• 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

Worked Example

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

This man's BMI falls just into the 'overweight' category (25.0 - 29.9). It is not possible to judge his health by this measure alone, but if his BMI has been trending upwards over recent time, this might suggest to a doctor that there is an underlying cause of his weight gain.

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• Nomograms have been?largely superseded?by rapid?online calculators?or?smartphone apps, but still have a use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)

Exam Tip