Body mass index
Choose a mass and a height and work out the body mass index, BMI = mass ÷ height². See which adult band it falls in, and learn why doctors use age charts, not these bands, for growing children.
Core concept
Body mass index compares mass with height: BMI = mass (kg) ÷ height² (m²); for adults 18.5 to under 25 is a healthy range, but children are still growing, so their BMI is checked on charts for their age and sex
Topic: Our body and health
Doctors and health workers use the body mass index (BMI) as a quick first check of whether a person's mass suits their height. BMI = mass in kilograms ÷ (height in metres)². A person of 45 kg and 1.5 m has BMI = 45 ÷ (1.5 × 1.5) = 45 ÷ 2.25 = 20. For adults, a BMI under 18.5 is underweight, 18.5 to under 25 is healthy, 25 to under 30 is overweight, and 30 or more is obese. BMI is only a guide: it cannot tell muscle from fat. Children and teenagers are still growing, so their BMI is compared on growth charts for their age and sex, not with the adult bands. Eating a balanced diet and being active every day matter more than any single number.
What you will learn
- Work out BMI from mass and height.
- Read the adult BMI bands.
- Explain why children's BMI is checked on age charts.
Before you start
- Know how to square a decimal number.
Materials
- A weighing scale and a measuring tape, if you want to try (privately)
Equipment
- None
Procedure
- Record 45 kg and 1.5 m.
- Change the mass to 35 kg and record.
- Change the mass to 65 kg and record.
How this simulation is simplified
- Adult bands: under 18.5 underweight (1), 18.5 to under 25 healthy (2), 25 to under 30 overweight (3), 30 or more obese (4) (WHO).
- These bands are for adults only; children and teenagers use BMI-for-age charts. This is a general guide, not medical advice.