Body mass index relates your weight to your height. It is the first filter used in clinic to classify excess weight, but it is not a diagnosis — below I set out exactly what it measures and what it misses.
BMI Calculator
Calculate your Body Mass Index to find out if your weight is in a healthy range. This tool is indicative and does not replace a medical evaluation.
What your result means
The World Health Organization classifies BMI as follows:
| BMI | Category |
|---|---|
| Under 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity class I |
| 35.0 – 39.9 | Obesity class II |
| 40 or above | Obesity class III |
From 30 upwards we are talking about obesity, which is a recognised chronic disease — not a cosmetic problem and not a failure of willpower. That distinction matters because it changes the entire approach to treatment.
What BMI does not measure
This is the point that causes the most misunderstanding, so it is worth being explicit.
BMI does not distinguish fat from muscle. A well-trained person can have a BMI of 28 with a perfectly healthy body fat percentage. The reverse also happens, and is more common: someone with a BMI of 24 can have low muscle mass and excess abdominal fat — what we call normal weight obesity, and it carries real metabolic risk.
BMI does not say where the fat is. That is where most of the risk actually lives. Visceral fat, the kind surrounding the abdominal organs, behaves very differently from fat on the hips and thighs. This is why waist circumference adds information BMI cannot give you:
- Increased risk in women: over 88 cm (35 in)
- Increased risk in men: over 102 cm (40 in)
Several Latin American societies use lower cut-offs for this population — 80 cm in women, 90 cm in men — because metabolic risk accumulates here with less abdominal fat than in the European reference populations these thresholds were derived from.
BMI does not read the same at every age. In adults over 65, a BMI slightly above 25 is associated with better outcomes, not worse. And in children and adolescents BMI is read as a percentile for age and sex, never against the adult table.
So what is it good for?
Exactly what it was designed for: classifying populations and opening a conversation. It is fast, free, reproducible, and correlates reasonably well with cardiometabolic risk at group level.
What I do in clinic is take BMI as a starting point and add what it lacks: waist circumference, body composition, blood pressure, lipid profile, fasting glucose and insulin, thyroid function and liver function. That set does draw the full picture.
When an endocrinology evaluation is worthwhile
Beyond the number, some signals justify a consultation:
- BMI of 30 or above, or 27 with an associated condition — prediabetes, hypertension, dyslipidaemia, fatty liver, sleep apnoea, PCOS
- Rapid weight gain with no change to your eating or activity
- Waist circumference above the cut-offs, even with a normal BMI
- A first-degree family history of type 2 diabetes
- Repeated weight loss followed by regain — which almost always has a physiological explanation rather than a disciplinary one
If your result concerns you, the logical next step is not a more restrictive diet. It is understanding why your body is where it is.
Common questions
Is BMI useful if I do a lot of strength training?
In people with high muscle mass, BMI overestimates body fat and loses much of its usefulness. In that case waist circumference is more informative, and a body composition measurement more informative still.
What is the ideal BMI?
The 18.5 to 24.9 range is considered normal in adults, but there is no universal ideal number. Age, muscle mass, fat distribution and metabolic status all change how the figure should be read.
At what BMI is medication for obesity indicated?
Guidelines consider drug treatment from a BMI of 30, or from 27 when at least one associated condition is present — prediabetes, hypertension or dyslipidaemia among them. Always alongside changes to diet and activity, never instead of them.
I'm used to pounds and feet. Does that change anything?
Only the arithmetic. BMI is the same measure worldwide: this calculator uses kilograms and centimetres, so convert first — 1 lb is 0.454 kg, and 1 inch is 2.54 cm. The categories and their interpretation are identical.
Further reading
Medical Weight Loss & GLP-1 Treatment
Physician-supervised GLP-1 treatment (semaglutide, tirzepatide, liraglutide) in Lima. English-speaking endocrinologist in San Borja — proper candidacy assessment, dose escalation and follow-up.
Getting Ozempic or Mounjaro in Peru, Safely
Semaglutide and tirzepatide are available in Peru — and so are counterfeits. How to verify what you are buying, what a legitimate prescription involves, and what supervision should actually look like.
5 min readTools point you in a direction. Diagnosis happens in the consulting room.
If your result raised more questions than it answered, book an evaluation in San Borja or a video consultation from wherever you are.