About the BMI Calculator
Body mass index is weight divided by height squared. The squared is the interesting part, because people are not two-dimensional.
This BMI calculator gives you the figure, the healthy weight range for your height rather than a single cut-off, and something most BMI calculators leave out: how much of your reading is an artefact of how tall you are.
Adolphe Quetelet devised the index in the 1830s and said plainly it was for describing populations, not for judging individuals. That caveat has been dropped somewhere along the way, and it matters.
How to Use the BMI Calculator
Enter your height and weight in metric or imperial — results come back in whichever you chose.
The threshold set offers the standard WHO bands and the lower ones used for South and East Asian populations, where the healthy band ends at 23 rather than 25.
Step-by-Step Example
175cm, 78kg.
78 ÷ (1.75)² = 25.5That is just inside the overweight band. The healthy range for 175cm is 56.7kg to 76.6kg, so it is 1.4kg above the top of it.
Which is a good illustration of why the single number is a poor way to present this. A figure of 25.5 sounds like a category; 1.4kg sounds like what it is.
The Height Bias Is Arithmetic, Not Opinion
Here is the part worth understanding, because it is provable rather than arguable.
Take any person and scale them up geometrically — every dimension 10% larger, proportions identical.
Weight goes with volume: × 1.1³ = 1.331 (up 33%)
Height goes with length: × 1.1 = 1.1 (up 10%)
BMI = weight ÷ height² → 1.331 ÷ 1.21 = 1.1A person 10% taller, built exactly the same, has a BMI 10% higher.
Not the same BMI. Ten per cent higher, purely from being larger.
The index therefore reads tall people as heavier and short people as lighter than their build warrants. The exponent 2 was chosen because it fitted population data reasonably, not because bodies scale that way — bodies scale by roughly 3. Proposals to use 2.5, or the ponderal index's 3, exist precisely because of this.
The calculator shows what your build would score at a different height, so the size of the bias in your own case is a number rather than a footnote.
It Cannot See What Weight Is Made Of
BMI has no way to distinguish muscle from fat. None. A lean, muscular person and a sedentary person of the same height and weight score identically.
Muscle is denser than fat, so a well-trained athlete frequently lands in the overweight band and occasionally in the obese one. That is not the index failing at its job — it is the index doing exactly what it does, which is divide two numbers that say nothing about composition.
If you carry significant muscle, a reading in the overweight band tells you very little.
Different Thresholds for Different Populations
The WHO recommends lower cut-offs for South and East Asian populations, because cardiometabolic risk — type 2 diabetes in particular — appears at a lower BMI.
| Band | Standard | Lower thresholds |
|---|---|---|
| Healthy | 18.5–25 | 18.5–23 |
| Overweight | 25–30 | 23–27.5 |
| Obese | 30+ | 27.5+ |
The same figure classifies differently. A BMI of 23.8 is a healthy weight on the standard scale and overweight on the lower one, and the calculator tells you what the other scale would have said.
This is not a minor adjustment. It shifts the healthy ceiling by two full points, which on a 175cm frame is about 6kg.
A Better Single Number
If you want one measurement rather than an index, use waist divided by height, kept under 0.5.
It works because it measures where weight is carried rather than how much of it there is, and abdominal fat is what most strongly predicts metabolic risk. It needs no assumption about build, it is not biased by height — the ratio cancels height out — and it needs nothing but a tape measure.
BMI is a reasonable screening figure with known biases. Waist-to-height is a better one. Both together are better than either.
Understanding Your Result
Your BMI gives the figure and the band it falls in.
Healthy weight for your height converts the band into actual weights, which is the more useful form.
Distance to that range says how far you are from the nearest edge, in the units you entered.
The height bias shows what the same build would score at a different height.
Worth knowing flags the threshold set, the muscle caveat, or what to measure instead.
When Should You Use This Calculator?
As a rough screen. That is what it is for and what it does reasonably well.
To find a target weight range. The range is more useful than the index.
Alongside a waist measurement. The two together cover what either misses.
Not to judge an athlete. Or anyone whose build is unusual in either direction.
Common Mistakes
Treating BMI as a measure of body fat. It cannot distinguish muscle from fat.
Ignoring the height bias. A taller person of identical build scores higher.
Using the standard thresholds for everyone. The lower set exists for good reason.
Reading a single number as a verdict. The healthy band is wide and the edges are not cliffs.
Applying it to children. They are assessed on age and sex percentile charts, not fixed thresholds.
Using it during pregnancy. It is not meaningful, and the adult bands do not apply.
BMI is a screening tool, not a diagnosis, and it says nothing about body composition, fat distribution, fitness or overall health. Nothing here is medical advice — discuss any concern about your weight with a doctor, who can interpret it alongside the rest of your health.