BMI (Body Mass Index) = weight (kg) ÷ height² (m²). It is a population-level screening tool for weight status in adults, not a substitute for a comprehensive health assessment.
BMI is a widely used screening measure for weight status, but it does not directly measure body fat and should not be used alone to assess individual health. This article explains how BMI is calculated, what the categories mean, and its important limitations.
BMI (Body Mass Index) is a number calculated from a person's weight and height. It is used as a screening tool to identify potential weight-related health risks in adults.
BMI = weight (kg) ÷ height² (m²)
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Example: a person weighing 70 kg and 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9
BMI is not a direct measure of body fat, body composition, or health. It is a population-level indicator — useful for identifying broad trends across groups, but limited when applied to any individual.
The following categories apply to adults aged 18 and over.
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy 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) |
Source: Australian Government Department of Health and Aged Care.
Research indicates that people of Asian background tend to have a higher proportion of body fat at the same BMI compared with people of European background. For this reason, lower thresholds are used for Asian populations:
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 23.9 | Healthy weight |
| 24.0 – 27.9 | Overweight |
| 28.0 and above | Obese |
BMI is used by health authorities worldwide because it is:
Public health organisations including the Australian Government, WHO, and CDC use BMI to track population weight trends and plan health interventions.
BMI is a screening measure, not a diagnostic tool. Its key limitations:
BMI cannot distinguish between fat mass and lean mass (muscle, bone, water). A highly muscular athlete may have a BMI in the "overweight" range while having very low body fat. Conversely, an older adult with normal BMI may have a higher proportion of body fat due to reduced muscle mass.
Where fat is stored matters for health risk. Abdominal (visceral) fat carries higher risk than fat stored elsewhere. Waist circumference is often used alongside BMI to give a more complete picture.
A person with normal BMI may still have risk factors such as high blood sugar, high blood pressure, or unhealthy lipid levels. Conversely, some people with higher BMI have healthy metabolic profiles.
Adult BMI categories do not apply to people under 18. BMI in children is interpreted using age- and sex-specific growth charts.
BMI can be less informative for:
The Australian Government recommends using waist measurement alongside BMI for a more complete picture of weight-related health risk.
General guidance for waist circumference:
| Risk level | Men | Women |
|---|---|---|
| Increased risk | ≥ 94 cm | ≥ 80 cm |
| Substantially increased risk | ≥ 102 cm | ≥ 88 cm |
Source: Australian Government Department of Health and Aged Care.
BMI is most useful when:
If you have concerns about your weight or body composition, speak with your doctor or a qualified dietitian. BMI is a starting point for a conversation, not a conclusion.
Vinora provides general health information and does not diagnose medical conditions or replace professional medical advice. If you have health concerns, please consult your doctor or other qualified healthcare professional.
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