BMI

1. **Choose your unit system** — click the Metric or Imperial toggle at the top of the calculator.
2. **Enter your height** — in centimetres (metric) or feet and inches (imperial).
3. **Enter your weight** — in kilograms (metric) or pounds (imperial).
4. **Click Calculate** — your BMI score and weight category (Underweight, Normal, Overweight, or Obese) are displayed instantly.
5. **Review your result** — read the on-screen explanation of what your BMI category means, and consult a healthcare provider for personalised guidance.

**Metric:** BMI = weight(kg) ÷ height(m)²

**Imperial:** BMI = [weight(lbs) ÷ height(in)²] × 703

• weight (kg) — Your body weight measured in kilograms (metric units).
• height (m) — Your height measured in metres (metric units). Convert centimetres to metres by dividing by 100 — e.g., 175 cm = 1.75 m.
• weight (lbs) — Your body weight measured in pounds (imperial units).
• height (in) — Your total height measured in inches (imperial units). Convert feet and inches to total inches — e.g., 5 ft 9 in = 69 in.
• 703 — A unit-conversion constant that makes the imperial BMI result numerically equivalent to the metric result.
• BMI — The resulting Body Mass Index value, expressed in kg/m². Compared to WHO thresholds: <18.5 Underweight, 18.5–24.9 Normal weight, 25.0–29.9 Overweight, ≥30.0 Obese.

BMI = 80 ÷ (1.75)² = 80 ÷ 3.0625 ≈ 26.1

Result: BMI ≈ 26.1 → **Overweight** (WHO category: 25.0–29.9)

A BMI of 26.1 falls in the Overweight range (25.0–29.9 kg/m²) according to WHO adult classifications.

• Below 18.5 → Underweight
• 18.5 – 24.9 → Normal (Healthy) weight
• 25.0 – 29.9 → Overweight
• 30.0 and above → Obese (Class I ≥30, Class II ≥35, Class III ≥40)

A result in the Overweight range may suggest an increased risk of certain health conditions, but BMI alone does not account for muscle mass, bone density, age, sex, or ethnicity. Always discuss your result with a qualified healthcare professional.

⚕️ Disclaimer: This calculator is for educational and informational purposes only. It does not constitute medical advice, diagnosis, or treatment. Consult a licensed healthcare provider before making any health-related decisions.

What Is BMI and Why Does It Matter?

Body Mass Index was developed by Belgian mathematician Adolphe Quetelet in the 1830s and was later adopted by the World Health Organization (WHO) and national health agencies as a simple, population-level screening metric. Because it requires only height and weight — both easily measured — it became a practical tool for identifying broad weight trends across large groups.

WHO Adult BMI Categories

| BMI Range | Category | |---|---| | Below 18.5 | Underweight | | 18.5 – 24.9 | Normal weight | | 25.0 – 29.9 | Overweight | | 30.0 – 34.9 | Obese Class I | | 35.0 – 39.9 | Obese Class II | | 40.0 and above | Obese Class III (Severe) |

Limitations of BMI

BMI is a screening tool, not a diagnostic measure. Key limitations include:

• Muscle mass: Athletes and bodybuilders often have a high BMI despite low body fat.
• Age: Older adults may have a healthy BMI but excess body fat due to muscle loss.
• Sex: Women naturally carry more body fat than men at the same BMI.
• Ethnicity: Research shows that Asian populations may face higher health risks at lower BMI thresholds; some guidelines use adjusted cut-offs.
• Body fat distribution: BMI does not distinguish between visceral (abdominal) fat and subcutaneous fat, even though their health implications differ significantly.

Complementary Measures

Healthcare providers often pair BMI with waist circumference, waist-to-hip ratio, or more precise tools like DEXA scans to get a fuller picture of body composition.

Children and Teens

For individuals under 18, BMI-for-age percentiles (not fixed cut-offs) are used, because healthy BMI ranges change as children grow. This calculator is designed for adults aged 18 and over.

• **Entering height in centimetres when the metric formula expects metres** — always divide cm by 100 before squaring (e.g., 170 cm → 1.70 m).
• **Using mixed units** — entering weight in kilograms but height in inches, or vice versa, produces a meaningless result. Always use the same unit system throughout.
• **Forgetting the 703 conversion factor** in the imperial formula — without it, the result will be roughly 4.88× too small.
• **Misinterpreting BMI as body fat percentage** — BMI is a ratio of weight to height, not a direct measure of fat.
• **Applying adult BMI categories to children** — for ages 2–18, use age- and sex-specific growth charts, not fixed WHO adult thresholds.
• **Rounding height or weight too aggressively** — even a 2 cm or 1 kg difference can shift a BMI by 0.5–1 point near category boundaries.

• Body Mass Index – WHO Global Database on Body Mass Index — World Health Organization (WHO)
• Assessing Your Weight – Healthy Weight, Nutrition, and Physical Activity — Centers for Disease Control and Prevention (CDC)
• Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults — National Institutes of Health (NIH) / National Heart, Lung, and Blood Institute
• About Adult BMI — Centers for Disease Control and Prevention (CDC)
• Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies — The Lancet (WHO Expert Consultation, 2004)

Only sources that have been reviewed are shown. Unverified citations are never published.