Tuesday, April 14, 2009

BMI confusion

Sara Buys wrote an interesting piece about Marilyn Monroe’s costumes in The Times Magazine on Saturday


One thing startled me. Since I got into a bit of a tiswas with the calculations I am going to show my workings in full, as they used to say on the exam papers

“Monroe was 5ft 5in”

5ft 5in = 65inches
x2.54 = 165cm
÷ 100 = 1.65 metres
Squared = 2.72

“Monroe was just over 8 stone”

8 stone x 14 = 112 lbs
÷ 2.2 = 50.9 kgs

So BMI = 50.9/2.72 = 18.7

which is distinctly underweight

But “she had a respectable BMI of 21”

So start with BMI =21
Assume height is correctly given as 5ft 5 in
Then weight = 21 x 2.72
= 57.1 kg
= 126 lbs
= 9 stone

I am not bothering to write all this here just to pick nits. I suspect the mistake might have been to assume that there are 16 lbs in 1 stone – easily done, I have done it myself

Since Monroe was American her weight would usually be quoted in pounds, but on this side of the water we still tend to think in terms of stones, despite years of metric-only education. So if people then multiply by 16 on the way to kilogrammes BMI will be overstated, which could explain some of the odder assertions we sometimes read