To ease the transition to metric measures, straight substitution of units is often used – kg for pounds, metres for yards, km for miles and so on. Ronnie Cohen argues that, as a result, we fail to take advantage of metric’s superiority in dealing with a range of numbers, including the very large (and small).
We often see large numbers used with multiples in the metric system. For example, it is normal to express large distances on earth in thousands of kilometres and large distances across space in millions of kilometres instead of using megametres or gigametres. I suspect that many see the kilometre as the metric counterpart to the mile and are probably unaware that the metric system has bigger multiples of length. The promotion of larger multiples of length, which imperial cannot match, would show one of the strengths of the metric system, namely the ability to express extremes at both ends of the scale.
Similarly, use of gigawatts instead of thousands of megawatts and greater use of kilotonnes and megatonnes would enable more Britons to become more familiar with SI prefixes and make metric more comprehensible for all.
In the UK, it is common to use the rule of 1000 and avoid intermediate multiples and that probably contributes to the image of the metric system as scientific and technical rather than a general-purpose measurement system that is suitable for all purposes. For example, millilitres and litres are commonly used and one is much more likely to see a product label for a drink that shows a volume of several hundred millilitres than a smaller number of centilitres or decilitres. These intermediate units are less frequently used in the UK. Perhaps a label that shows 5 dl or 50 cl would look more comprehensible and human in scale than 500 ml.
The rule of 1000 is commonly followed in DIY stores where far more products are labelled in millimetres than centimetres, probably because architects and builders work in millimetres. I have seen private height restriction signs and heights of buses shown in millimetres and metres but never in centimetres. It seems odd to use millimetres for such purposes when we consider that we would be unlikely to express our height in millimetres. We might do so in centimetres or metres but never in millimetres.
The way that metric units are commonly used in the UK is probably a legacy of the use of imperial where metric units are often seen as equivalents of imperial units. Poor choices in the use of multiples have been exploited by anti-metric campaigners. The users who make those poor choices, not the system itself, are at fault. Sensible use of the appropriate multiples would make it possible to use smaller numbers to express measurements and might promote acceptance of metric in the UK.
Let’s see how better use of multiples can be put into practice with practical examples. On 16 June 2012, The Economist published a special report about the Arctic, which contained some measurements with huge numbers. Here are a few quotes from the report with examples:
- “Then, in 2007, the sea ice crashed, melting to a summer minimum of 4.3m sq km, close to half the average for the 1960s and 24% below the previous minimum, set in 2005.” 4.3m sq km can be expressed as 4.3 square megametres.
- “According to a 2008 study by the US Geological Survey, the Arctic may hold 90 billion barrels of oil and 1669 trillion cubic feet of natural gas, respectively 13% and 30% of the world’s estimated undiscovered reserves.” 90 billion barrels of oil can be expressed as 14.3 teralitres of oil. 1669 trillion cubic feet of natural gas can be expressed as 47.26 thousand cubic kilometres of natural gas.
- “According to an estimate made in 2009, terrestrial permafrost holds about 1.7 trillion tonnes of carbon, roughly twice as much as the atmosphere.” 1.7 trillion tonnes can be expressed as 1.7 teratonnes.
With the imperial system, we are often forced to use extremely large numbers for expressing very large quantities because the imperial system has a limited range at both ends of the scale and struggles when describing the extremely large and extremely small. This practice is often seen with metric units but it is not necessary.