What BMI Actually Tells You (And What It Gets Completely Wrong)

A Quick Recap: What BMI Actually Is

Body Mass Index is calculated by dividing your weight in kilograms by your height in metres squared. A person who is 80kg and 1.75m tall has a BMI of 26.1. The resulting number slots into a category: under 18.5 is underweight, 18.5–24.9 is healthy, 25–29.9 is overweight, and 30 or above is obese.

Calculate yours right now with our BMI calculator — then keep reading to understand what the number is actually telling you.

BMI was developed by Belgian mathematician Adolphe Quetelet in the 1830s. He was studying population statistics, not individual health. The formula was later adopted by insurance companies in the 20th century as a cheap, fast screening proxy for mortality risk. Its clinical use today is a legacy of administrative convenience, not scientific precision.

What BMI Gets Right

To be fair to the metric: at the population level, BMI correlates reasonably well with body fat percentage and with health outcomes including cardiovascular disease, type 2 diabetes, and all-cause mortality. People with very high BMIs (above 35) tend, on average, to carry significantly more body fat and have genuinely elevated health risks.

For screening purposes — identifying people who might benefit from further assessment — BMI is cheap, fast, and requires no equipment. That's why it hasn't disappeared from clinical practice despite its well-documented shortcomings.

Where BMI Falls Apart

The problems start when BMI is applied to individuals rather than populations, and when it's used as a definitive measure rather than a rough screen.

It can't distinguish muscle from fat. A lean, muscular person and an obese person with the same weight and height will have identical BMIs. Elite rugby players, soldiers, and competitive CrossFit athletes routinely score as “overweight” or “obese” despite having very low body fat percentages. The formula has no way to tell the difference.

It ignores where fat is stored. Visceral fat — the fat stored around internal organs in the abdominal cavity — is far more metabolically dangerous than subcutaneous fat stored under the skin. Two people with identical BMIs can have wildly different visceral fat loads and accordingly different health risks. Waist circumference and waist-to-hip ratio capture this risk; BMI does not.

It wasn't calibrated for all ethnicities. The original BMI thresholds were developed using European populations. Research has since shown that people of Asian descent tend to carry a higher proportion of body fat and face elevated metabolic health risks at lower BMIs. Many health bodies now recommend lower BMI thresholds for Asian populations — around 23 for overweight and 27.5 for obese — but the original European thresholds remain the default in most clinical settings.

It ignores age and sex. Women naturally carry more body fat than men at the same BMI. Older adults naturally lose muscle mass, meaning an elderly person can have a “healthy” BMI while actually being sarcopenic (dangerously low muscle mass). BMI treats a 25-year-old male athlete and a 70-year-old woman as interchangeable.

Better Measures to Use Alongside BMI

No single number captures health. Here are the measurements worth tracking alongside BMI:

• Waist circumference: Above 94cm for men and 80cm for women signals elevated metabolic risk. This is a direct proxy for visceral fat, which BMI completely ignores.
• Waist-to-height ratio: Divide your waist circumference by your height. Under 0.5 is the target. This measure accounts for body size in a way raw waist measurement doesn't.
• Body fat percentage: The actual proportion of your body that is fat tissue. Measured via DEXA scan (gold standard), hydrostatic weighing, or smart scales (less accurate). Use our body fat calculator to estimate yours.
• Resting heart rate and blood pressure: Functional cardiovascular markers that BMI doesn't capture at all.
• Blood biomarkers: Fasting glucose, HbA1c, triglycerides, and HDL cholesterol give direct insight into metabolic health that no external measurement can provide.

Check our ideal weight calculator for a range based on multiple methodologies rather than BMI alone.

So Should You Ignore BMI Entirely?

No — but treat it for what it is: a rough, population-level screening tool with significant individual blind spots. A BMI in the healthy range with a large waist circumference is a warning sign worth taking seriously. A BMI in the overweight range with a low waist-to-height ratio and good blood markers is a very different situation to someone with the same BMI but poor metabolic health.

If you're tracking your fitness or weight loss progress, BMI is a useful single data point in a broader picture. It just isn't the picture itself.

For fitness tracking tools and home body composition scales, browse smart scales and fitness tools on Amazon AU — a good smart scale measures body fat percentage, muscle mass, and BMI simultaneously.