Body Mass Index


Arnold Schwarzenegger

Michelle Davis

It is true that if you look like either of these two people then your BMI will be pretty meaningless, so, before you read any further, please undress and either stand in front of a mirror, or in front of a good friend, and ask them whether, in all honesty, you look much like either of these pictures.

If the answer is “no”, and if you are concerned for your weight or you health, then BMI can be a useful metric, provided that you understand it.  So what is  the body mass index?

Consider this: the taller you are, the more you would expect to weigh.

The shorter you are the less you would be expected to weigh.  But what about a short and very fat person?  They may weigh more than a tall, thin person.

BMI is the measure that evens that out, so that we can compare like with like.  It is your body mass (or weight: only a physicist needs to know the difference between mass and weight) divided by the square of your height.  For all the gruesome detail, check the Wikipedia article, “Body Mass Index“.

You derive the number directly if working in kilograms and meters: you need to multiply by 703 if working in lbs and inches.  And then those numbers fall into broad bands just to give a description to where you are, and to turn the raw numbers into descriptive words.  Here are some charts to give you an idea.

BMI, Kg and Metres

BMI, Pounds and inches


Now you think that would be all very simple … but it isn’t.  Let me tell you my own story, by way of illustration.  Back in June 2009 I weighed 22 stones (308lbs, 140kg).  I am 6’4″ tall.  That falls off of the chart above, but let’s use the calculator that’s in the right margin of this page.  It gives a BMI of 37.5, classed as “Obese Class II”.  Back in 2009 the description was more blunt.  It was called “morbidly obese”.  I accepted the verdict.

The pharmacist who was guiding me into weight loss using Lipotrim asked me what I thought I should choose as my goal weight.  I didn’t know, so he looked on the BMI chart (I had never come across it before).

Look across from 6’4″ until you get to the green zone.  Somewhere between 190lbs and 200lbs looks about right.  In English stones, 14 stones is 196lbs.  So he suggested 14 stones as a goal weight.  ROFL, LMAO, and other such tags came to mind.  That was patently ridiculous!  I hadn’t been that weight since I was in my early 20s, which was before the metric system had even been invented!

He wasn’t fazed by my mirth, and asked what I thought was reasonable.  Well, once, back in the mid-1970s, I had made a concerted effort to lose weight and had come within a whisker of 15 stones (210 lbs, 95.5 kg).  But that was 35 years earlier, and I was older and wider now (I wish I could say older and wiser, but I’ll stick to the truth), so I figured I’d shoot for 16 stones (224 lbs, roughly 100 kg).

That would get me down from Morbidly Obese, through Obese, to merely overweight.  And I wasn’t looking at his chart.  And he didn’t want to put me off by holding me to what I obviously thought was an unachievable target.  To help someone lose 84 lbs (6 stones, 38 kg) was a huge improvement, after all.

But here’s the thing.  When I hit 16 stones I did the test above: the jumping up and down naked thing.  Not only did I not look like Arnie, I didn’t even look like the 7-stone (97 lb) weakling in the Charles Atlas ads of my youth.  I still looked like a fat guy, just not as fat as I had been.  I realised that the BMI scale is pretty accurate.

Yesterday evening a friend wrote to me, convinced that the BMI calculator over on the right is wrong; giving false information.  So let’s check it out.  First I’ll use my measurements.  6’4″ is 76 inches.

22 stones comes to 308 lbs.  So, my height squared is 76 x 76 = 5776.  308 ÷ 5776 = 0.05332409972299168975069252077562, times 703 = 37.48.  Which is what the calculator gives.  So it works for this tall, heavy man.

Now, my friend is a woman, and she’s shorter and lighter.  She tells me that she is 5’5″ (65 inches, 165 cm), and that she currently weighs 154 lbs (11 stones, 70kg).  Let’s do the sums: 65 squared is 4225, and 154 ÷ 4225 gives a BMI of 26.23, which makes her in the lower third of the “overweight” band.  It also gives her a range of goal weights to aim for.  The lighter end of the normal band for her height is 110lbs (7 stone 12, or 50kg) and the upper end is 140lbs (10 stones, 64 kg), which is quite a range.  Only you (or your best friends) can tell you whether you are truly big-boned, or whether you are kidding yourself.

And, of course, the BMI won’t tell you if you are fit.  Even when you are at a BMI rated as “normal”, it is interesting to see what happens if you jump up and down naked in front of the mirror.  Can people bounce coins of your butt, or would they vanish.  As you head towards your goal weight, getting some muscle tone will help a lot, and BMI has nothing to say about that!

How surprised people are when told of the goal weight that would give a “correct” BMI is, I believe, a measure of how overweight we have all become.

(An earlier version of the calculator in the right margin gave incorrect values lower down the range.  I have switched to a different calculator.)

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