Skills: Calculating BMI (DP IB Biology: SL)

• Body Mass Index (BMI) is a rough and ready measure of a person’s mass in relation to their height
• The calculation of BMI is as follows

• A BMI below 18.5 is considered underweight
• A BMI 18.5-24.9 is considered normal
• A BMI of 25.0-29.9 is considered overweight
• A BMI of 30.0-39.9 is considered obese
• A BMI of 40.0 or more is considered morbidly obese
• BMI is a crude measurement as it works against individuals who are heavily muscular but who are also extremely lean; their BMI might be an overestimate
• BMI may also be misleading in the case of elderly people who have lost a lot of muscle mass; their BMI might be an underestimate

Step 1: Ensure that mass and height are expressed in the correct units 

Mass units are kg - this is correct

Height units are in metres - this is correct

Step 2: Use the formula

A nomogram can help to calculate BMI

• A nomogram is a two-dimensional chart that allows rapid estimation of BMI by reading off two scales, one for mass and one for height
• A line is drawn, between the two scales, that intersects a third axis in the middle
  • This reveals the BMI

• This removes the need for a calculation and requires no mathematical expertise
• Because the relationships between mass, height and BMI are all fixed, they can be represented on a chart like this
• Nomograms have been largely superseded by rapid online calculators or smartphone apps, but still have a use
  • For example, doctors and health workers will often have a BMI nomogram on the wall of their office for rapid reference when consulting a patient

A BMI nomogram. A line can be drawn from the height scale to the mass scale. Where the line intersects the BMI scale is the person’s BMI (26 in this case)