What BMI tells you, and what it doesn't
What is BMI and how is it calculated?
Body mass index is your weight divided by your height squared:
An 82 kg person at 1.80 m has a BMI of 25.3. That is the entire calculation — it uses two numbers and knows nothing else about you.
What do the categories mean?
| BMI | Category |
|---|---|
| under 18.5 | Underweight |
| 18.5 – 24.9 | Healthy range |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obesity |
These bands come from population studies of health risk [1]. They describe how risk trends across large groups — they are not a diagnosis of any individual.
What BMI genuinely cannot see
It has no way to distinguish muscle from fat. A lean, heavily trained person can land in the "overweight" band on muscle alone, while someone with very little muscle can sit inside the healthy range while carrying a lot of fat.
It is also blind to where fat sits, and that matters more than the total. Fat around the organs carries considerably more metabolic risk than the same weight on hips and thighs — which is why waist circumference adds information BMI cannot [2].
And the bands were derived largely from European-ancestry populations. Risk begins to rise at lower BMI values in South Asian and some East Asian populations, so several health bodies apply lower thresholds for them [3].
Then why is it used at all?
Because it needs only a scale and a tape measure, and at population scale it predicts risk well enough to be useful. It is a screening signal, not a verdict — cheap, fast, and roughly right about groups while frequently wrong about individuals.
What should I use instead?
Alongside, not instead. Waist circumference, how your clothes fit, your strength trend, blood markers from a check-up, and the direction your weight is moving over months all say more about you than a single index does. A number that is not changing while your waist is shrinking is good news, and BMI cannot tell you that.
Does JISM use BMI for anything?
For two guardrails, not for your targets. Your calorie and macro numbers are built from resting energy, activity and your goal — BMI is not an input.
Where it does intervene: a weight-loss target is refused below a BMI of 18.5, because coaching a deficit for someone already underweight is harmful regardless of what they asked for. And above a BMI of 30, protein is dosed on adjusted body weight instead of total weight — fat tissue has no protein requirement, so multiplying grams-per-kilo by total mass produces targets that are neither necessary nor achievable.
Keep reading
How your daily calories are calculated
Mifflin-St Jeor, the activity ladder, and the 25% deficit cap.
How much protein do you actually need?
Grams per kilogram by goal, the ceilings, and what to multiply them by.
Sources
- World Health Organization. Obesity: preventing and managing the global epidemic, WHO Technical Report Series 894 — the BMI category thresholds.
- Ross R, et al. Waist circumference as a vital sign in clinical practice: a consensus statement from the IAS and ICCR Working Group on Visceral Obesity. Nat Rev Endocrinol 2020;16:177–189.
- WHO Expert Consultation. Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. Lancet 2004;363:157–163.
- Devine BJ. Drug Intell Clin Pharm 1974 — the ideal-body-weight formula behind adjusted body weight.
- Dickerson RN. Dosing on adjusted body weight in obesity. Nutr Clin Pract 2017.