BMI and Weight Decisions
Calculate, check units, then separate the number from any health conclusion.
Continue where you stopped
The core idea
Staying Healthy · Lesson 2 of 5 · about 25–35 min
What you need to understand
Calculate, check units, then separate the number from any health conclusion.
Definitions
- BMI
- body mass in kilograms divided by height in metres squaredFor example: BMI is a limited indicator, not a diagnosis.
Explanation
Use only the fictional measurements supplied in an exercise. BMI = mass in kg ÷ height² in m². Convert centimetres to metres before squaring: 160 cm is 1.60 m, so the denominator is 1.60 × 1.60 = 2.56 m². The final unit is kg/m².
BMI is a screening measure, not a diagnosis of health. It does not directly measure fitness, diet or body composition. For children and adolescents, interpretation uses age- and sex-specific growth references, not adult cut-offs. Without the relevant supplied reference, calculate the value and state the limitation instead of inventing a health category.
Undereating can leave too little energy or nutrients for growth and normal functioning. Excessive exercise without enough food or recovery can cause fatigue or injury. A fictional case with these behaviours calls for support and appropriate advice, not a weight-loss target or a more restrictive plan.
Pause and say it: Calculate, check units, then separate the number from any health conclusion.
Common mistake
Tempting wrong idea: BMI is height squared divided by mass.
Why it fails: A single BMI cannot rank overall health or justify restricting food. Do not collect personal measurements for this lesson.
Use this instead: This reverses the division: 1.50² ÷ 54 ≈ 0.0417 m²/kg. BMI requires 54 ÷ 1.50² = 24 kg/m².
Practical work
What to show: Calculate from supplied fictional data, state what is missing and reject unsafe restriction or excessive exercise.
Before you finish: Do not invent an age, growth reference or weight-loss target.
Practical: BMI and health-claim evaluation
How do calculation choices and missing information affect interpretation of supplied BMI data?
Safety: Use fictional data; do not request classmates' measurements or make personal diagnoses.
- Change
- supplied person or claim
- Measure
- calculated BMI and strength of conclusion
- Keep the same
- same formula, metres and kilograms, same rounding rule
View the apparatus, method and sample results
Apparatus
fictional supplied mass and height data, calculator, spreadsheet or graph paper, health-claim cards.
Method
- Convert every height to metres and calculate height squared.
- Calculate BMI = mass ÷ height² and retain one guard digit.
- Check units and compare calculations between partners.
- For each health claim, list evidence supported by BMI and information that is missing.
- Write a conclusion that avoids diagnosing any named individual.
| Case | Mass / kg | Height / m | BMI / kg m⁻² |
|---|---|---|---|
| A | 54 | 1.60 | 21.1 |
| B | 72 | 1.80 | 22.2 |
| C | 68 | 1.55 | 28.3 |
Limitation: BMI does not directly measure body composition, fitness, diet, age-related context or disease.
Improvement: Combine appropriately collected indicators and qualified medical assessment when individual conclusions are needed.
Worked Examples
All people and measurements in these exercises are fictional. Do not supply your own or a classmate’s measurements.
Modelled example 1
Calculate BMI
Problem
A person has mass 54 kg and height 1.60 m. Calculate BMI.
Study the worked solution
Reason from the evidence
Method
Square the height in metres before dividing the mass in kilograms.
Reason
Calculate carefully, then state the limitation before drawing a health conclusion.
Working
- Square height: 1.60² = 2.56 m².
- BMI = 54 kg ÷ 2.56 m².
- BMI = 21.1 kg/m² to three significant figures.
State the conclusion
Working
BMI is 21.1 kg/m²; interpret it only as one screening indicator.
Guided practice 2
Calculate, then interpret carefully
Problem
A student has mass 45 kg and height 1.50 m. Calculate the student’s BMI. Give one reason why this number cannot describe the student’s health on its own.
Show the squared height and your limitation
Hints
Hint 1: square first
Multiply 1.50 m by 1.50 m before dividing.
Hint 2: think beyond one number
Health also depends on factors such as diet, activity, body composition and medical evidence.
View solution step by step
Calculate BMI
Method
Use mass divided by height squared.Reason
The stated BMI relationship uses height in metres twice.
Working
BMI = 45 ÷ (1.50 × 1.50) = 45 ÷ 2.25 = 20 kg/m²
Limit the conclusion
Method
State what the calculation leaves out.Reason
A calculated index is only one piece of evidence about health.
Working
BMI alone does not show the student’s fitness, diet, body composition or overall medical condition.
Common misconception 3
Height must be squared
Learner response
For mass 64 kg and height 1.60 m, a learner writes “BMI = 64 ÷ 1.60 = 40 kg/m²”. Spot and correct the error.
Correct the substitution and conclusion
View solution step by step
Use height twice
Method
Square 1.60 m before dividing.Reason
The denominator is height², not height.Working
BMI = 64 ÷ (1.60²) = 64 ÷ 2.56 = 25 kg/m²
Examiner practice 4
Evaluate a claim based on BMI
Examination question
A person has mass 70 kg and height 1.75 m. Calculate the person’s BMI. A friend says, “This result proves the person is healthy.” Evaluate the claim. [4 marks]
Show the calculation, unit and a balanced judgement
View solution step by step
Calculate the index
2 marksMethod
Square the height, then divide the mass by that value.
Reason
BMI compares mass with height squared, not height alone.
Working
BMI = 70 ÷ (1.75²) = 70 ÷ 3.0625 = 22.9 kg/m²
Evaluate the evidence
2 marksMethod
Reject the word “proves” and explain why.Reason
BMI is a limited indicator rather than a complete health test.
Working
The result is one useful indicator, but it does not prove health because it does not account for all relevant evidence, such as body composition, diet, activity or medical measurements.
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Award the method, answer and evaluation separately.
Challenge 5
Use more than one piece of health evidence
Problem
Two people have the same BMI. Person A is active, eats a balanced diet and has normal medical measurements. Person B is inactive and has persistently high blood pressure. Explain why their equal BMI values do not support the conclusion that their health is identical.
Separate the calculation from the health judgement
Hints
Hint 1: same value
Say what BMI does compare.
Hint 2: different evidence
Use the activity and blood-pressure evidence in your judgement.
View solution step by step
Weigh all the evidence
Method
Recognise that the shared BMI is only one similarity.
Reason
Different habits and medical measurements provide additional evidence about risk and wellbeing.
Working
The equal BMI values compare mass with height, but they do not erase the differences in activity and blood pressure. A sound health judgement must use these other observations too.
Guided practice
Try it with support
A fictional learner has mass 54 kg and height 1.60 m. Calculate BMI, then explain why it cannot rank the learner's overall health.
- Square height in metres.
- Name information BMI does not measure.
Check the guided answer
Answer: BMI = 54 ÷ 1.60² = 21.1 kg/m². It does not directly measure diet, fitness, body composition, rest, hygiene or disease and must use an age-appropriate reference.
Check: A BMI calculation is one limited indicator, not a diagnosis of overall health.
Practise and continue
Practise this
A fictional case supplies mass 64 kg and height 160 cm. Calculate BMI and explain why an adult cut-off cannot classify a child.
Need a hint?
- Convert height before squaring.
Check your answer
Answer: 160 cm = 1.60 m. BMI = 64 ÷ 1.60² = 25 kg/m². Child interpretation needs the relevant age- and sex-specific reference; the number alone is not a diagnosis.
Check: Do not invent an age, growth reference or weight-loss target.
Think like a scientist
A fictional poster says a single BMI proves health and recommends skipping meals and exercising without rest. Correct the two errors.
Check the reasoning
BMI is only one screening indicator. Restricting food or exercising without adequate recovery can cause harm; the poster cannot justify that advice.
Remember: Use only the supplied fictional data; do not assess classmates.
One-minute check
- Hide the page and explain bmi and weight decisions in your own words.
- Give a new example that is different from the worked example.
- Correct this common mistake: “BMI is height squared divided by mass.”