as a trend only, in the same conditions each time. The BHF advises measuring at the same time of day in similar conditions.¹
• Ignore single-day jumps. Chutex says on the Vita page that its bio-impedance readings are indicative and vary with hydration, meals, recent exercise and time of day.¹⁹
• Ignore any number you are tempted to treat as a target. Which? notes that scale makers cannot agree on an ideal body fat percentage.⁴

One safety point. The BHF says people with a pacemaker should avoid electronic body fat scales completely.²⁰ If you have any implanted electronic device, ask the team that looks after it before using one.

Frequently asked questions