Covid-19, weight and your bathroom scales

Reports of a possible link between obesity and morbidity have prompted one of our occasional contributors, Martin Vlietstra, to look at issues around measuring body weight in the home.

Whilst we still do not know much about the Covid-19 virus, press reports suggest that many if the victims were either frail or were overweight. Metric Views cannot comment on the accuracy of these reports but nevertheless it is instructive to look at the principles of logging one’s weight and BMI (body-mass index).

The newspapers that reported on the degree of obesity amongst Covid-19 victims often quote the formula for calculating somebody’s BMI as being “their weight expressed in kilograms divided by the square of their height expressed in metres”. The same newspapers then use stones to describe people’s weights and feet and inches to describe their heights. For example, many newspapers have stated that in 2018 Boris Johnson weighed 16½ stone and was 5ft 9in tall. This is not very helpful for working out his BMI. These figures are equivalent to approximately 105 kilograms and 1.75 metres, giving people of his stature a BMI of a little over 34. (According to medical specialists, an ideal BMI is between 18 and 25; 25 to 30 is classed as being overweight; 30 to 40 is obese and above 40 is morbidly obese).

Although many apps exist for smartphones that will calculate one’s BMI using imperial units, imperial units have a hidden problem of rounding.  People who use imperial units for their weight often round it to the nearest half-stone, whereas those who use metric units will round to the nearest kilogram.    If somebody of Mr Johnson’s stature were to increase their weight by half a stone, their BMI would increase by 0.96 which is quite a big step, especially if one is monitoring one’s BMI. If however they used kilograms, the increase (or decrease) in BMI if their weight increased (or decreased) by one kilogram would be 0.33 kg, a much more manageable step.

A similar rounding problem exists in the way in which people record their heights. Those using imperial units tend to round their height to the nearest inch while those using metric units tend to round to the nearest centimetre.  If somebody made an error of one inch when measuring their height (or rounded the odd half-inch up rather than down), the difference in BMI of somebody of Mr Johnson’s stature would be 1.04, whereas if the difference was one centimetre, the resultant difference in BMI would be 0.39, again a much smaller difference.

Of course, if one used stones and pounds, rounded one’s weight to the pound and rounded one’s height nearest half inch to calculate one’s BMI, the difference between metric and imperial units would not be so dramatic, but it would mean using a degree of precision that is often not used by the person in the street.

If you want to buy a new set of bathroom scales in order to monitor your weight, what should you look out for?  Digital scales have a large number of features such as memories, self-zeroing facilities and the like which affect the price, but what about the actual weighing mechanism. Mechanical bathroom scales that can give the same precision as digital models are prohibitively expensive, so I will only look at digital scales.  All digital scales have a load cell that give a voltage output, an associated analogue to digital convertor (ADC) and a microprocessor to handle the output from the ADC.  The output from the ADC has a specified number of bits (often 10 or 12), with each bit representing the smallest increment that the scale will display.  Some scales only have one range which means that all measurements will have the same precision while other scales have an “auto-range” facility whereby the electronics will automatically switch between different ranges, depending on the scale. My bathroom scales (which I bought 20 years ago), has three ranges: below 50 kg where one bit represents 0.1 kg, 50 to 100 kg where one bit represent 0.2 kg and 100 to 150 kg where one bit represents 0.5 kg.  Most models have metric/imperial switches, but on the cheaper models the conversion is done in the firmware, resulting is some awkward (though invisible) rounding errors. Only the more expensive models have imperial auto-ranging (where one bit can represent 8oz, 4oz or 2oz) alongside metric auto-ranging.  On examining the packaging of a number of different models, I found that the manufacturers were reluctant to publish any real details about the precision of their models.

 Where does that leave us? If you wish to record weight changes on a spreadsheet, then metric units win hands down (something that Gunter realised in 1620 when he invented the link and the chain with 100 links in a chain, 10 chains in a furlong and 10 square chains in an acre thereby making it easier to measure land area).  If you want an economy of the way in which you express your weight and height, then again metric units win – you get a better precision by using a round number of centimetres and kilograms than by using a round number of inches and [half]-stones. Finally, all digital bathroom scales have been designed with the metric market in mind – the imperial market being a “bolt-on”. Your choice of model of bathroom scale will depend on the additional features that you want and how much you wish to pay.

14 thoughts on “Covid-19, weight and your bathroom scales”

  1. I have never understood why the British media continue to use imperial measurements. Even decent newspapers like The Guardian and The Independent give weights in stones and heights in feet and inches. British TV documentaries are just as bad. I’ve seen two TV documentaries lately. One presented by Michael Mosely and the other by Miriam Margolyes. Both used imperial measurements or a confusing mixture of imperial and metric rather than just metric. I like both those presenters and I was interested in the subject but I ended up turning the programs off before they were finished because I couldn’t be bothered to get out my calculator to convert the measurements. I’m quite insulted that I should have to. I made the effort many years ago to acquaint myself fully with SI measurements, the official system of measurement in the UK, and I’m happy with that decision. I don’t see why I should have to re learn an inferior method of measurement because other people are too lazy to change and I’m angry that the media expect me to. The media just perpetuate the ignorance.


  2. I have often wondered why people who insist on weighing themselves in stones and pounds don’t use hundredweights as well!
    Some years ago I managed to buy metric only bathroom weighing scales from IKEA.


  3. This UK practice of measuring weight and height in stones and feet seem to be one of the more bizarre and inexplicable features of our mixed measurement muddle,even by D(a)fT standards. Not only the media but also doctors and nurses in the NHS.
    One figure could stand out from all others, 100 kg is a thick red line over which none should tread. Given humans are getting taller, another stand out figure is that 2 m is the new height of a tall person. What could be simpler? “Never in my lifetime” is still the cry of the imperial faithful, whoever they are these days.
    Bizarre on bizarre they then use these incalculable figures to work out our metric BMI!
    Anyone for a dose of logic here?


  4. I’m not a big fan of BMI as BMI does not distinguish an obese unhealthy person from a person who exercises and has a large muscle structure. A 100 kg man with a height of 1.75 m who is who is muscular and has little to no fat would be lumped in with an obese man loaded with fat.

    In the point on rounding, most people are innumerate to the point they don’t know how to round and just truncate the extra digits. This is why marketers take advantage of this ignorance and advertise prices with values ending in nine so the person ignores the 9. A price of 4.99 is treated as 4 and not 5.

    Also, the social distance of 2 m when imperialised to 6.562 feet is put at 6 feet rather than rounded up to 7 feet.


  5. Bathroom scales suffer the same problem as things like tape measures… shops will continue to sell them with imperial units because they don’t want to lose sales to those who don’t; even IKEA, despite Rob’s comment above, don’t appear to sell metric-only any more (mildly off-topic I recently purchased a set of saucepans from them with measures on the inside, the ones on the ‘right’ side (if you’re right handed) showed measures in ‘cups’ and the other side were metric but spelt ‘liters’).

    I’ve gone out of my way to buy metric-only measuring devices for my home, having got various scales, jugs, tapes and rulers on visits to France. Sadly my metric-only bathroom scales had a problem and my wife helpfully went out and got some new ones which for the first few months kept getting switched back to stones though I think she’s given up. The lack of imperial-measuring devices in the kitchen doesn’t appear to have caused any issues and the fact I can directly link the weight of the food I eat with my body weight has helped me keep my own BMI in check.

    A simple law to ban the sale of imperial measuring devices would clear this up in no time!


  6. Alex,

    There is no reason the Chinese manufacturers of tape measures and scales don’t ship to England the same metric only products they ship to elsewhere. The English have to be paying more for their oddity as the Chinese have to isolate and send different products to England than they would do to Europe.

    Somewhere in the supply chain someone has decided that England will be offered all dual equipment and everyone else metric only. The shops may have no choice but to choose only from a limited selection of dual products and metric only is not a choice. Those customers who want metric only thus by-pass the shops and buy on-line. I’m sure if it was possible the shops would offer a selection of metric only along with dual to satisfy both set of customers, but can’t. So now they lose business to those who buy metric only online bypassing their shop.

    They had such a law in Australia in the beginning but were forced to repeal it decades later, but by the time they did the people had adopted metric quicker and now ignore imperial if it is presented to them.


  7. Daniel:

    One could be forgiven for thinking that the Chinese are putting into practice Kruschchev’s famous phrase that the Communists would sell the Capitalists the rope with which they hang themselves. The Chinese know that America and Britain refuse to accept the metric system in many aspects of life and are simply cashing in on that. Divide and rule. China wants to see a weak US, a weak UK and a weak EU. Trump wants a weak China and a weak EU too, hence his support for the UK leaving. We can’t all win, but exploitation of one’s opponents’ weaknesses is nothing new. A country without a proper, single system of measurement used and understood by everyone, as taught in school, is arguably a weak country.


  8. ‘Home Schooling’
    During lockdown many parents have become ‘teachers’, or ‘support workers’/ ‘teaching assistants’ helping their children do work set by their school.

    I can imagine a mini exercise “weigh yourself”, [without the specific inclusion of kg in the request].
    Sadly, the answer given by (some) children will be the imperial reading from the bathroom scale.


  9. Bill,

    That’s where the exercise would have to specify the student measure their mass specially in kilograms with instructions to either use a metric scale or flip the switch to the kilogram mode. Then afterwards the instructions could continue with a request to calculate the students weight in newtons at sea level, on mount Everest, on the moon, Saturn, in the dead of space, etc.


  10. @Bill
    A number of years ago my wife worked in a nursery school. One of their on-going problems was parents teaching their children the alphabet, but using upper-case letters whereas the schools teach lower-case letters first.

    Your observation about use (or non-use) of metric units is probably well-founded, particularly among the virulent anti-metric sectors of our society.


  11. Martin,
    It’s a bit off topic; your comment about the teaching of the alphabet, reminds me about ‘poor’ handwriting. The wrong use of a tittle on top of an upper-case (capital) letter I. [A tittle, is the dot, which in English is put on the top of a lower-case letter i, and sometimes a lower case letter j.]
    I’m no expert on alphabets/fonts, I think in the Turkish alphabet there are two capital letter I’s, one has a dot, the other is dotless.


  12. A number of newspapers have reported that the PM is taking steps to reduce his weight following his recent illness. We frequently hear that his government is “following the science”, and also that science indicates that obesity increases the risk from the virus. Pity that the UK’s measurement muddle (science – who cares?) does not help when deciding whether or not to try to lose weight.


  13. Ramsden,

    People who cling to obsolete units are also the type to cling to bad or pseudoscience. Losing weight (or mass) being a path to health is part of the pseudoscience. In truth, losing mass is just the opposite. When one starts a diet, the rapid loss of mass is the result of the body going into famine mode and shedding itself of muscle cells that it doesn’t want to feed and keeping the fat cells. Fat is less dense than muscle tissue so when muscle tissue is lost the mass loss is quite noticeable when steeping on the scale.

    The ideal way to lose fat and not muscle is to first start an exercise program that increases the demand for muscle tissue and then slowly changing your eating habits to force the body use up the fat cells without going into famine. The result would be an increase in body mass due to the increase of muscle cells.

    The US is the leading country with high obesity and is also the country most fixated with weighing themselves just to see the pounds on the scale and having the desire to repeat those numbers to others.


  14. Daniel,

    You’re wrong about the initial rapid weight loss during the first week on a diet: you don’t lose muscle mass then as the body doesn’t go into “famine mode” that quickly. The initial weight loss is actually from glycogen, along with water with which glycogen is stored in the body.


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