BMI Calculator
Enter your weight and height to calculate your Body Mass Index (BMI), see your weight category, ideal weight range, and BMI Prime — all in your browser, nothing is uploaded.
Optional: Waist circumference (for waist-to-height ratio)
Waist-to-height ratio (WHtR) = waist ÷ height. Values under 0.5 are generally considered healthy.
BMI Categories & Health Context
| BMI Range | Category | BMI Prime | Health Context |
|---|---|---|---|
| < 18.5 | Underweight | < 0.74 | May indicate malnutrition, nutrient deficiency, or other health issues. Associated with reduced bone density and immune function. |
| 18.5 – 24.9 | Normal weight | 0.74 – 1.00 | Associated with lowest risk of weight-related health conditions in most population studies. General target range for healthy adults. |
| 25.0 – 29.9 | Overweight | 1.00 – 1.20 | Moderately elevated risk of cardiovascular disease, type 2 diabetes, and hypertension. Risk depends on fat distribution and fitness level. |
| 30.0 – 34.9 | Obese (Class I) | 1.20 – 1.40 | High risk of obesity-related conditions. Weight management through diet and exercise is typically recommended. |
| 35.0 – 39.9 | Obese (Class II) | 1.40 – 1.60 | Very high risk. Medical supervision and structured intervention are usually advised. |
| ≥ 40.0 | Obese (Class III) | ≥ 1.60 | Extremely high risk. Also referred to as severe or morbid obesity. Bariatric or intensive medical intervention may be considered. |
Source: World Health Organization (WHO) classification. Some health authorities use lower thresholds for Asian populations (overweight ≥ 23, obese ≥ 27.5).
Worked Example
Person: Weight = 72 kg, Height = 1.78 m (178 cm)
- Height in metres: 178 cm ÷ 100 = 1.78 m
- Height squared: 1.78² = 3.1684 m²
- BMI = 72 ÷ 3.1684 = 22.7
- Category: Normal weight (18.5–24.9)
- BMI Prime: 22.7 ÷ 25 = 0.91 (below 1.0 = within normal range)
- Ideal weight range: 18.5 × 3.1684 to 24.9 × 3.1684 = 58.6 kg to 78.9 kg
What is Waist-to-Height Ratio (WHtR)?
The waist-to-height ratio (WHtR) is calculated by dividing your waist circumference by your height (both in the same unit). It is considered a better predictor of cardiovascular risk than BMI alone because it captures central adiposity (belly fat), which is more strongly linked to metabolic disease than overall weight.
| WHtR | Interpretation |
|---|---|
| < 0.40 | Underweight range |
| 0.40 – 0.49 | Healthy range |
| 0.50 – 0.59 | Overweight/increased risk |
| ≥ 0.60 | Obese/high risk |
The general rule of thumb: "Keep your waist to less than half your height."
Frequently Asked Questions
What is BMI and how is it calculated?
BMI (Body Mass Index) is a number calculated from your weight and height: BMI = weight (kg) ÷ height (m)². For example, if you weigh 70 kg and are 1.75 m tall: BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 ≈ 22.9. It was developed in the 19th century by Adolphe Quetelet and is widely used as a quick population-level screening tool. It does not directly measure body fat.
What do the BMI categories mean?
The WHO standard BMI categories for adults are: Underweight (BMI < 18.5), Normal weight (18.5–24.9), Overweight (25.0–29.9), Obese Class I (30.0–34.9), Obese Class II (35.0–39.9), and Obese Class III (≥ 40.0). These categories are based on statistical associations with health outcomes in large population studies. Some health authorities use slightly different cut-offs for Asian populations (e.g., overweight at ≥ 23).
Is BMI accurate for athletes and muscular people?
No. BMI cannot distinguish between fat mass and muscle mass. A heavily muscular athlete may have a high BMI (even "Obese") while carrying very little body fat. Conversely, someone with a "Normal" BMI could have high body fat and low muscle mass ("normal-weight obesity"). For a more complete picture, consider body fat percentage measurements (DEXA scan, skinfold calipers), waist circumference, and waist-to-height ratio alongside BMI.
Does BMI apply to children?
Standard adult BMI categories do not apply to children and teenagers. For those under 18, BMI is compared against age- and sex-specific growth charts (BMI-for-age percentiles). A paediatrician or healthcare provider should interpret BMI in children. Separate calculators using CDC or WHO growth reference data are needed for accurate assessment in children.
What is BMI Prime?
BMI Prime is BMI divided by 25 (the upper limit of "Normal"). A BMI Prime of 1.0 means you are exactly at the top of the normal range. Less than 1.0 = underweight or normal; greater than 1.0 = overweight or obese. It is a dimensionless ratio that makes it easy to compare BMI relative to the ideal cut-off without memorising the category thresholds.
What are the limitations of BMI?
BMI is a screening tool, not a diagnostic measure. Key limitations: (1) It does not measure body fat directly — muscle and bone are denser than fat, so two people with the same BMI can have very different body compositions. (2) It does not account for fat distribution — abdominal fat (central obesity) carries higher health risks than fat elsewhere, and this is better captured by waist circumference or waist-to-height ratio. (3) It does not adjust for age, sex, or ethnicity at the individual level. (4) It is not a substitute for a clinical examination. Always consult a healthcare professional for a complete health assessment.