Centimetres, kilograms and their kin dramatically simplify arithmetic, making clear the benefits of the metric system. Its competitors are almost out of business, embraced only by eccentric nations such as Liberia, Myanmar, and the US.

Of course, this may simply be orneriness. When the world's economic superpower decides that transferring to the metric system is not worth the effort, and seems to suffers no ill effect, perhaps we might ponder a moment before we preach.

What is the supposed benefit of the metric system? Suppose we had 1573 economists, each 180 centimetres tall, and we have them stand on each other's shoulders. They may still not reach a conclusion (ha ha), but we can easily calculate that their combined height is 180 x 1573= 283140 centimetres, which is about 2831 metres, or about 2.8 kilometres.

The point, of course, is that the metric system is a base-10 system, with 100 centimetres in a metre, 1000 metres in a kilometre, and so on. We then avoid the use of mixed units (such as 5 feet 11 inches), making the multiplication much easier, and then converting to more suitable units is simply a matter of shifting the decimal point. If instead we try to do the same calculation with economists who are 5' 11'' tall (eventually concluding a height of about 1.8 miles), the extra work required is obvious.

On the other hand, this example also makes clear that the metric system is *only* a benefit if we are actually performing arithmetic, and if we are mixing or converting our units. If we simply want to remark upon the height of Ken The Economist, there is absolutely no benefit to the metric system. It is simply a question of which units are more familiar, or which have more psychological meaning. In this regard, one could argue that feet and inches are (or at least once were) preferable.

When do we actually do arithmetic with units? Not as often as one may be led to believe. The obvious example is the use of dollars and cents in commerce. Here the benefits of base 10 are compelling, which is why base 10 monetary units are now used worldwide.

But in most other contexts, the use of base 10 units is quite arbitrary. Take temperature, for example, where Australia has switched from the Fahrenheit scale to the Celsius scale. Why? Because water freezes at 0 degrees Celsius and water boils at 100 degrees Celsius. And who, exactly, could care less?

If the use or non-use of metric systems is contentious, one point is absolutely clear: make sure that everyone is using the same units. For example, if you are building a rocket using metric units (kilograms), try to make sure those calculating the thrust do not use Imperial units (pounds). Otherwise your ''orbiting'' rocket may simply plunge into your ''orbited'' planet, which is exactly what happened with NASA's Mars Climate Orbiter in 1999.

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