How to calculate BMI — and what it does not tell you
Body mass index is one number from two measurements, which is both why it is used everywhere and why it is so often misread. Here is how it is calculated, and — more usefully — what it can and cannot tell you.
The formula
Metric:
BMI = weight in kilograms ÷ (height in metres)²
Imperial:
BMI = (weight in pounds ÷ (height in inches)²) × 703
The 703 is just the unit conversion; both give the same answer.
A worked example
Someone 170 cm tall weighing 68 kg:
height = 1.70 m
1.70² = 2.89
BMI = 68 ÷ 2.89 = 23.5
That falls in the healthy band under the standard classification.
The standard bands
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
The more useful way to read this is backwards. Rather than asking which band you are in, work out the weight range that puts you in the healthy band at your height — multiply 18.5 and 24.9 by your height in metres squared. At 1.70 m that is 53.5 kg to 72.0 kg, which is a target rather than a verdict.
Why the thresholds are lower in India
Health risks associated with a given BMI appear at lower values in South Asian, Chinese and other Asian populations — the same BMI tends to come with more visceral fat and a higher risk of type 2 diabetes and cardiovascular disease. Several health authorities, including in India, use a lower threshold of around 23 for overweight rather than 25, and around 25 for obesity rather than 30.
If that applies to you, read the standard bands as conservative.
What BMI genuinely cannot do
It was devised in the 1830s by a statistician describing populations. It was never intended to assess an individual, and it takes no account of what the weight is made of.
- Muscle weighs more than fat. A trained athlete routinely scores as overweight while carrying very little body fat.
- Muscle loss hides. An older adult who has lost muscle can score in the healthy band while carrying a great deal of fat.
- Location is ignored. Fat around the abdomen carries far more health risk than the same amount elsewhere, and BMI cannot see the difference.
- Children are different. They are assessed against age- and sex-specific percentile charts, because a healthy BMI changes considerably through growth. A fixed band is meaningless for a child.
A better single number
Waist-to-height ratio tracks health risk more closely than BMI, because it reflects where the fat is rather than only how much there is. A widely used rule of thumb: keep your waist under half your height. At 170 cm that means a waist under 85 cm.
It takes one tape measure and no arithmetic, and for most people it is the more informative of the two.
How to use it properly
BMI is a screening tool. It is good for spotting a trend in yourself over months, and reasonable as a first look at a population. It is poor as a judgement about any individual on any given day, and it should never be the only thing you look at.
If a number here concerns you, that is a reason to talk to a doctor — who will look at your waist, your blood pressure, your blood work and your history, none of which BMI knows anything about.
Work out yours
The BMI Calculator takes metric or imperial input, shows which band the result falls in on a visual scale, and — more usefully — gives the healthy weight range for your height. It calculates in your browser, so nothing about your body is uploaded anywhere.