It's simple and consistent
The relationship between small and large units is the same whatever quantity is being measured. That relationship is based on 10 and its powers (100, 1000, etc) to make the conversion effortless.
The importance of the number 10
Multiplying and dividing by 10, 100, 1000, etc is particularly easy. To do so all that is required is to move the decimal point the appropriate number of places right or left.
The comparison between metric and imperial
Imperial has too many factors
Imperial units relate to each other in a way that is complicated with too many different ratios.
|Metric is ...||Easy|
|1 tonne||= 1 000 kg|
|1 kg||= 1 000 g|
|1 g||= 1 000 mg|
|Imperial is ..||Messy|
|1 imperial ton||= 20 cwt|
|1 cwt||= 8 stone|
|1 stone||= 14 lb|
|1 lb||= 16 oz|
Imperial units are inflexible
The larger imperial units are not subdivided into particularly convenient smaller ones so the choice of precision is limited. For example, people often quote personal weight to the nearest stone which can be a wide margin of accuracy. Let us suppose that a person weighed around 12 stone. How would a 5% margin be expressed in pounds? Anyone who is mathematically competent can do it but why should something so basic be so awkward?
The decimal structure of the metric system overcomes this because we only require the percentage of one number and it is so easy to convert between kilograms and grams. For example, 5% of say 75 kg is easy to work out with such strategies as take 10% and halve it to give 3.75 kg. We can then round this to say the nearest 100 g (i.e. 0.1 kg) to reflect the precision displayed by modern digital bathroom scales.
Metric prefixes enable the use of whole numbers only
The system of prefixes for unit names makes it possible to express measurement data in whole numbers only because there are always convenient submultiples. For example, much of industry use the millimetre for linear dimensions rather than the centimetre with the decimal point. This has the advantage of clarity and simplicity in working drawings etc and reduces the chances of errors of interpretation caused by a poorly reproduced or inconspicuous decimal point.
This kind of practice is seldom possible with imperial measures. In situations where the inch is too large to be used without subdivision fractions are used involving cumbersome notation that detract from the clarity of working drawings.
Calculations in metric require fewer steps
When doing measurement calculations it is essential to express all the data in the same units. The awkward unit ratios in imperial means that extra calculation steps are involved to do this conversion.
As an example suppose we want to find how much paint is needed to cover a wall. The wall's measurements are either 4.37 m long and 2.39 m high, or 14 feet 4 inches by 7 feet 10 inches; what is its area?
The decimal basis of metric means we can find the answer straight away (with the aid of a calculator) as 4.37 x 2.39 = 10.4443 or approximately 10.4 m2.
Now we have to multiply 14 feet 4 inches by 7 feet 10 inches to get the result in square feet.
We cannot do this directly without either converting to inches or decimalising the measurements in feet. Either way requires extra arithmetic:
14 feet 4 inches = (14 x 12) + 4 = 172 inches
7 feet 10 inches = (7 x 12) + 10 = 94 inches
Then 172 x 94 = 16 168 square inches = 16 168 ÷ 144 = 112.278 or approximately 112 square feet.
4 inches = 4 ÷ 12 = 0.333 feet
10 inches = 10 ÷ 12 = 0.833 feet
14.333 x 7.833 = 112.271 or approximately 112 square feet.
You might object that working in feet alone is, roughly, accurate. But why not choose a simpler, more direct method - metric - that allows any degree of precision without any extra effort?