BMI is a quick screening number that can point you toward a conversation about your health, but it was never designed to describe any single person on its own.
What BMI actually measures
Body Mass Index, or BMI, is a single number that compares your weight to your height. Doctors and researchers use it as a quick screening tool across large groups of people. It gives a rough sense of whether someone carries more weight than is typical for their height, and it is easy to work out with a scale and a tape measure.
What it does not do is measure fat directly, or tell you where that weight sits, or say anything about how fit and healthy you are. Keep that in mind as we go.
How it is calculated
The sum is simple. You take your weight in kilograms and divide it by your height in metres, squared.
Say you weigh 90 kg and you are 1.70 m tall. First multiply your height by itself: 1.70 x 1.70 = 2.89. Then divide: 90 divided by 2.89 gives a BMI of about 31. You do not need to do the maths by hand. Most online calculators and every registered doctor can work it out in seconds.
Reading your number
The standard categories used around the world, and here in South Africa, are these:
- Below 18.5 is classed as underweight.
- 18.5 to 24.9 is the healthy weight range.
- 25 to 29.9 is classed as overweight.
- 30 and above is classed as obesity.
These bands are guides, not verdicts. Two people with the same BMI can have very different health, which is exactly why the number needs context.
Where BMI falls short
The biggest limit is that BMI cannot tell muscle from fat. It only knows total weight.
Muscle and fat weigh differently
A rugby forward or a regular gym-goer can carry a lot of muscle. Muscle is dense, so it adds weight without adding fat. That person might land in the overweight or obese band on paper while being genuinely fit. The reverse also happens. Someone with little muscle and more fat can sit in the healthy range yet carry risk that the number hides.
It ignores where fat sits
Fat around the middle carries more health risk than fat on the hips and thighs. BMI treats both the same. This is one reason a doctor may also measure your waist. For many people, waist size adds useful information that BMI alone misses.
It does not fit everyone equally
BMI was built mainly from older studies of European populations. It can read differently across ages, body types and ethnic groups. Older adults lose muscle naturally, which shifts the picture. For some groups, health risks rise at a lower BMI than the standard bands suggest. A number on its own cannot account for any of this.
Why it is still worth knowing
None of this makes BMI useless. It is cheap, fast and consistent, and it flags patterns worth a closer look. Think of it as the first line of a conversation rather than the conclusion. If your BMI sits above the healthy range, that is a reason to talk to a doctor, not a diagnosis in itself.
A fuller picture usually includes your waist measurement, your blood pressure, your blood sugar and cholesterol, your family history and how you actually feel day to day. An HPCSA-registered doctor puts those pieces together in a way no calculator can.
BMI and treatment eligibility
BMI does play a defined role when doctors consider whether GLP-1 medication may suit someone. As a general guide, treatment is usually considered at a BMI of 30 or above, or from a BMI of 27 when there is a weight-related condition such as type 2 diabetes, high blood pressure, high cholesterol, sleep apnoea or fatty liver.
This class of treatment is not suitable for everyone. It is not used in pregnancy or while breastfeeding, or for anyone with a personal or family history of medullary thyroid cancer or MEN 2. Results also vary from person to person, and the eligibility line is a starting point, not the whole assessment.
The bottom line
Your BMI is one clue among several. Work it out, note which band it falls in, then use it to open a proper conversation about your health rather than to judge yourself by a single figure.
This is general information and not medical advice. A registered doctor reviews your full health picture and decides what, if anything, suits you as an individual.
If your BMI sits in the overweight or obese range and you are wondering what your options are, a short eligibility check with a registered doctor is a sensible next step.
The short version
- BMI is your weight in kilograms divided by your height in metres squared, and 30 or above is classed as obesity.
- It cannot tell muscle from fat or show where weight sits, so a fit, muscular person can read as overweight.
- Treat BMI as a starting point that opens a conversation, alongside waist size, blood pressure and blood sugar.
- Doctors generally consider GLP-1 medication at a BMI of 30, or 27 with a weight-related condition, but results vary and a registered doctor decides.
See if this could be an option for you
A two-minute eligibility check, reviewed by a doctor registered with the HPCSA. No obligation, and you only pay if a doctor decides treatment is right for you.
Check if you qualifyWondering if this applies to you?
The eligibility check is free and takes about two minutes.
This article is general information, not medical advice, and it cannot take the place of a consultation. GLP-1 medicines require assessment by a doctor registered with the HPCSA. Speak to a doctor about your own circumstances. See our medical disclaimer and editorial policy.