BMI Test (Body Mass Index): How to Calculate and Interpret It

Body mass index (BMI) is probably the best-known (and most misunderstood) health indicator out there. It's calculated in 10 seconds, requires no equipment, and shows up in any medical report. The problem is that it doesn't distinguish between fat and muscle, so in athletic populations it can lead to absurd conclusions.

That said, it's still a useful piece of data as a first population-level screen and for detecting relevant weight changes over time.

What does this test measure?

BMI relates body weight to height through a simple formula, producing a value used to classify a person into weight categories (underweight, normal weight, overweight, obesity).

It doesn't measure body composition, fat distribution, or muscle mass: it's a population-level estimate, not an individual one.

Equipment needed

• Scale
• Stadiometer or tape measure for height
• Calculator (or the formula by hand)

Test protocol

1. Measure body weight (kg), ideally barefoot, in light clothing, and in the morning while fasting
2. Measure height (m) barefoot, with the back straight against a wall or stadiometer
3. Apply the formula: BMI = weight (kg) / height (m)²
4. Example: a person weighing 75 kg and 1.75 m tall → BMI = 75 / (1.75 × 1.75) = 75 / 3.0625 ≈ 24.5
5. Compare the result with the WHO classification table

Common mistakes

• Measuring weight with heavy clothing or shoes on
• Rounding height (a 1-2 cm difference noticeably changes the result)
• Interpreting BMI as a measure of "fat" in very muscular people (a bodybuilder can have an "obese" BMI without excess fat)
• Using it as the sole health criterion, ignoring context (age, body composition, circumferences)

How to interpret the result

BMI (kg/m²)	Classification (WHO)
< 18.5	Underweight
18.5 - 24.9	Normal weight
25.0 - 29.9	Overweight
30.0 - 34.9	Obesity class I
35.0 - 39.9	Obesity class II
≥ 40.0	Obesity class III

Important: in people with high muscle mass (strength athletes) or very low muscle mass (sarcopenia), BMI loses a lot of accuracy. In those cases, always supplement it with an actual body composition test.

Relationship with other tests

BMI makes much more sense when combined with:

• Body composition test (skinfolds) or bioimpedance test — to find out whether that "extra" weight is fat or muscle
• Waist-to-hip ratio test — to understand fat distribution, not just the total amount

One more data point within the full picture

BMI alone doesn't tell much of the story, but as a contextual data point within a broader tracking process (weight, circumferences, body fat %) it remains useful, especially for seeing long-term trends. With Movalytics BMI is calculated automatically from your client's weight and height, which you've already logged, and it appears integrated alongside the rest of the body composition tests on their profile, with no manual calculation needed.

Frequently asked questions

Why can a muscular athlete have an "obese" BMI? Because muscle weighs more than fat for the same volume. BMI only uses weight and height, so it can't distinguish whether that extra weight is muscle or fat. Is BMI different for men and women? The formula and WHO categories are the same for both sexes, although typical fat and muscle mass distribution differs between them. Does BMI work for children? Not in the same way. For children and adolescents, age- and sex-adjusted BMI percentile charts are used instead of adult categories.