BMI Calculator

What BMI is, and what it is not

Body Mass Index is weight divided by height squared. That is the whole of it. It was devised in the 1830s by Adolphe Quetelet as a way to describe populations, not to diagnose individuals, and it is still most reliable used that way.

The standard WHO categories are:

Where BMI is misleading

BMI does not know what your weight is made of. That produces predictable errors:

• Muscular people are frequently classified as overweight or obese despite

low body fat. A rugby forward and a sedentary person of the same height and weight get the same BMI.

• Older adults lose muscle and gain fat at a stable weight, so BMI can look

unchanged while body composition worsens.

• Height extremes are distorted because the formula squares height, which

under-reads very tall people and over-reads very short people.

• Ethnicity matters. Health risk rises at lower BMI in South Asian and East

Asian populations, and several health bodies use a lower overweight threshold of around 23 for these groups.

BMI is a screening signal, not a diagnosis. A number in the overweight band is

a reason to look further, not a conclusion on its own.

Better questions than "what is my BMI"

• Waist circumference is a stronger predictor of metabolic risk than BMI

because it reflects abdominal fat specifically. Above roughly 94 cm for men and 80 cm for women, risk begins to rise.

• Waist-to-height ratio is simpler still: keep your waist under half your

height.

• Body fat percentage measures what BMI is trying to approximate — see the

body fat calculator.

Using this sensibly

Track the trend rather than the single reading. A BMI moving steadily in one direction over months tells you something real; a reading that differs by 0.4 from last week tells you about hydration and what you last ate.

If you want a figure that drives actual decisions, the TDEE calculator and calorie calculator are more actionable, because they output something you can change.