What is BMI and what does your score actually mean?

📅 April 2026⏱ 4 min read🏷 Fitness

BMI — Body Mass Index — is the most widely used measure of healthy weight. It's calculated from just two numbers (height and weight), produces a single score, and has been the standard in medical and public health settings for decades. It's also widely criticised. Here's an honest look at both sides.

How BMI is calculated

BMI = weight (kg) ÷ height (m)²

For example, someone who is 175cm tall and weighs 70kg has a BMI of 70 ÷ (1.75 × 1.75) = 22.9.

In imperial units: BMI = (weight in lbs ÷ height in inches²) × 703

The BMI categories

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 and above	Obese (Class III)

💪 Calculate Your BMI

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Why BMI is useful

BMI is fast, free, and requires no equipment. It correlates reasonably well with body fat percentage at the population level, which is why it's used in large-scale health research. A BMI consistently above 30 is associated with increased risk of type 2 diabetes, cardiovascular disease and some cancers. It's a useful first filter.

What BMI misses

BMI treats everyone's body the same — and bodies aren't the same.

Muscle mass: Muscle is denser than fat. An athlete with low body fat may have a high BMI. A lean person with little muscle may have a "healthy" BMI but poor metabolic health.
Fat distribution: Where you carry fat matters as much as how much you carry. Abdominal fat carries more cardiovascular risk than fat on the hips and thighs — BMI doesn't distinguish these.
Age: Body composition changes with age. Older adults naturally lose muscle mass, so the same BMI can represent different body fat levels at different ages.
Ethnicity: Research shows some ethnic groups face higher health risks at lower BMI thresholds. Several health organisations use adjusted categories for South Asian populations.

What to do with your BMI

Use it as one data point among several, not a verdict. If your BMI is in the healthy range and you exercise regularly, eat well and have normal blood pressure, there's little reason for concern. If your BMI is outside the healthy range, that's worth discussing with a doctor — alongside waist circumference, blood markers and activity level, not in isolation.