Written by Dr. Ashutosh A Shah, M.B.B.S., M.S., M.Ch., D.N.B., Board Certified Plastic Surgeon, Body Reform Clinic, Surat. Reg. no. [REG NO]. 22+ years in plastic and reconstructive surgery.
Medically reviewed by Dr. Ashutosh A Shah · Published 30 September 2026 · Last reviewed 30 September 2026
BMI is weight in kilograms divided by height in metres squared. It is a screening tool that flags who may need further assessment, not a measure of health on its own. It cannot tell muscle from fat or show where fat is stored, which is why it is used alongside other measurements.
BMI was designed to describe populations. Most of the arguments about it come from using it to describe one person.
It is genuinely useful for what it was built for, and genuinely poor at several things it gets used for. Knowing which is which makes the number far easier to interpret.
What is BMI and how is it calculated?
Your weight in kilograms divided by your height in metres squared. That is the whole calculation, and noticing what goes into it tells you most of what you need to know about its limits.
Two inputs: weight and height. Nothing else. No measure of what the weight is made of, no measure of where it sits on the body, no measure of anything happening inside it.
The NHS gives the general adult categories as: "a BMI of 25 to 29.9 is overweight a BMI of 30 or more is obese".
Those categories are not universal, and the next few sections are largely about why.
What is BMI genuinely useful for?
Sorting large numbers of people quickly into those who probably need a closer look and those who probably do not. That is a real job and BMI does it cheaply and consistently.
Where it earns its place:
- Population level work, where the aim is to describe a group rather than an individual, which is what it was designed for.
- A starting point in a consultation, where it tells a clinician whether to measure further.
- Tracking change in one person over time, where the errors stay roughly constant and the direction of travel is still informative.
- Consistency, since two people measuring the same person get the same answer, which is not true of several more sophisticated methods.
- Thresholds in guidelines, where a reproducible number is needed to make criteria workable.
The common criticism that BMI is unscientific misses this. It is a screening instrument and it performs adequately as one. The problem is almost always what it is asked to do next.
Why can two people with the same BMI have very different health?
Because BMI measures weight, and weight is made of different things in different people. The NHS states the central limitation directly: "The BMI calculator cannot tell the difference between muscle and fat."
Its full explanation: "For example, if you have a lot of muscle, it's possible the BMI calculator may put you in the overweight or obese category even if you do not have a lot of body fat."
What follows from that, in both directions:
- A muscular person can be categorised as overweight or obese with little body fat, which is the best known failure and the one most often cited.
- Someone with a normal BMI can carry a great deal of fat and relatively little muscle, which is less often discussed and is the more common situation.
- Two people at the same BMI can store fat completely differently, which BMI cannot see at all.
- Age changes the composition behind the same number.
The second point deserves more attention than it gets. A great deal of commentary about BMI focuses on athletes being misclassified, which is real but rare. A normal BMI with a high waist measurement is considerably more common and is the case where BMI is most likely to be falsely reassuring.
Why do BMI thresholds differ for Indian and other Asian populations?
Because risk appears at a lower BMI, so the same number means something different. This is not a minor adjustment and it is the most practically important thing on this page for an Indian reader.
The NHS sets it out explicitly: "If you are from a South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background you're at risk of health problems with a slightly lower BMI, so: a BMI of 23 to 27.4 is overweight a BMI of 27.5 or higher is obese".
Compare the two sets. The general obese threshold is 30. For these backgrounds it is 27.5. A person at BMI 28 is below the general threshold and above this one, and the number on the app has not changed at all.
Indian surgical guidance runs in the same direction, and states the reasoning openly. The Obesity and Metabolic Surgery Society of India, in its clinical guidelines for bariatric and metabolic surgery, says: "Asians, and in particular the Indian population needs a more aggressive approach in treating obesity and its co-morbidities as it is associated with higher morbidity and mortality at a relatively lower BMI."
Its surgical criteria follow from that, and they are lower than the Western equivalents:
- "BMI above 37.5 kg/m2 without co-morbidities"
- "BMI above 32.5 kg/m2 with obesity related co-morbidities"
- "BMI > 30 kg/m2 with life threatening obesity related disorders like uncontrolled diabetes, cardiovascular disease / cardiomyopathy and severe obstructive sleep apnoea"
Two independent bodies, one describing general health risk and one describing surgical criteria, arriving at lower thresholds for the same populations. That agreement is what makes this worth acting on rather than debating.
If you have been reading BMI advice written for Western populations, and a great deal of what circulates online is, the categories you have been applying to yourself are probably the wrong ones.
What does waist measurement add that BMI cannot?
Where the weight sits, which BMI cannot express at all. The NHS gives a target that requires no calculation: "You should try to keep your waist size to less than half your height."
That is unusually practical. It needs a tape measure and no arithmetic, and because it scales with height it does not need separate figures for different builds.
| Measure | What it captures | What it misses | Why it is used |
|---|---|---|---|
| BMI | Weight relative to height | Muscle versus fat, and where fat sits | Fast, consistent, workable in guidelines |
| Waist measurement | Where weight is carried around the middle | Total body composition | Adds what BMI structurally cannot |
| Waist to height | The same, scaled to build | Composition elsewhere on the body | NHS target needing no calculation |
| Blood tests | What is actually happening metabolically | Nothing about body shape | Measures consequences rather than proxies |
| Blood pressure | A direct clinical measurement | Everything about composition | Independent of any body measurement |
Notice the bottom two rows. Blood tests and blood pressure measure what everyone is actually worried about when they worry about BMI, and they do not depend on body measurement at all.
Why is where fat is stored more important than how much there is?
Because guidance treats it as a separate question with its own target, which is the clearest evidence that total weight does not capture it. Waist is not measured for completeness. It is measured because it tells you something weight and height cannot.
An honest note about how far this page will go. The biological explanation for why abdominal fat differs from fat elsewhere is widely described, and I did not find a source I could verify to the standard used for everything else here. So this page does not explain the mechanism.
What it can say, from the guidance itself, is narrower and still useful:
- Waist has its own target, separate from BMI, in NHS guidance.
- That target is expressed as a ratio to height, not as a weight, which means it is measuring something weight does not.
- A normal BMI with a large waist is a recognised combination, and it is the one where BMI alone is most misleading.
- Both are measured in practice, which is itself the answer to whether one replaces the other.
If you take one practical action from this page, measure your waist and compare it with half your height. It takes a minute and it tells you something your BMI cannot.
Which measurements belong alongside BMI in a proper assessment?
Waist, blood pressure, and blood tests appropriate to the picture. BMI on its own is a starting point, not an assessment.
What a reasonable assessment includes:
- BMI, interpreted against the right thresholds for your background rather than the general ones.
- Waist measurement, compared with half your height.
- Blood pressure, which is quick and independent of any of this.
- Blood tests where the picture warrants them, which a clinician decides, covering the metabolic questions that body measurements only approximate.
- History, including what has changed and over what period, which no single measurement captures.
- What you can actually do, since an assessment that ends in a number nobody can act on has not achieved anything.
The reason this matters practically: a person can be reassured by a normal BMI while several of these are abnormal, and a person can be alarmed by a high BMI while the rest are fine. Only measuring the first thing produces both errors.
When does a BMI figure actually change a clinical decision?
Mainly where guidelines use it as a threshold, and the clearest example is eligibility for surgery, where the figures above are applied alongside the rest of an assessment rather than on their own.
Who those criteria are actually for, what else is assessed, and what the process involves is covered in our page on who bariatric surgery is actually for, which is the right place for that question rather than this one.
Why is BMI a poor way to talk to yourself about your body?
Because it was built to describe populations, and applying a population instrument to yourself is a category error rather than a personal verdict. A number designed to sort thousands of people into rough groups was never meant to tell one person anything about their worth or their effort.
Three things worth separating:
- BMI is a screening measure. Screening measures are built to be over-inclusive on purpose, because the cost of a false flag is a further test and the cost of a miss is worse.
- A number above a threshold is a prompt to look further, not a diagnosis and not a judgement.
- A number below a threshold is not a clearance, which is the half of this that rarely gets said, particularly by anyone with something to sell.
That third point is why this page argues in both directions. A page that only tells you a high BMI might not be a problem, while leaving a normal BMI unexamined, is still using the number to sort people. Both errors are the same error.
What is more useful than a BMI figure, if you want something to hold onto: what has changed recently, how you actually feel, your waist compared with half your height, and what your blood pressure and blood tests show. Those describe you. BMI describes a group you happen to fall into.
How is this assessed at the clinic in Surat?
By measuring more than one thing, and by interpreting BMI against the thresholds that apply to Indian populations rather than the general ones. In our practice in Surat, people frequently arrive having already been given a number by an app and having drawn a conclusion from it.
The conclusion is often wrong in one of two ways. Someone with a BMI in the twenties has decided they are fine, without anyone having measured a waist or checked a blood pressure. Or someone has been alarmed by a figure that is being read against Western categories when the relevant ones sit lower.
So the first thing that happens is measurement rather than discussion: waist, blood pressure, and blood tests where the picture calls for them. That takes very little time and it changes the interpretation of the BMI figure often enough to be worth doing in every case.
Where all of it is reassuring, that is the answer and nothing follows from it. Where it is not, the conversation is about what the other measurements showed, not about the BMI number, because the number was only ever the prompt to look. Related pages: why weight comes back, protein during weight loss, and body contouring after bariatric surgery.
Next step
The useful action is not to recalculate your BMI. It is to measure your waist against half your height, and to have blood pressure and appropriate blood tests done, because those describe what is actually happening. Book a consultation with Dr. Ashutosh Shah at Body Reform Clinic, Surat, if you would like that done properly rather than estimated.
This page is for education and is not a substitute for assessment. BMI is a screening measure and a figure above or below any threshold is not a diagnosis in either direction. Thresholds quoted for Indian and other populations come from the sources named and linked. Please consult Dr. Ashutosh A Shah or your own doctor about your situation.
Frequently Asked Questions
What is BMI and how is it calculated?
BMI is your weight in kilograms divided by your height in metres squared. It uses only those two inputs, which is why it cannot account for muscle, body composition or where weight is carried. The NHS gives general adult categories of 25 to 29.9 as overweight and 30 or more as obese.
Is BMI an accurate measure of health?
It is an accurate screening tool and a poor measure of individual health. The NHS notes it cannot tell the difference between muscle and fat. It flags who may need further assessment rather than describing anyone's health, which is why other measurements are taken alongside it.
Why are BMI thresholds lower for Indians?
Because health risk appears at a lower BMI. NHS guidance sets overweight at 23 to 27.4 and obese at 27.5 or higher for South Asian and several other backgrounds. Indian surgical guidance gives the same reasoning: higher morbidity and mortality occur at a relatively lower BMI.
What is a healthy waist measurement?
NHS guidance is to keep your waist size to less than half your height. That needs a tape measure and no calculation, and it scales with build, so it works without separate figures for different heights. It measures something BMI structurally cannot.
Can you be healthy with a high BMI?
BMI alone cannot answer that, which is the point. It cannot distinguish muscle from fat or show where weight is carried. A figure above a threshold is a prompt for further assessment, not a diagnosis, and waist, blood pressure and blood tests carry the actual answer.
Does BMI work for muscular people?
Poorly. The NHS notes that someone with a lot of muscle may be put in the overweight or obese category even without much body fat. The reverse error matters more in practice: a normal BMI with a large waist is common and BMI is falsely reassuring there.
What should be measured instead of BMI alone?
Waist measurement compared with half your height, blood pressure, and blood tests appropriate to the picture, alongside history of what has changed. BMI stays useful as a starting point, interpreted against the thresholds that apply to your background rather than general ones.
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About Dr. Ashutosh Shah
Dr. Ashutosh Shah, M.S., M.Ch. (Plastic Surgery), has over 22 years of experience in body contouring after weight loss. He leads Elegance Clinic, Surat, with centres at Adajan and Vesu, and is known for honest counseling and natural looking results.