What does BMI measure?

BMI (Body Mass Index) measures the ratio of weight to height squared (kg/m²). It is a population-level screening tool used to categorise weight ranges correlated with health risks — not a direct measure of body fat or health.

Is BMI accurate for everyone?

No. BMI does not account for muscle mass, bone density, age, sex, or ethnicity. Muscular individuals may have a high BMI without excess body fat. The WHO and NHS use it as a starting screen, not a diagnosis.

What are the BMI categories according to WHO?

WHO defines: under 18.5 as underweight, 18.5–24.9 as normal weight, 25–29.9 as overweight, and 30+ as obese. These thresholds are population-level and have different interpretations for different ethnic groups.

What should I use instead of BMI?

Waist-to-height ratio, waist circumference, and DEXA scans are more accurate for body composition. For clinical assessment, a healthcare professional will typically combine BMI with other measurements and context.

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Understanding BMI: What It Measures, Its Limitations, and Alternatives

Last updated: 2 June 2026

BMI (Body Mass Index) is a widely used screening tool that estimates whether a person's weight is appropriate for their height. It is calculated as weight in kilograms divided by height in metres squared (kg/m²). While useful at the population level, it has well-documented limitations as an individual health assessment tool.

WHO BMI Categories

BMI Range	Category
Under 18.5	Underweight
18.5 – 24.9	Normal weight
25.0 – 29.9	Overweight
30.0 and above	Obese

Why BMI Has Limitations

BMI was originally designed by the Belgian mathematician Adolphe Quetelet in the 1830s as a statistical tool for studying populations, not for assessing individual health. Its limitations are now well-established in medical literature:

Muscle mass: Athletes and muscular individuals may have a high BMI without excess body fat. The formula cannot distinguish muscle from fat.
Bone density: Higher bone density increases weight without increasing health risk — BMI treats this identically to excess fat mass.
Ethnicity: Some ethnic groups (notably South Asian populations) carry higher metabolic risk at lower BMI values. WHO and NICE in the UK have different thresholds for these groups.
Age and sex: BMI does not account for changes in body composition with age, or differences in fat distribution between sexes.

What to Use Alongside BMI

Healthcare professionals typically use BMI as one data point among several:

Waist circumference: A waist over 94 cm (men) or 80 cm (women) indicates elevated health risk regardless of BMI.
Waist-to-height ratio: A ratio under 0.5 is generally associated with lower cardiometabolic risk.
DEXA scan: The gold standard for measuring actual body fat percentage and muscle mass distribution.

Calculate Your BMI

Use our BMI calculator to calculate your BMI in metric or imperial units. It shows your result alongside WHO categories and notes the limitations of the measure.

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Frequently Asked Questions

What does BMI measure?: BMI (Body Mass Index) measures the ratio of weight to height squared (kg/m²). It is a population-level screening tool used to categorise weight ranges correlated with health risks — not a direct measure of body fat or health.
Is BMI accurate for everyone?: No. BMI does not account for muscle mass, bone density, age, sex, or ethnicity. Muscular individuals may have a high BMI without excess body fat. The WHO and NHS use it as a starting screen, not a diagnosis.
What are the BMI categories according to WHO?: WHO defines: under 18.5 as underweight, 18.5–24.9 as normal weight, 25–29.9 as overweight, and 30+ as obese. These thresholds are population-level and have different interpretations for different ethnic groups.
What should I use instead of BMI?: Waist-to-height ratio, waist circumference, and DEXA scans are more accurate for body composition. For clinical assessment, a healthcare professional will typically combine BMI with other measurements and context.