Body mass index
v1pesoalturaimcHow it is worked out
Work out your body mass index from your weight in kilograms and your height in metres.
Body mass index divides weight by height squared. It is a population-level reference and cannot tell muscle from fat, so it is a rough bearing rather than a diagnosis: anyone who wants to know what it means for them should ask a health professional.
The expression that runs
peso / altura ^ 2The fields arrive with an example filled in: put your own numbers in and run it again. You can also edit the formula to fit your case.
What if the formula were yours?
This calculator is not hand-written: it is an Axioma formula. Open it in the editor to see how it is built, adapt it to your case and create the ones you need.
From weight and height to the index
The index divides weight in kilograms by height in metres squared. With 70 kilos and 1.75 metres: 1.75 squared is 3.0625, and 70 over 3.0625 gives 22.86.
Squaring the height is what makes bodies of different sizes comparable, because the weight of a person grows roughly with the square of their height rather than with height itself.
What the ranges say
For adults, the World Health Organization places underweight below 18.5, normal weight between 18.5 and 24.9, overweight between 25 and 29.9, and obesity from 30 upwards. The 22.86 of the example falls in the normal range.
Those limits are conventions for talking about populations, not thresholds where something changes abruptly: 24.9 and 25.1 are the same person with the same body.
What the index cannot see
It does not tell muscle from fat, and it says nothing about where the fat is. Someone who trains for strength can land in the overweight range with little fat, while a sedentary person can sit at a normal index with excess abdominal fat, which is the kind most associated with cardiovascular risk.
It also does not read the same at every age or in every situation: for children and adolescents it is interpreted with percentile charts by age and sex, and in pregnancy or older age the index loses meaning. It is a screening tool, not a diagnosis: a number outside the range is a reason to see a health professional, not a conclusion.
Frequently asked questions
- Does it work for children and teenagers?
- Not in this form. Bodies change a great deal with age, so the same index is read against percentile charts by age and sex, and that reading belongs to whoever is caring for the child.
- Why does an athlete come out as overweight?
- Because muscle weighs more than fat and the index only knows total weight. In bodies with a lot of muscle the number rises without any excess fat.
- What else is worth looking at?
- Waist circumference adds what the index cannot see, since it points at abdominal fat. Neither replaces a professional assessment, which also looks at medical history and blood work.