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Science · Classes 9-10 · Chapter 1 · 1.2

Body Mass Index (BMI).

Sixty kilograms: is that a lot, a little, or just right? It depends on how tall you are. BMI squeezes weight and height into one number.

1. What BMI is, and its formula

The human body keeps growing until the age of 20-24 years. After that the height does not increase, and food only repairs the body and keeps it healthy.

The index that shows the balance between height and weight is called the Body Mass Index (BMI). If the weight is balanced with the height, the body can be considered nutritionally healthy.

The formula divides the weight by the square of the height, with the height in metres:

BMI = weight (kg) / [height (m)]²

The book's example: with weight 80 kg and height 1.8 m, BMI = 80 / (1.8 × 1.8) = 80 / 3.24 ≈ 24.7. Now pick out the right formula from four.

formula check tap a chip

Which one is the BMI formula?

Wrong. Adding mixes kilograms with metres, and shows no balance at all.

2. The bands of Figure 1.02

The bands in Figure 1.02 are the same for male and female: below 18.5 is less weight; 18.5-24.9 is correct weight (healthy); 25-29.9 is over weight; 30 and above is fatty.

BMI is an indicator of the fat in our body. 18.5-24.9 is healthy and normal; less than that is under weight, more is obese. BMI is not the only or the most accurate indicator of health, but it is widely used. Drag the handle and see which number falls in which band.

band gauge drag the handle

Which band is this BMI in?

These are the book's rough indicators, not a judgement of a person.

The mark above moves BMI alone. Now take both handles: drag weight and height separately, the figure changes and the mark crosses the bands.

drag and see Try the height too

Weight, height and BMI

The person is imaginary. BMI is a rough indicator, not a judgement of anyone.

3. Work it out yourself

If the height comes in centimetres, convert it to metres first: 160 cm = 1.6 m. With weight 60 kg, BMI = 60 / 2.56 ≈ 23.44. Another: weight 50 kg and height 1.5 m give BMI = 50 / 2.25 ≈ 22.22, which falls in the 18.5-24.9 band. The calculator below carries these three, and takes your own numbers too.

calculator convert cm to m first

weight (kg) / height (m)²

BMI is a rough indicator, not a verdict on you. What you type goes nowhere. The notebook is yours.

The book's group work: calculate the BMI of every learner in the class and show it in a graph (Figure 1.03). BMI goes on the horizontal axis, the number of learners on the vertical axis.

4. Check yourself

Three questions in the exam's style.

check yourself pick one answer

Question 1

What is BMI?

The second needs both the formula and the numbers.

check yourself pick one answer

Question 2

The weight of Ariba is 60 kg and the height is 1.5 m. What is the BMI?

In the last one, watch the edges of the bands.

check yourself pick one answer

Question 3

The weight of Eva is 50 kg and the height is 1.5 m. What is the BMI?

The whole note on seven cards.

cut from the note tap to turn over

Seven cards

QuestionWhat is the formula of BMI?
Answerbody weight (kg) / [height (m)]²
QuestionWhat is the BMI for weight 80 kg and height 1.8 m (the book's example)?
Answer80 / (1.8 × 1.8) = 80 / 3.24 ≈ 24.7
QuestionAccording to Fig 1.02, what BMI is healthy and normal?
Answer18.5-24.9.
QuestionIn Fig 1.02, what is a BMI of 25-29.9 called?
AnswerOver weight.
QuestionBMI is an indicator of what, and is it the only indicator of health?
AnswerFat in the body; no, it is not the only or most accurate indicator, but it is widely used.
QuestionWhat is the BMI for weight 60 kg and height 160 cm?
Answer160 cm = 1.6 m; 60 / 2.56 ≈ 23.44
QuestionWhat is the BMI for weight 50 kg and height 1.5 m?
Answer50 / 2.25 ≈ 22.22

Source: Science, Classes 9-10 (NCTB), Chapter 1 “Healthy Life, Better Living”, section 1.2.