BMI Calculator: What Is BMI and How to Calculate It
BMI — Body Mass Index — is a number derived from your height and weight that classifies body size into underweight, normal, overweight, and obese ranges. It's the most widely used screening tool in clinical practice, but it has important limitations that are worth understanding before acting on a number.
The BMI formula
BMI is calculated differently depending on which unit system you use:
| System | Formula |
|---|---|
| Metric | BMI = weight (kg) ÷ height (m)² |
| Imperial | BMI = 703 × weight (lb) ÷ height (in)² |
Worked examples
Example 1 (metric): A person who weighs 70 kg and is 1.75 m tall.
BMI = 70 ÷ (1.75)²
= 70 ÷ 3.0625
= 22.9Example 2 (imperial): A person who weighs 154 lb and is 5′9″ (69 inches) tall.
BMI = 703 × 154 ÷ (69)²
= 703 × 154 ÷ 4761
= 108,262 ÷ 4761
= 22.7BMI classification (WHO standard)
| BMI range | Category | Health risk |
|---|---|---|
| Below 16.0 | Severe underweight | High risk of malnutrition, bone density loss, heart problems |
| 16.0 – 16.9 | Moderate underweight | Elevated risk, clinical evaluation recommended |
| 17.0 – 18.4 | Mild underweight | Slightly elevated risk |
| 18.5 – 24.9 | Normal weight | Lowest risk range |
| 25.0 – 29.9 | Overweight | Moderately increased risk of metabolic disease |
| 30.0 – 34.9 | Obese Class I | High risk |
| 35.0 – 39.9 | Obese Class II | Very high risk |
| 40.0 and above | Obese Class III (severe) | Extremely high risk |
Asian BMI cut-offs
The WHO classification above is based primarily on European populations. Multiple studies have shown that people of South Asian, East Asian, and Southeast Asian origin have higher rates of metabolic disease (type 2 diabetes, cardiovascular disease) at the same BMI compared to European populations. Several health authorities — including India's National Institute of Nutrition — recommend adjusted cut-offs:
| Category | Standard WHO cut-off | Recommended Asian cut-off |
|---|---|---|
| Overweight | ≥ 25.0 | ≥ 23.0 |
| Obese | ≥ 30.0 | ≥ 27.5 |
Limitations of BMI
BMI is a population-level screening tool, not a diagnostic metric. It has known blind spots:
- Muscle vs fat — BMI doesn't distinguish between muscle and fat tissue. A muscular athlete can have a BMI of 28–30 (overweight by classification) with very low body fat. Conversely, a sedentary person with a "normal" BMI of 23 can carry excess visceral fat.
- Fat distribution — where you carry fat matters more than total fat. Visceral fat (around the abdomen) is more metabolically active and dangerous than subcutaneous fat. Waist circumference is a better predictor of cardiovascular risk than BMI alone.
- Age — older adults tend to have more body fat at the same BMI due to muscle loss (sarcopenia). BMI slightly overestimates health for older people.
- Sex — women naturally carry more body fat than men at the same BMI. The 18.5–24.9 range is identical for both sexes despite this biological difference.
- Pregnancy — BMI is not applicable during pregnancy.
Use BMI as a first-pass indicator. For a more complete picture, pair it with waist circumference (risk increases above 88 cm for women / 102 cm for men by WHO criteria) and body fat percentage measured by DEXA scan or bioimpedance analysis.
Healthy weight range by height
| Height | Healthy weight (BMI 18.5–24.9) |
|---|---|
| 155 cm (5′1″) | 44 – 60 kg (98 – 132 lb) |
| 160 cm (5′3″) | 47 – 64 kg (104 – 141 lb) |
| 165 cm (5′5″) | 50 – 68 kg (111 – 149 lb) |
| 170 cm (5′7″) | 54 – 72 kg (118 – 159 lb) |
| 175 cm (5′9″) | 57 – 76 kg (125 – 168 lb) |
| 180 cm (5′11″) | 60 – 81 kg (133 – 178 lb) |
| 185 cm (6′1″) | 63 – 85 kg (140 – 188 lb) |
Calculate your BMI instantly with the DevBench BMI Calculator — enter height and weight in metric or imperial and get your BMI, category, and healthy weight range.
Try it yourself
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