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    BMI Calculator

    Methodology

    This page sets out how the results shown on the site are obtained, which texts they rest on and what they do not allow you to conclude.

    1. The formula used

    Body mass index is weight in kilograms divided by height in metres squared. An adult of 70 kg and 1.75 m gets 70 ÷ (1.75 × 1.75), that is 22.9. The formula is the one adopted by the World Health Organization; we apply no correction of our own.

    2. Where the thresholds come from

    The adult categories, underweight below 18.5, normal up to 25, overweight up to 30, obesity above, come from the WHO international classification. For children and adolescents the raw index means nothing: growth charts by age and sex are used instead, and the result is expressed as a percentile, not a fixed category.

    3. What BMI does not measure

    BMI relates a mass to a height. It ignores body composition: a heavily muscled athlete can exceed 25 with no excess fat, while an older person can stay below 25 with high fat and low muscle mass. It also ignores where fat sits, although waist circumference is a better marker of cardiovascular risk.

    BMI is therefore not a diagnosis. It helps place a population, and helps open a conversation with a health professional, never to replace one.

    4. Updates and corrections

    Each page carries the date of its last real change, taken from the repository history, not today's date: an unchanged page must not look fresh. The WHO thresholds have not moved in decades; the prevalence figures quoted are reviewed whenever a new national survey is published.

    If a figure looks wrong to you, write to contact@bmi-imc.com naming the page. We check it against the source, correct it if the error is confirmed, and carry the correction across to every page using the same data.

    Official sources