In a previous post, I alluded to some of the disadvantages of the metric system, and I think it’s something worth expanding on in its own post. The metric system is very useful and has its advantages, which I have included in this post. However, I don’t go into much detail with the pros here as there are other sites that have done that and I don’t think enough attention has been drawn to its cons!



Metric base units
Metric base units

All metric units are easily converted between each other by simply moving the decimal places. For example, converting from metre to millimetre is as simple as moving the decimal three places to the right. 

Prefixed naming convention

All metric units are clearly related to each other using prefixes. For example if a unit begins with kilo we immediately know that it’s in multiples of 1000 of the base unit (1km = 1000m etc).

Precise whole units

Metric units can be as precise as you need them to be by simply moving down to the next prefixed unit. For example, if 1 gram is not precise enough you can use milligrams. 


In the SI metric system, we are encouraged to use as few units as possible to allow for greater simplicity and ease of understanding. For example, the litre, tonne and hectare are not part of the System of International Units (SI) as they are not necessary, which in turn reduces the chances of confusion by having too many units. By the way, the litre, tonne and hectare (along with other units) are legacy units that are permitted to be used alongside the SI metric system.


One size doesn’t fit all

On the face of it, the metric system looks like a logical, simple, easy to understand system of measuring things. And it is, in a lot of areas like science, maths and engineering. The issue is when you try to apply it to everyday uses, like weighing out carrots for sale in a supermarket, or measuring the amount of buttermilk you need in your brown bread recipe. By the way, if you’re looking for a good brown bread recipe, this one on the odlums site is pretty good. Anyway, back to the exciting topic of the metric system. The trouble is that metric units can quite often be either too big or too small for everyday use.

The metric system is designed to be as simple and to have as few units as is possible. For example, in weight (or mass) there are two possible units you can use; the gram or the kilogram. A gram is a tiny weight and a kilogram is quite big. To put it into perspective, a gram is about a ¼ teaspoon of sugar, and a kilogram is two bags of sugar. Quite a big jump. The same for volume, we’ve got a millilitre the size of ⅕ of a teaspoon of water and a litre the size of two regular sized bottles of water. So what happens when you use these for our bag of carrots or the buttermilk in our recipe? The gram is so small that you end up with large numbers like 750g for a regular sized bag of carrots, 400g for tins of stuff like tomatoes and 500g for bags of sugar. Same with the millilitre, 250ml of this, 500ml of that, and so on.

While this works fine, it does make for unnecessarily large numbers. It seems there may be a little room between 1 and 1000 for some more units that would make our measurements into smaller numbers, which are easier on the eye. More on this in future posts…

Rubbish sounding words

The idea of using prefixes on metric base units seems like a good one on the face of it, but you do end up with pretty rubbish sounding words. This is because in the metric system, the names of the units are artificially created and stuck to a rigid system using prefixes, and the resulting word may not sound good in everyday use. For example; Gram sounds okay, but kilogram is a longish, not particularly pleasant sounding word. The same goes for millilitre, millimetre and kilometre. They’re not exactly words that roll off the tounge. Even the not-so-bad sounding words litre and metre still aren’t anything to write home about, and don’t sound quite the same as pint or yard.

If I was a scientist I wouldn’t give a toss about this, but I think in the units we use to weigh and measure things in our everyday lives, it’s worth using words that sound good when we use them.

Arbitrary scale

While the origins of traditional units of measurement are based on things we can relate to on a human scale, like a thumb, foot, cup, stone etc; metric units are on much more of a scientific scale. For example, the definition of a metre is;

The length of the path travelled by light in a vacuum in 1299 792 458 of a second 

All other units of length (and by extension area and volume) are based off this definition of a metre. The disadvantage here is that, while this is great in scientific and technical areas where you have measuring equipment, the derived units are not immediately relative to anything useful on a human scale. I’m talking particularly about the smaller units here, centimetre and millimetre. When estimating the length of something without a measuring tape, it’s not as easy to visualise this with centi/millimetres, whereas you might do better visualising how many of your thumbs or your feet or paces or your full body length something is. Quite often, we estimate measurements by eye – which I don’t think metric units are really designed for.

Plus I think this arbitrary scale teaches us that we need to have measuring equipment to measure something, whereas it might be better and more fun to teach kids about measuring things using whatever they can think of based on what’s around them. This might get kids to understand size and scale a bit better from the beginning. And then of course move on to using measuring tapes and rulers! 

The prefixes can be cumbersome and unnecessary in everyday use

Despite the prefixes being one of the pros mentioned above, they can also be a con. Again, I’m excluding scientific and technical areas here, but think about how we measure things; we usually only use a couple of units in a particular scenario. For example, if measuring a small length (no smart comments please!) using the metric system, you will only ever use millimetres or centimetres. You probably don’t need to know at that particular time how the units you’re using relate to the metre, kilometre, megametre etc. The same goes for figuring out how far away your trip the beach is going to be. We all have a preconceived notion of how much 1km or 50km or 100km (or the same in miles) is based on prior knowledge. When using kilometres, we don’t care about metres or millimetres.

To illustrate this further, let’s convert the only measurement that hasn’t been metricated yet – time. The metric system does have a definition for the second but not any other units of time (makes me wonder how kilometres per hour works) so let’s use the second as the base unit. From there we move up the scale to deca, kilo, mega, and giga to match something close to what we currently use. I stop at the equivalent of one week here and am also skipping hecto because it doesn’t fit any units close to normal time. What you end up with is shown in the table below.

 Unit  Value Normal time equivalent
 Second  1 second  1 second
 Hectosecond  100 sec  1 m 40 s
 Kilosecond  1,000 sec  16 m 40 s
 Megasecond  1,000,000 sec  1 wk 4 days 13 hr 46 min 40 sec

As you can see, the multiple use of prefixed words becomes cumbersome and would be awkward in everyday language. This is a limitation of the prefixed base unit idea, while it does work, it tends to get confusing if you need to use a lot of them. You could apply the same logic to lengths (a decametre, hectometre etc). We don’t want to use too many of these prefixes to avoid confusion, which makes me wonder; outside of the scientific circle, what’s the point of them if we only use a couple of them at a time? Could we not just use different words that aren’t directly related to one another? It’s not that hard to remember a couple of words.


So there you have it, the pros and (mostly) cons of the metric system. I think based on the points above, we can say that the metric system is great in science, maths or any situation where you need precision; but it is not really at its best when we use it for everyday things like in recipes or at the supermarket. Which of course is fine, nothing is perfect!

The pros and cons of the metric system
  • Michael Glass

    In practice, the common metric units are trimmed down in usage. For example, the kilogram is shortened to kilo, and the supposed difficulty in asking for 500g of carrots is something that people take in their stride. It’s the same with the litre and millilitre. In speech this becomes 5 mils and in writing, 5 mL of 5ml – not a great strain. In Australia, kilometre become a “k.” and in US army parlance it becomes a click. As for the kilogram being too large for common use, I am surprised at that. I buy kilo packets of carrots and capsicums (peppers). The litre is quite a common size, and many liquids come in 2 litre containers, including milk, fruit juice and liquid laundry detergent.

    I would also point out that the Magna Carta was right in decreeing that there should be one measure of corn and wine in England. Because of this, England had standard units long before the rest of Europe. It really is unnecessarily cumbersome to have one scale for ordinary things and another for science and industry. The metric system will do just fine for both purposes.

    • Hi Michael,

      Thanks for your comment. A couple of points;

      – Why do you need to shorten kilometre to K, kilogram to kilo and millilitre to mil?
      – My point on the ‘large’ kilogram was more to do with the gap between the gram and kilogram and its use in smaller products. The kilogram is just fine in size for large products.
      – To your point on the different scales of the two systems, I think a possible solution there would be to align the traditional units to exact metric values and have one ‘merged’ system. That’s something I may explore in a future post.

      • Michael Glass

        The idea of merging the two systems is an interesting one. Actually, Napoleon tried it with his :Mesures usuelles: where the old measures were recalibrated with the newly developed metric measures. They ended up being swept away. Read about it here:

        Why do we shorten kilometres to ks? For the same reason we shorten mosquitoes to mozzies, swimming costumes to cossies, utility vehicles to utes and a whole raft of other ones, some of which you probably use, such as “pram” for perambulator and zoo for zoological gardens. It’s just what we do in Australia (Oz).


  • Mark Williams

    On a point of order:

    Unit nameMagnitude`Normal’ time equivalent:
    decasecond10 s~8.27 microfortnight,
    hectosecond100 s~82.7 microfortnight,
    kilosecond1000 s~827 microfortnight,
    megasecond1000000 s~827 millifortnight,
    gigasecond1000000000 s~827 fortnight.

    So time has been metricated. Calendars have not, but they can and have been decimalised. For example, the Julian date has the solar day as the base unit—usually, but not always, 86.4 ks at present—and sub-divided decimally. Computer software commonly uses at least the integer day number part as an intermediate when doing date arithmetic or converting between religious calendars.

    I did not find most of your `cons’ opinions convincing, but agree with you about the km/h thing. Most people can accurately visualise a kilometre—unlike an imperial mile, IMO—but few can reliably keep track of a whole hour and fewer still divide by it. In the places where km/h are most commonly encountered and if you were starting from scratch, use of m/s would be more widely intuitive.

  • metricadvocate

    Your “time” example table appears to reflect a complete misunderstanding of metric prefixes, 1 das = 10 s, 1 ks = 1000 s, etc. Since you interpret all the prefixes incorrectly, your conventional times don’t match either.
    Also note that the minute, hour day are not SI units but are explicitly approved for use with the SI, with the understanding that they are not coherent units and formulas have to be modified in calculations. I suppose if you are scared of numbers larger than 10, you should stick with the compound units of Imperial. On the other hand, Imperial is full of units which people no longer remember the meaning of, drams, poles, chains, furlongs, fortnights, stones (well Americans never knew what a stone was or why some hundredweights are 112 lb, WTF?)

    • Your “time” example table appears to reflect a complete misunderstanding of metric prefixes, 1 das = 10 s, 1 ks = 1000 s, etc. Since you interpret all the prefixes incorrectly, your conventional times don’t match either.

      I did indeed get that wrong, I’ll get it fixed up for you. In the meantime, please accept my apologies

      • metricadvocate

        Changed but not “fixed.” Note that 1 Ms > 11 days, and 1 Gs > 11574 days (31+ years). Note that giga is 10^9.

  • Sam

    Your example about the litre being too big and the millilitre being too small ignores the obvious fact of the other prefixes.

    For example, at bars and restaurants drinks tend to be measured in decilitres (100 ml), in supermarkets you tend to see both millilitres and centilitres for small items.

    Surprisingly where I live we tend only to use the gram and kilogram, no mention of the other prefixes, but I have seen them used in other countries (1 hectogramme of cheese please).

    In terms of distances, on the road kilometres and metres are only used, but for other sports like skiing all the units are in hectometres.

    • In that case, please refer to my other point about the use of prefixes making the words sound rubbish

      • dan_2000

        Only to those who have made an effort to avoid using metric units and find themselves on the outside looking in. In most cases, the 4 prefixes around unity have limited use. We prefer to scale using the prefixes in powers of three. You seem to be looking for reason to debase SI, so you pick examples outside of the normal use of the system. If you are trying to convince the world to de-metricate, give it up, you are fighting a losing battle. As time goes on those clinging to obsolete units are the ones that will suffer hardship.