We have almost — almost — banished ridiculous measuring systems based on the length of the king's foot or the Babylonians' favourite number, or whatever, in favour of a nice, consistent metric. A few backwaters remain cough US cough but most of the world uses SI (International System) units for science and buying milk.
For example, this gives us the liquid volume unit the litre. A thousandth of a litre of water, which I hear the kids are calling a 'millilitre', weighs a gram and has a volume of one cubic centimetre. This leads painlessly to the useful result that a litre of water weighs a kilogram, and has a volume equivalent to a 10cm sided cube — or a thousandth of a metre cubed. So a metre cubed of water weighs one thousand kilograms, which we call a ton.
This utility for conversion purposes, easy multiplying and general compatibility with scribbling-on-back of envelope problem-solving is why this unit system is generally win (I now only use imperial/english1 to talk to my mum or NASA).
But now, there seems to be another measurement system for us to get to grips with. I refer of course to the 'friendly unit' or 'FU'. Commonly used in science communication, this system uses units such as the football pitch, the Olympic swimming pool and the human hair to get the man in the street to relate to area, volume, and small lengths respectively. Clearly I haven't spent enough time in the street though, as I for one am lost with them.
Admittedly, I am unsporting in the extreme. The last time I was on a football pitch was when I was 11, and everything seemed pretty big back then. But this means I have to look up the dimensions of the football pitch in good old fashioned metres to find out what the hell people are talking about, which seems to rather defeat the object. And oh joy, when I did look it up, I found that football pitches can be anywhere between 4,050 and 10,800 m². Repeat after me: a varying unit is not a helpful unit!2
My lack of interest in war substitutes means that I am again clueless when we get volumes described in 'Olympic swimming pools'. I think, and Wikipedia agrees, that an Olympic swimming pool is 50m long. Okay. To translate that into a volume you have to multiply that by the depth and width. I assume those people who enjoy flailing in gyms have an idea how wide it is, but you can't see how deep it is due to water being reflective and refractive — judging distance underwater is hard, as is being over 2m tall so you can touch the bottom. Upshot is we are now describing a volume in terms of a visible area. Way to clarify.
However, even ignoring my above bitching as personal prejudice, there is no excuse for the 'bag of sugar' or 'bag of flour' as an FU. Do the people who write these things actually cook? Ever?3 If they had, or if they had even wandered down the baking aisle looking for a packet to heft as research/weaponry, they would know that sugar and flour come in variously sized bags, Where the size is determined by weight. You are now describing a 500g object as weighing the same as a 500g bag of sugar. This is a tad redundant4.
I have already mentioned elsewhere the width of human hair issue, but I would like to further add that the width of a hair depends on: the age of the person concerned, the colour of the hair (red thinnest, then blonde, brown, then black), whether the measurement is taken at the root or the tip, and the weather. According to Brian Ley, the source given in the previous link, these things can cause the hair width to vary between 17 and 181 micrometres. Given this, what distance exactly are you describing when you say 'a quarter of the width of a human hair'? Just give it to me straight in nanometres for godssake.
You may be able to tell that I feel the FU system is slightly silly. Fair enough to give you something to hang your thoughts on, but if you actually want to know how long, wide, heavy or massive something is, nothing beats using actual measurements. It's easier, multi-purpose, international5 and saves time. The only reason not to is if you're unfamiliar with them, which is certainly not going to be helped by refusing to use them in colloquial writing.
Go forth and get used to metric.
1. Americans, for some reason, refer to the imperial system (inches, feet, grains, fathoms) as 'english' measurments. Don't attribute them to us, guys — we've been working on metrification since the 1800's and while admittedly it's gone pretty slowly, we now prosecute people for using them exclusively. True. ←
2. Which, I am sad to say, has its parallels in the world of real units for real people who sometimes leave the street. For example, an American pint is only four fifths of the volume of an English pint — perhaps this explains their tendency to overindulge when in Blighty? And even American gallon-to-gallon conversions can be tricky, since naturally the dry gallon is 16% larger than the wet one. ←
3. I do: I baked a cake in the shape of the internet for my boyfriend's birthday, but didn't have a fine enough icing nozzle to label the IP addresses. So it was a schematic with occupied space in Bailey's icing and unallocated areas in Kahlua flavour. ←
4. Except in the analogous case of an ounce of gold or an ounce of feathers, since precious metals are measured in Troy weight, where ounces are not ounces because jewellers are trying to screw you over. ←