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Body composition vs BMI: what’s the difference and why it matters
Written byJodi M Johnson
Reviewed byOlivia Rosenvinge
Think BMI tells the full story? One of our nutritionists reveals why body composition gives a clearer picture of your health – and why it matters more than you think
Summary
1Why body composition matters
“The number on the scale doesn’t tell you the full story. Two people can weigh the same but have very different bodies. One…”
2BMI vs body composition
“BMI doesn’t consider what your weight is made of. It assumes that excess weight means excess fat, but our bodies are…”
3How to change your body composition
“Improving your body composition isn’t about dropping weight, it’s about shifting the balance between fat and muscle in a…”
Most of us have a rough idea of what we weigh or where we sit on the BMI scale – but those stats don’t tell the whole story. Body composition may reveal more of what’s really going on inside.
By looking at what your body is actually made of – fat, muscle, water and minerals – you can uncover insights the scales alone can’t provide. We spoke to our nutritionist, Olivia Rosenvinge, to explain why understanding body composition could completely change the way you view your health.
What’s body composition?
Body composition measures the proteins, fats, minerals and water that make you, you. But in the diet and fitness world, the spotlight often falls on pounds – lost or gained – rather than what’s actually going on inside.
Why body composition matters
“The number on the scale doesn’t tell you the full story. Two people can weigh the same but have very different bodies. One may have more muscle, the other more fat. That’s why body composition is so important,” says Olivia.
Body composition gives a far more meaningful picture of both your health and how your body actually looks.1
If you're strength training, knowing your muscle and fat percentages can be a real game-changer. It helps you track real progress – like getting stronger or leaner – rather than relying on a number on the scales that can jump up and down for all sorts of reasons.
Another big benefit of knowing your body composition is that it helps you understand whether you need to lose, maintain or gain fat and muscle to support your health. It shifts the focus to look at what your body is actually made of: fat, muscle, bone, water and not just your total weight.
“Focusing on weight alone can be misleading, especially if you’re trying to gain muscle or lose fat, but the scale stays the same. Body composition can give a clearer, more meaningful picture of your health and progress,” says Olivia.1
This kind of insight also makes it easier to set realistic, personalised health and fitness goals that actually reflect what your body needs (not just what the scales say).
BMI vs body composition
BMI is one of the most commonly used ways to assess health, based on a simple calculation of your height and weight. It then puts you into one of these categories: underweight, healthy, overweight or obese.
“BMI doesn’t consider what your weight is made of. It assumes that excess weight means excess fat, but our bodies are much more complex.
“As we’re made up of muscle, bone, organs and water, someone with lots of muscle, like an athlete, could be labelled ‘overweight’ or ‘obese’ even if they’re in great shape,” says Olivia. While it can be a useful general guide, it has some pretty big limitations.
On the flip side, someone who looks slim and falls into the ‘healthy’ BMI range might actually have a high amount of body fat, especially around their organs, and be at greater health risk.2
“For most people, aiming for a BMI between 18.5 and 24.9 can be a useful guide.3 But it doesn’t work for everyone.” BMI also doesn’t take into account differences between ethnic groups or diverse body types, so it doesn’t always reflect everyone accurately. That’s where body composition comes in, it looks beyond the numbers to show you what’s really going on beneath the surface.3
“So, is BMI helpful? Yes, but only as a rough guide. When combined with waist measurements and, even better, body composition data, you get a much more accurate picture of health.” says Olivia.
How to use body composition to track your health
Using body composition to track your health is all about getting a clearer picture of what’s really going on inside your body. Rather than obsessing over a single number on the scale, it helps you focus on what truly matters – like how much muscle you’re building, how much fat you’re losing and whether your body is properly hydrated.
“Monitoring your body composition can give you valuable insights into your health and progress, especially if you're working on changing your body. Instead of focusing on weight loss alone, you can track whether you’re reducing fat, gaining or preserving muscle, and even see how visceral fat (the fat around your organs) is changing, which is key for long-term health,” says Olivia.4
You might aim to increase muscle if you’re strength training, reduce fat for better metabolic health or simply work on maintaining a good balance overall.
What’s really powerful about body composition is how it reveals changes that weight alone can’t. Your weight might stay exactly the same, but if you’ve lost fat and gained muscle, your body is changing in all the right ways. “It helps you know if a fitness or nutrition plan is actually working, it may help prevent muscle loss during weight loss and helps you celebrate progress that might not show up on the scale,” says Olivia.5,6
That’s why tracking over time is so important. A single snapshot might not tell you much, but regular check-ins can show progress that the scales miss, helping you stay motivated and make informed choices that actually support your long-term health.
How to calculate body composition: which method is best?
At-home methods
Bioelectric Impedance Analysis (BIA) might sound a bit technical, but it’s actually a simple and non-invasive way to measure body composition – and you might already have access to it at your gym or even in your bathroom scales at home.
“BIA is found in many smart scales. Quick and easy, but results can vary based on hydration and other factors,” says Olivia.7
It works by sending a tiny, harmless electrical current through your body to measure how much water you’re holding. Because muscle contains more water, the current passes through it easily, while fat slows it down. By measuring how quickly the signal moves, the device can estimate things like body fat, muscle mass and overall hydration. It’s a clever bit of science that helps you get a much clearer picture of what your body is made of.7
Skinfold callipers – often called the pinch test – are a hands-on way to estimate your body fat percentage. “They work by gently pinching the fat just under your skin at various points on your body to measure its thickness. It’s a simple method that’s fairly easy to access, but it does rely on having someone who knows what they’re doing to get the most accurate results,” says Olivia.8
It’s also worth knowing that it might not be as reliable for everyone, especially if fat is distributed differently around the body. Still, when done properly, it can offer a useful snapshot of your body composition.
Lab-based methods
Air displacement plethysmography is a bit like hydrostatic weighing, but without getting wet. Instead, you sit in a special chamber (“known as a BodPods”, says Olivia) that measures how much air your body displaces. This change in pressure is used to calculate your body volume and from that, your body fat. Olivia adds that they’re “comfortable and fairly accurate”. 9
“DEXA is highly accurate and gives detailed information about fat, muscle and bone. It’s usually done in clinics,” says Olivia.
DEXA uses two different X-ray beams to measure your bone density, muscle mass and fat. It’s often used in clinical settings and provides a very detailed breakdown of your body composition.10
MRI takes it a step further by using powerful magnets to scan your body in detail. It can measure both fat mass and muscle mass, and even tell the difference between the fat stored just under your skin and the more harmful visceral fat around your organs.11
“MRIs are the gold standard, especially for visceral fat, but they’re mostly used in research or medical settings,” says Olivia.11
Hydrostatic weighing works by measuring your weight on land and then again underwater. By comparing the two, it can calculate your body’s volume and give a very accurate estimate of your total body fat. It’s often seen as one of the gold-standard methods for measuring body composition. Olivia says, “Hydrostatic weighing is very accurate but not widely available”.12
How to change your body composition
“Improving your body composition isn’t about dropping weight, it’s about shifting the balance between fat and muscle in a healthy way.
Olivia shares some tips on how we can do this:
- strength training helps you build and maintain muscle
- eating enough protein can support muscle growth
- cardio supports fat loss and overall health
- getting good sleep and managing stress helps to regulate hormones that impact fat storage and muscle recovery
“Consistency is key and sustainable habits lead to lasting change. Weight is just one number. Body composition gives you a fuller, more meaningful picture of your health. Whether you’re working on fitness, fat loss or simply want to feel better in your body, understanding what you're made of matters far more than just how much you weigh”, says Olivia.
The final say
Understanding your body goes beyond the number on the scales or a one-size-fits-all calculation like BMI. While these can offer a rough guide, it's your body composition that tells the real story about your health and fitness.
With expert advice and smarter tools available, it's easier than ever to shift the focus from simply losing weight to building a stronger, healthier you. So whether you're just starting out or refining your routine, remember: it's not about being lighter, it's about being better balanced – and this can be achieved by prioritising what really matters.
And now you’re all clued up on body composition and BMI, learn more about how many calories you burn at rest? Your BMR explained.
This article provides informational advice and is not a substitute for medical care. Curated by experts for accuracy, we take great care to ensure the information is up-to-date and relevant. However, you should always consult your GP or healthcare professional before using supplements or alternative products, particularly if you have medical conditions or are under supervision.
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2. Van der Kuip M, de Meer K, Oosterveld MJS, Lafeber HN, Gemke RJBJ. Simple and accurate assessment of energy expenditure in ventilated paediatric intensive care patients. Clinical Nutrition. 2004 Aug;23(4):657–63.
3. NHS . Diagnosis - Obesity [Internet]. NHS. 2023. Available from: https://www.nhs.uk/conditions/obesity/diagnosis/
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5. Cruz P, Johnson BD, Karpinski SC, Limoges KA, Warren BA, Olsen KD, et al. Validity of Weight Loss to Estimate Improvement in Body Composition in Individuals Attending a Wellness Center. Obesity. 2011 Nov;19(11):2274–9.
6. Willoughby D, Hewlings S, Kalman D. Body Composition Changes in Weight Loss: Strategies and Supplementation for Maintaining Lean Body Mass, a Brief Review. Nutrients [Internet]. 2018 Dec 3;10(12):1876. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6315740/
7. Wang Y, Beydoun MA, Liang L, Caballero B, Kumanyika SK. Will All Americans Become Overweight or Obese? Estimating the Progression and Cost of the US Obesity Epidemic. Obesity. 2008 Oct;16(10):2323–30.
8. Bistrian B. Skinfold Calliper - an overview | ScienceDirect Topics [Internet]. www.sciencedirect.com. 2012. Available from: https://www.sciencedirect.com/topics/nursing-and-health-professions/skinfold-calliper
9. Fields DA, Goran MI, McCrory MA. Body-composition assessment via air-displacement plethysmography in adults and children: a review. The American Journal of Clinical Nutrition [Internet]. 2002 Mar 1;75(3):453–67. Available from: https://academic.oup.com/ajcn/article/75/3/453/4689336#109557184
10. Shepherd JA, Ng BK, Sommer MJ, Heymsfield SB. Body composition by DXA. Bone [Internet]. 2017 Nov;104(104):101–5. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5659281/
11. Després JP. Body Fat Distribution and Risk of Cardiovascular Disease. Circulation. 2012 Sep 4;126(10):1301–13.
12. Hydrostatic Weighing - an overview | ScienceDirect Topics [Internet]. www.sciencedirect.com. Available from: https://www.sciencedirect.com/topics/medicine-and-dentistry/hydrostatic-weighing
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