BMI Is Flawed, But Here's Why It's Still Useful (and What to Use Alongside It)

BMI gets dragged online a lot. Fair enough too. It is a very blunt formula trying to summarise a very non-blunt thing, namely your entire body, with one number. That is a bit like reviewing a suburb based only on the letterbox.

But the internet has overcorrected. BMI is flawed, not useless. For most Australian adults, it is still a handy first-pass screening tool. The mistake is treating it like a final diagnosis instead of what it actually is, which is a cheap, quick and imperfect starting point.

If you want the short version, use our BMI Calculator first, then check your body fat estimate and your BMR if your goal is weight loss, maintenance or muscle gain. That combo tells a much better story than one lonely BMI number ever could.

What BMI actually measures

BMI stands for Body Mass Index. The formula is simple:

BMI = weight in kilograms ÷ height in metres squared

So if you weigh 75 kg and you are 1.75 m tall, your BMI is 24.5.

For most adults, the standard screening categories are:

• Underweight: below 18.5
• Healthy weight: 18.5 to 24.9
• Overweight: 25 to 29.9
• Obesity: 30 and above

Those categories are widely used in Australia, including by GPs and public-health reporting. The important bit is that BMI is a screening tool. It helps flag risk. It does not diagnose health on its own.

Why people say BMI is broken

The criticism is mostly valid.

• It cannot tell muscle from fat. A muscular person can land in the overweight range while being lean and fit.
• It ignores fat distribution. Carrying more fat around the abdomen is generally a bigger health risk than carrying it elsewhere.
• It does not adjust neatly for age, sex, ethnicity or training history.
• It works better for populations than for edge-case individuals.

This is why athletes love calling BMI rubbish. And to be fair, if you are built like a second-rower or spend half your life under a barbell, BMI can absolutely insult you for no good reason.

But here is the catch. Most people are not high-performance athletes. For the general adult population, BMI still tracks broad health risk reasonably well, especially at the higher end.

Why BMI is still useful

BMI has stuck around for a reason.

1. It is fast

You only need height and weight. No scan, no clinic visit, no fancy gear, and no standing half-naked on a machine that looks expensive enough to judge you.

2. It is consistent

Because the formula is standardised, BMI is useful for tracking change over time and comparing broad trends across the population. That consistency is why health agencies still use it in reporting.

3. It still correlates with risk

The Australian Institute of Health and Welfare reports that around 65.8% of Australian adults were overweight or obese in 2022 (ABS National Health Survey). BMI is not perfect, but it is still good enough to identify a very real public-health problem. Higher BMI categories are associated with higher rates of conditions like type 2 diabetes, sleep apnoea and cardiovascular disease.

Think of BMI like a smoke alarm. It does not tell you whether the problem is a burnt crumpet or a kitchen fire, but it is still worth paying attention when it goes off.

What to use alongside BMI

This is where most people go wrong. BMI should not work solo.

Waist circumference

If you only pair BMI with one extra measure, make it waist size. Abdominal fat is linked with greater cardiometabolic risk, and BMI misses that completely. Two people can share the same BMI and have very different waist measurements and health profiles.

Body fat percentage

If you lift weights, play sport, or just suspect BMI is being a bit rude, body fat percentage gives a better estimate of composition. Our Body Fat Calculator uses the Navy method, which is still an estimate, but usually a more useful estimate for muscular or active people than BMI alone.

BMR and calorie planning

If your goal is changing your weight, BMI tells you where you are now. It does not tell you what to do next. That is where BMR comes in. Basal Metabolic Rate is an estimate of how many calories your body uses at rest. It is not your full daily calorie burn, but it is a solid starting point for building a realistic eating plan instead of just vibing your way through MyFitnessPal.

Real health markers

Blood pressure, blood glucose, cholesterol, sleep, movement, alcohol intake and family history all matter. A person can have a healthy BMI and still be unhealthy. A person can also have a higher BMI and have decent metabolic markers. Context matters, which is both useful and slightly annoying.

When BMI is less useful

There are a few situations where BMI deserves some scepticism:

• Very muscular adults, because lean mass can push BMI up
• Older adults, because muscle loss can hide under a normal BMI
• Children and teenagers, who use age-specific growth charts rather than adult BMI ranges
• Pregnancy, where standard interpretation is limited
• Some ethnic groups, where metabolic risk can increase at lower BMI thresholds than standard categories suggest

If you are in one of those buckets, BMI is still data, but it is not the whole answer.

How to use BMI properly in real life

1. Calculate it as a first screen, not a judgment on your worth as a human.
2. Check waist size and body fat if you want more context.
3. Look at trends over months, not tiny daily fluctuations.
4. Use BMR or TDEE if your goal is to change your weight in a practical way.
5. Talk to a GP if the result lines up with other symptoms or risk factors.

That is the grown-up version. The less useful version is calculating BMI once, declaring it either gospel or fake news, then eating chips in a state of philosophical protest.

The bottom line

BMI is flawed. Everyone saying that part is right.

But it is still useful when you understand what it can and cannot do. It is a quick screening tool, not a complete health assessment. Used properly, it helps start better questions. Used badly, it becomes an oversimplified label that misses the nuance.

So do not worship BMI, but do not bin it either. Use it as one tool in the kit, then add body fat, waist size, BMR and real-world health markers on top. That is how you turn a blunt number into something actually useful.