Complete guide
Reviewed July 2026Body Mass Index (BMI) is the most widely used screening measure for weight status. It relates your weight to your height with a single number that doctors, insurers and public-health agencies use to flag potential underweight, overweight and obesity — in seconds, with no equipment beyond a scale and a tape measure.
Enter your weight and height above and this calculator returns your BMI along with its WHO category. Below, you'll find the formula, category tables (including the lower Asian-population cut-offs many calculators omit), worked examples, and an honest discussion of what BMI can and cannot tell you.
BMI is a screening tool, not a diagnosis. A high or low value is a reason to look deeper — with waist measurements, body-fat estimates or a clinician — not a verdict on your health.
What is BMI?
BMI was devised in the 1830s by Belgian statistician Adolphe Quetelet and adopted in the 1970s as a practical population-screening index. It assumes that, for adults, weight scales roughly with the square of height — so dividing weight (kg) by height squared (m²) yields a number comparable across people of different heights.
Because it needs only two measurements, BMI became the universal shorthand for weight classification. The World Health Organization (WHO), the US CDC and most national health services define underweight, normal weight, overweight and obesity using the same BMI thresholds for adults aged 20 and over.
BMI formula and calculation
BMI = weight (kg) ÷ height² (m²) Imperial: BMI = 703 × weight (lb) ÷ height² (in²)
The metric formula divides weight in kilograms by height in metres squared. The imperial version multiplies by 703 to produce the same scale from pounds and inches.
Step-by-step example
- Weight = 70 kg, height = 170 cm.
- Convert height to metres: 170 cm = 1.70 m.
- Square it: 1.70 × 1.70 = 2.89 m².
- Divide: 70 ÷ 2.89 = 24.2.
- BMI 24.2 falls in the 18.5–24.9 'normal weight' band — near its upper edge.
| Category | BMI range (kg/m²) |
|---|---|
| Severe underweight | below 16.0 |
| Underweight | 16.0 – 18.4 |
| Normal weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obesity class I | 30.0 – 34.9 |
| Obesity class II | 35.0 – 39.9 |
| Obesity class III | 40.0 and above |
Asian and South-Asian cut-offs
Research shows that people of Asian descent tend to carry more visceral fat and face higher cardio-metabolic risk at lower BMI values. WHO's expert consultation therefore recommends action points of 23 (increased risk) and 27.5 (high risk) for Asian populations, and several countries — including India, Singapore and Japan — use overweight thresholds of 23–25 and obesity thresholds of 25–30 in clinical practice.
Healthy weight range for your height
To find your own range: multiply your height in metres squared by 18.5 for the lower bound and 24.9 for the upper bound. The calculator does this automatically for the height you enter.
| Height | Healthy weight range |
|---|---|
| 150 cm (4'11") | 41.6 – 56.0 kg |
| 160 cm (5'3") | 47.4 – 63.7 kg |
| 165 cm (5'5") | 50.4 – 67.8 kg |
| 170 cm (5'7") | 53.5 – 71.9 kg |
| 175 cm (5'9") | 56.7 – 76.3 kg |
| 180 cm (5'11") | 59.9 – 80.7 kg |
| 185 cm (6'1") | 63.3 – 85.2 kg |
What BMI gets right — and where it fails
Strengths
- Fast, free and reproducible anywhere — ideal for screening and tracking trends over time.
- At population level, higher BMI correlates strongly with risk of type 2 diabetes, hypertension, heart disease and several cancers.
- Standardised categories make results comparable across clinics, studies and countries.
Limitations
- It cannot distinguish muscle from fat: muscular athletes routinely register 'overweight' BMIs at low body-fat levels.
- It says nothing about fat location. Visceral (abdominal) fat drives metabolic risk far more than subcutaneous fat — waist circumference and waist-to-height ratio capture this; BMI doesn't.
- Standard cut-offs fit adults 20–65; they mislead for children (who need age-specific percentiles), pregnant women, the elderly and people who have lost muscle mass.
- Ethnicity shifts the risk curve — hence the Asian cut-offs above.
- 'Normal' BMI with high body fat and low muscle (sometimes called normal-weight obesity) still carries elevated risk that BMI misses entirely.
Better together: pair BMI with a waist measurement
A practical, evidence-backed upgrade is to check waist-to-height ratio alongside BMI: keeping your waist circumference below half your height is associated with lower cardio-metabolic risk at any BMI. If your BMI is borderline but your waist is over that line, treat it as the more important signal.
Common mistakes when using BMI
- Measuring height in shoes or weight in heavy clothing — small errors are squared in the height term.
- Applying adult categories to children or teenagers — use age-and-sex percentile charts instead.
- Treating a single reading as a trend; weigh under consistent conditions (morning, after the bathroom, before eating) and track the average.
- Chasing a BMI target through crash dieting — losing muscle lowers BMI while worsening body composition.
- Ignoring a rising waistline because BMI is stable — fat can replace muscle at constant weight.
Frequently asked questions
Glossary
- BMI
- Body Mass Index — weight in kilograms divided by height in metres squared.
- Visceral fat
- Fat stored around abdominal organs; the strongest fat-related driver of metabolic disease.
- Waist-to-height ratio
- Waist circumference divided by height; keeping it under 0.5 is a widely used risk guideline.
- Body-fat percentage
- Share of total body weight that is fat tissue; measured by DEXA, calipers or bio-impedance.
- Normal-weight obesity
- Normal BMI combined with high body-fat percentage and its associated risks.
- Percentile (children)
- A child's BMI ranked against peers of the same age and sex; the basis for paediatric weight categories.
- WHO
- World Health Organization — source of the standard adult BMI classification.
- Obesity classes
- WHO subdivisions of obesity: class I (30–34.9), II (35–39.9), III (40+).
Key takeaways
BMI compresses weight-for-height into one instantly comparable number, and for most adults it's a useful first screen: under 18.5 flags underweight, 25+ flags overweight, 30+ flags obesity (23/27.5 action points for Asian populations). Its blind spots — muscle vs fat, fat location, age and ethnicity — mean it should start conversations about health, not end them. Pair it with a waist measurement, and act on trends rather than single readings.
Enter your height and weight above to get your BMI and category — then check whether your waist is under half your height for the fuller picture.