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Quick guide to BMI

Body Mass Index (BMI) relates weight to height to place body size in reference categories. It is a population guideline and does not replace individual clinical assessment.

How it is calculated

BMI = weight (kg) / height (m)². Imperial inputs are converted to metric before performing the calculation.

Form fields

• Weight: enter your current body weight in kilograms (or pounds when using the imperial system).
• Height: enter your height in centimetres (or inches when using the imperial system).

Interpreting results

BMI and health risk

Research involving over 10.6 million participants across 239 prospective studies demonstrates a J-shaped relationship between BMI and all-cause mortality, with the lowest risk in the 20–25 range (Global BMI Mortality Collaboration, Lancet 2016). A BMI above 25 is associated with increased risk of type 2 diabetes, cardiovascular disease, hypertension, and certain cancers, while a BMI below 18.5 is linked to higher risk of respiratory disease, malnutrition, and osteoporosis. The Prospective Studies Collaboration (Lancet 2009) found that each 5 kg/m² increase above 25 raises overall mortality by approximately 30%. It is important to remember that BMI is a population-level screening tool, not a diagnostic instrument — it must always be interpreted alongside clinical assessment by a qualified healthcare professional.

Waist circumference: a key complement

The National Heart, Lung, and Blood Institute (NHLBI) recommends measuring waist circumference alongside BMI to better assess cardiometabolic risk. A waist circumference above 102 cm (40 in) in men or 88 cm (35 in) in women is associated with substantially increased risk of type 2 diabetes, dyslipidaemia, and cardiovascular disease, independent of BMI (NHLBI Clinical Guidelines, 1998). A 2020 consensus statement in Nature Reviews Endocrinology proposed that waist circumference should be treated as a vital sign in clinical practice because of its strong, independent association with morbidity and mortality (Ross et al., 2020). Unlike BMI, waist circumference directly reflects abdominal (visceral) fat, which is the adipose tissue most strongly linked to metabolic complications.

BMI in special populations

BMI thresholds should be interpreted differently across populations. For children and adolescents aged 2–19 years, the WHO and CDC use age- and sex-specific percentile charts rather than fixed cutoffs. In older adults, the extra risk linked to a higher BMI is smaller: in the Global BMI Mortality Collaboration, each 5 kg/m² above 25 was associated with 52% higher mortality when BMI was measured at age 35–49, but 21% higher when measured at age 70–89 (Lancet 2016). A meta-analysis of 97 studies even found slightly lower mortality with overweight (BMI 25–29.9) than with normal weight (HR 0.94; Flegal et al., JAMA 2013) — the so-called "obesity paradox" — but when the analysis was limited to never-smokers without prior disease, overweight was associated with higher mortality (Global BMI Mortality Collaboration, 2016). For people of Asian descent, a WHO Expert Consultation kept the international cutoffs but found that many Asians already have a high risk of type 2 diabetes and cardiovascular disease below a BMI of 25, and proposed additional public-health action points at 23 and 27.5 (WHO Expert Consultation, Lancet 2004). During pregnancy, BMI should not be used for weight monitoring — the Institute of Medicine gestational weight gain guidelines are more appropriate. Athletes with high muscle mass may show elevated BMI without excess body fat; body composition analysis (DEXA, bioimpedance) provides a more accurate assessment in these cases.

Limitations of BMI

BMI does not distinguish between muscle and fat nor account for fat distribution. Very muscular individuals, older adults or people with specific medical conditions may receive values that do not fully reflect their actual health status.

Practical tips

• Combine BMI with waist circumference, body composition and professional review.
• Sustained habit changes (activity, nutrition, rest) deliver lasting improvements.
• Avoid drastic decisions based only on the BMI number. If in doubt, consult your doctor or a registered dietitian.