BMI Calculator
Body Mass Index in metric or imperial — updates as you type.
Your BMI
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How BMI is calculated
Body Mass Index is a simple ratio of weight to height. In metric units:
BMI = weight (kg) ÷ height (m)²
In imperial units: BMI = 703 × weight (lb) ÷ height (in)².
BMI categories (adults)
| Category | BMI range |
|---|---|
| Underweight | Below 18.5 |
| Healthy weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obesity | 30.0 and above |
BMI is a general guide and does not account for muscle mass, body composition or other factors. It is not medical advice — consult a healthcare professional for personal guidance.
The formula in both unit systems
Body Mass Index compares your weight with the square of your height. The squaring is what makes it a shape-independent number rather than just a weight.
Someone 5′9″ (69 in) weighing 165 lb:
165 ÷ (69 × 69) × 703 = 165 ÷ 4,761 × 703 = 24.4
That sits in the healthy range, just under the 25.0 threshold.
What the categories mean
The adult cut-offs below are the World Health Organization’s, and they are the ones used by most health services.
| BMI | Category | General note |
|---|---|---|
| Under 18.5 | Underweight | May indicate under-nutrition; worth discussing with a clinician |
| 18.5 – 24.9 | Healthy weight | Population-level lowest-risk band |
| 25.0 – 29.9 | Overweight | Elevated average risk of metabolic conditions |
| 30.0 – 34.9 | Obesity class I | Clinical follow-up commonly advised |
| 35.0 – 39.9 | Obesity class II | Higher associated risk |
| 40.0 and over | Obesity class III | Highest associated risk |
Where BMI is useful — and where it fails
BMI was designed in the 19th century to describe populations, not individuals, and that is still what it does best. It is cheap, repeatable and needs only a tape measure and scales, which is why epidemiologists still use it. But it cannot tell muscle from fat, and it says nothing about where fat is stored.
- Muscular builds often score as overweight. A rugby player at 30 BMI may carry very little fat.
- Older adults lose muscle mass, so a “healthy” BMI can hide low lean tissue.
- Children and teenagers need age-and-sex percentile charts, not adult cut-offs.
- Pregnancy makes BMI meaningless as a body-composition measure.
- Ethnicity matters. Health bodies in several countries apply lower thresholds — often 23 for overweight — for people of South Asian, Chinese and other Asian descent.
Better companions to BMI
Waist circumference and waist-to-height ratio capture abdominal fat, which is the part most strongly linked with metabolic risk. A simple guide many clinicians use: keep your waist under half your height. Combining that with BMI gives a far more honest picture than either alone.
Not medical advice
BMI is a screening number, not a diagnosis. Any decision about weight, diet or exercise should involve a qualified health professional who can see the whole picture.
BMI FAQ
How is BMI calculated?
What is a healthy BMI range?
Is BMI accurate for athletes?
Does BMI work for children?
Should BMI thresholds differ by ethnicity?
What is a better measure than BMI?
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