Before the metric system

For more details on this topic, see History of measurement.

The Roman empire used the pes (foot) measure. This was divided into 12 unciae ("inches"). The libra ("pound") was another measure that had wide effect on European weight and currency long after Roman times, e.g. lb, £. The measure came to vary greatly over time. Charlemagne was one of several rulers who launched reform programmes of various kinds to standardise units for measure and currency in his empire, but there was no real general breakthrough.
In medieval Europe, local laws on weights and measures were set by trade guilds on a city-by-city basis. For example, the ell or elle was a unit of length commonly used in Europe, but its length varied from 40.2 centimetres in one part of Germany to 70 centimetres in The Netherlands and 94.5 centimetres in Edinburgh. A survey of Switzerland in 1838 revealed that the foot had 37 different regional variations, the ell had 68, there were 83 different measures for dry grain, 70 measures for fluids and 63 different measures for "dead weights".^[19] When Isaac Newton wrote Philosophiae Naturalis Principia Mathematica in 1687, he quoted his measurements in Parisian feet so readers could understand the size. Examples of efforts to have local intercity or national standards for measurements include the Scottish law of 1641, and the British standard imperial system of 1824, which is still commonly used in the United Kingdom. At one time Imperial China had successfully standardised units for volume throughout its territory, but by 1936 official investigations uncovered 53 values for the chi varying from 200 millimetres to 1250 millimetres; 32 values of the cheng, between 500 millilitres and 8 litres; and 36 different tsin ranging from 300 grams to 2500 grams.^[20] However, revolutionary France was to produce the definitive International System of Units which has come to be used by most of the world today.
The desire for a single international system of measurement came from growing international trade and the need to apply common standards to goods. For a company to buy a product produced in another country, they need to ensure that the product would arrive as described. The medieval ell was abandoned in part because its value could not be standardised. One primary advantage of the International System of Units is simply that it is international, and the pressure on countries to conform to it grew as it became increasingly an international standard. However, it also simplified the teaching and learning of measurement as all SI units are based on a handful of base units (in particular, the metre, kilogram and second cover the majority of everyday measurements), using decimal prefixes to cover all magnitudes. This contrasts with pre-metric units, which largely have names that do not relate directly to one another (e.g. inch, foot, yard, mile) and are related to one another by inconsistent ratios which must simply be memorised, (e.g. 12, 3, 1760). As the values in an SI expression are always decimal (i.e. without vulgar fractions) and mixed units (such as "feet and inches") are not used with SI, measurements are easy to add or multiply. Moreover, scientific measurement and calculation are greatly simplified as the units for electricity, force etc. are part of the SI system and hence are all interrelated in a coherent manner (e.g. 1 J = 1 kg·m²·s⁻² = 1 V·A·s). Standardization of measures has contributed significantly to the industrial revolution and technological development in general^{[according to whom?]}. SI is not the only example of international standardization; several powerful international standardization organizations exist for various industries, such as the International Organization for Standardization (ISO), the International Electrotechnical Commission (IEC), and the International Telecommunication Union (ITU).