I response to a query this morning I mentioned the current BMI calculation is somewhat crude and tends to be inaccurate for both short and tall people making them thin and fat respectively. To this end I have reproduced a letter to the Economist from 2013 below. I will try and post a link to the revised calculation later as I keep finding pop-up infested ones!
SIR - The body-mass index that you (and the National Health Service) count on to assess obesity is a bizarre measure. We live in a three-dimensional world, yet the BMI is defined as weight divided by height squared. It was invented in the 1840s, before calculators, when a formula had to be very simple to be usable. As a consequence of this ill-founded definition, millions of short people think they are thinner than they are, and millions of tall people think they are fatter.
Nick Trefethen
Professor of numerical analysis
University of Oxford
Written by
MichaelJH
Heart Star
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I agree with the letter writer that BMI isn't always accurate. There's also the question of body shape, is the person being measured an ectomorph, an endomorph, or a mesomorph?
There seems to be some movement amongst doctors towards the far simpler system of instead looking at a person's waist measurement. The rationale is that too much fat around the waist means there will be a blanket of fat within the body cavity, enveloping and seeping into vital organs. The simplest method of analysis is that your waist size should be no more than half your height. So a six foot person, 72" tall, should have a waist measurement of 36" or less.
It's worth pointing out that assuming your waist size is the same as your trouser size is almost certainly wrong. Trouser manufacturers flatter with generous sizing, usually in the range of 2-6"! So use a measuring tape for an accurate waist measurement.
In terms of the connection between growing waist sizes and growing levels of heart disease, I've read that the average woman fifty years ago had a waist measurement of 24-25", yet today the average is 34-35", I'm sure the trend is equally alarming for men too. It's frightening how fast we're digging our graves with our teeth!
What is also misleading is, not only have women got bigger but clothes manufacturers have altered sizing to flatter us. Age 20 my measurements were 36-26-38 size 14 . Today age 66 they are 35-29-39 but I wear size 12 sometimes 10. Crazy.
I have noticed that with the cheaper menswear outlets. Also that to make up numbers the manufacturers seem to put random labels in. Once I brought two pairs of identical trousers after trying one pair on. The second pair turned out to be ~8" larger!
Unfortunately, this is the one area where all BMI calculators fall over (muscle weighs more than fat discussion). Heavily muscled people tend to be heavier and thus fall foul of the BMI categorisations). The BMI including this new one, is a good general guide but it is only a guide.
It is perfectly fine for most people by which i mean those who don't play at prop for england. That link gives me two very similar values, both scrawny. It's not the sort of thing where you can say oh look I've gone down by one i must be ok. It does work for fat people who refuse to admit it, telling them this says you are obese might sink in. The link to code is a bit silly, just height over weight squared
I am 186 pounds and 5 10.5 in ht. My BMI is 26, overweight. Apparently I would need to be 156 lbs for ideal weight. That is a loss of 30lbs. I currently have 14% body fat and 10% visceral fat. Anything less would be unhealthy. Maybe I should lose one or two limbs.
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So shocked 
Time to give up
Higher BMI, lowering the exercise without lowering the calorie intake ... silly boy!!
A response would be nice!
