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Working out body mass index
Age 14 · Maths
Body mass index (BMI) is your mass in kilograms divided by your height in metres multiplied by itself. The answer is a number that falls into one of four weight bands for adults.
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In the picture
- The scale: body mass in kilograms
- The height gauge: height in metres
- The square tile: height × height, the number you divide by
- The coloured bar: the weight band the BMI falls into
Examples
- An adult of 70 kg and 1.75 m: 1.75 × 1.75 = 3.0625, and 70 ÷ 3.0625 ≈ 22.9, which is in the healthy band (18.5 to 24.9).
- An adult of 90 kg and 1.80 m: 1.80 × 1.80 = 3.24, and 90 ÷ 3.24 ≈ 27.8, which is in the overweight band (25 to 29.9).
Key takeaways
- BMI = mass (kg) ÷ height (m)², so always square the height before dividing.
- The adult bands are under 18.5, 18.5 to 24.9, 25 to 29.9, and 30 or more. For people under 18, doctors compare BMI with age charts instead.
Quiz: Working out body mass index (Age 14)
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To work out BMI, what do you divide the body mass in kilograms by?
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An adult’s BMI works out at 27. Which weight band does it fall into?
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How do doctors judge the BMI of someone under 18?
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