A student weighs 54 kg and is 1.5 m tall. His BMI is: (a) 18.0 kg/m² (b) 24.0 kg/m² (c) 36.0 kg/m² (d) 81.0 kg

Concept understanding — BMI Classification

BMI Classification: A Concept-First Explanation

Think about how we describe a person's build in everyday language. Someone might be "lean," "stocky," "heavyset," or "slim." These words are useful but imprecise — what one person calls "stocky," another might call "average." BMI Classification is a system that replaces these vague descriptions with a standardised, objective scale.

The Core Idea

BMI stands for Body Mass Index. It is a single number that relates a person's weight to their height. The fundamental intuition is simple: a taller person naturally carries more weight than a shorter person, so we cannot judge body size by weight alone. BMI adjusts for height, giving a number that can be compared across people of different statures.

The classification system then takes that number and places it into one of several categories. These categories are not arbitrary — they are based on large-scale population studies that link each range to different levels of health risk.

The Classification Categories

The standard classification divides BMI values into these ranges:

Underweight — a BMI below a certain threshold
Normal weight — a BMI within a healthy range
Overweight — a BMI above the normal range but below obesity
Obese — a BMI above a higher threshold, further subdivided into classes

Important

BMI Classification is a screening tool, not a diagnostic one. It tells you whether a person's weight falls into a range associated with higher or lower health risks, but it does not measure body fat directly, nor does it account for muscle mass, bone density, or fat distribution.

Why It Matters

The value of BMI Classification lies in its simplicity and its statistical power. For populations, it is an excellent predictor of trends — rising obesity rates, for instance, show up clearly in BMI data. For individuals, it provides a quick, non-invasive starting point for a conversation about health.

A person classified as "normal weight" is, on average, at lower risk for conditions like heart disease, diabetes, and joint problems. Someone classified as "obese" faces elevated risks for these same conditions. But the key word is "average" — a muscular athlete may have a high BMI with excellent health, while a person with a "normal" BMI may have poor metabolic health. …

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BMI is a simple ratio of weight to height squared; for this student the calculation gives exactly 24.0 kg/m², which falls in the Normal range by WHO standards.

This is a straightforward MCQ that tests whether you know the BMI formula and can apply it without a calculator slip. BMI (Body Mass Index) is the most widely used screening tool to classify underweight, normal weight, overweight, and obesity in adults. It matters because it gives a quick, inexpensive estimate of body fatness and associated health risk — though it does not measure fat directly.

The formula is fixed:

BMI = (weight (kg))/([height (m)]²)

Now plug in the given values. Weight = 54 kg, height = 1.5 m.

Square the height: 1.5 × 1.5 = 2.25 m².
Divide weight by that square: 54 ÷ 2.25.

Do the division carefully. 2.25 × 24 = 54 exactly, so the result is 24.0 kg/m².

That matches option (b).

Now check the other options to see where a student might land if they make a common error:

(a) 18.0 kg/m² — This is what you get if you mistakenly divide height by weight, or if you square 1.5 incorrectly as 3.0 (giving 54/3 = 18). A plausible slip.
(c) 36.0 kg/m² — This comes from forgetting to square the height: 54/1.5 = 36. A very common mistake.
(d) 81.0 kg/m² — This is the square of the height (2.25) multiplied by 36, or some other arithmetic confusion; no sensible path leads here. …

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Q.A student weighs 54 kg and is 1.5 m tall. His BMI is:

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