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 ValueNormal 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