One decimal scale removes a whole class of measurement error

The argument

The case for metric begins with what a system of measurement is for: carrying a quantity from one place, person or calculation to another without altering it. Metric units are related by powers of ten, so moving between them shifts a decimal point and nothing else. Proponents argue that this is not a matter of taste but of error. Every conversion in a system of unrelated units is a chance to misremember a factor, and the factors are many and arbitrary: inches to a foot, feet to a yard, yards to a mile, ounces to a pound, pounds to a stone, each a separate thing to learn and a separate thing to get wrong. The same point holds inside a calculation rather than between units. Because the metric units of length, volume and mass are defined in terms of one another, a volume follows from a length and the mass of a volume of water follows from the volume, so quantities that would otherwise need a lookup table fall out of the definitions. Proponents say this is why laboratories, medicine, engineering and manufacturing adopted metric whatever the surrounding country used: when work involves chaining many quantities together, a coherent system removes the steps at which a mistake can enter. Proponents also argue that the learning cost runs the opposite way from how defenders of imperial present it. A child who learns one prefix has learned it for every unit, whereas an imperial education is a long memorisation of unrelated conversions that carry no pattern. The conclusion is that metric is better not because it is modern but because it does the job a measurement system exists to do with fewer places to go wrong.

Premises

Counter-arguments

Critics reply that the error argument describes a problem of the arithmetic classroom rather than of practice. People who work in imperial units do not spend the day converting between them in their heads; they work within one unit and reach for a tool when they need another, exactly as metric users do. They add that coherence has its own costs: a system defined by powers of ten gives awkwardly sized everyday units, so its users end up inventing informal measures of their own, and a scale whose chief virtue is arithmetic is poorly matched to tasks where division into thirds and quarters is what the work actually requires. On that view metric wins on paper and the win does not reach the workshop.

Rejecting the premises

[Rejecting P1] Moving a decimal point is easy only once the quantity is already expressed in metric, and where records, drawings and products are not, the conversion step remains and brings its own factors with it. [Rejecting P2] The imperial factors are learned in childhood and applied without effort thereafter, so counting each one as a standing source of error overstates how often any of them is actually used. [Rejecting P3] Coherence between units matters to calculation rather than to measuring, and most measurement consists of reading an instrument rather than chaining derivations. [Rejecting C] At most it follows that metric is better suited to calculation, not that it is the better system for every use to which measures are put.