France has a unique history of units of measurement due to the radical decision to invent and adopt the metric system after the French Revolution.
In the Ancien régime, before 1795, France used a system of measures that had many of the characteristics of the modern Imperial System of units. There was widespread abuse of the king's standards to the extent that the lieue could vary from 3.268 km in Beauce to 5.849 km in Provence. In the revolutionary era, France used the first version of the metric system. This system was not well received by the public. Between 1812 and 1837, the mesures usuelles was used – traditional names were restored, but were based on metric units: for example, the livre (pound) became 500 g. After 1837, the metric system was reintroduced and has remained the principal system of use to this day.
In the pre-revolutionary era (before 1795), France used a system of measures that had many of the characteristics of the modern Imperial System of units, but there was no unified system of measurement. Charlemagne and successive kings had tried but failed to impose a unified system of measurement in France. (In England, by contrast, the Magna Carta decreed that "there shall be one unit of measure throughout the realm.")
The names and relationships of many units of measure were adopted from Roman units of measure and many more were added – it has been estimated that there were seven or eight hundred different names for the various units of measure. In addition, the quantity associated with each unit of measure differed from town to town and even from trade to trade to such an extent that the lieue (league) could vary from 3.268 km in Beauce to 5.849 km in Provence. It has been estimated that, on the eve of the Revolution, a quarter of a million different units of measure were in use in France. Although certain standards, such as the pied du roi (the king's foot) had a degree of pre-eminence and were used by savants across Europe, many traders chose to use their own measuring devices, giving scope for fraud and hindering commerce and industry.
As an example, the weights and measures used at Pernes-les-Fontaines in southeastern France differ from those catalogued later in this article as having been used in Paris. In many cases, the names are different, while the livre is shown as being 403 g, as opposed to 489 g – the value of the livre du roi. (The Imperial pound is about 453.6 g.)
The French Revolution and subsequent Napoleonic Wars marked the end of the Age of Enlightenment. The forces of change that had been brewing manifest themselves across all of France, including the way in which units of measure should be defined. The savants of the day favoured the use of a system of units that were inter-related and which used a decimal basis.
There was also a wish that the units of measure should be for all people and for all time and therefore not dependent on an artefact owned by any one particular nation. Talleyrand, at the prompting of the savant Condorcet, approached the British and the Americans in the early 1790s with proposals of a joint effort to define the metre. In the end, these approaches came to nothing and France decided to "go it alone".
Decimal time was introduced in the decree of 5 October 1793 under which the day was divided into 10 "decimal hours", the "hour" into 100 " decimal minutes" and the "decimal minute" into 100 "decimal seconds". The "decimal hour" corresponded to 2 hr 24 min, the "decimal minute" to 1.44 min and the "decimal second" to 0.864 s.
The implementation of decimal time proved an immense task and under the article 22 of the law of 18 Germinal, Year III (7 April 1795), the use of decimal time was no longer mandatory. On 1 January 1806, France reverted to the traditional timekeeping.
The metric system of measure was first given a legal basis in 1795 by the French Revolutionary government. Article 5 of the law of 18 Germinal, Year III (7 April 1795) defined five units of measure. The units and their preliminary values were:
Decimal multiples and submultiples of these units would be defined by Greek prefixes - "myria", "kilo", "hecta" (100), "deka" - and Latin prefixes - "deci", "centi" and "milli". Using Cassini's survey of 1744, a provisional value of 443.44 lignes was assigned to the metre which, in turn, defined the other units of measure.
The final value of the metre had to wait until 1799, when Delambre and Mechain presented the results of their survey between Dunkirk and Barcelona that fixed the length of the metre at 443.296 lignes. The law 19 Frimaire An VIII (10 December 1799) defined the metre in terms of this value and the kilogram as being 18,827.15 grains. These definitions enabled the construction of reference copies of the kilogram and metre, which were to be used as standards for the next 90 years.
At the same time, a new decimal-based system for angular measurement was implemented. The right angle was divided into 100 grads, which in turn was divided in 100 centigrads. An arc on the earth’s surface formed by an angle of one centigrad was one kilometre.
Main article: Mesures usuelles
The metric system was introduced into France in 1795 on a district by district basis, starting with Paris. However, by modern standards, the introduction was poorly managed. Although thousands of pamphlets were distributed, the Agency of Weights and Measures, which oversaw the introduction, underestimated the work involved. Paris alone needed 500,000 metre sticks, yet one month after the metre became the sole legal unit of measure, there were only 25,000 in store. This, combined with other excesses of the Revolution made the metric system unpopular.
Napoleon ridiculed the metric system, but as an able administrator, he recognised the value of a sound basis for a system of measurement. Under the décret impérial du 12 février 1812 (imperial decree of 12 February 1812), he introduced a revised system of measure – the mesures uselles or "customary measures" for use in small retail businesses. However, all government, legal and similar works still had to use the metric system and the metric system continued to be taught at all levels of education. The names of many pre-metric units were reintroduced, but were redefined in terms of metric units. Thus the toise (fathom) was defined as being two metres with six pied (feet) making up one toise, twelve pouce (inches) making up one pied and twelve lignes making up one pouce. Likewise, the livre (pound) was defined as being 500 g, each livre comprising sixteen once and each once eight gros and the aune as 120 centimetres.
La loi du 4 juillet 1837 (the law of 4 July 1837) effectively revoked the use of mesures usuelles by reaffirming the laws of measurement of 1795 and 1799 to be used from 1 May 1840. However, many units of measure, such as the livre, remained in colloquial use for many years and the livre still does to some extent.
When this legislation was introduced, the metric system was beginning to take hold across Europe. Switzerland and the German state of Baden had both defined their Fuß (foot) as being 300 mm and the German state of Hessen-Darmstadt has defined its Fuß as being 250 mm. Moreover, the Netherlands, Belgium, Greece, Lombardy and Venice had all adopted the metric system, albeit with local names for the "metre", "kilogram" and so on. The metric system was given a boost when the German Zollverein (Customs Union) introduced the Zollpfund of 500 g in 1850.
The Great Exhibition of 1851 in London was followed by international exhibitions in Paris in 1855 and 1867. The 1867 exhibition had a stand showing how the diverse units of measure were converging onto the metric system – a system that had been developed in France and whose standards were in the custody of the French government, but available for world use.
In 1870, while France was preparing to host an international conference to discuss international cooperation in the sphere of units of measurement, the war broke out. France was humiliated by Prussia's military action, but in 1872 France seized the diplomatic initiative and re-issued the invitations for the 1870 conference. The conference met in 1875 and concluded with the signing of the Treaty of the Metre. The principal agreements under the treaty were:
Thus the French metre and kilogram passed into international control.
During the early part of the twentieth century, the French introduced their own units of power - the poncelet, which was defined as being the power required to raise a mass of 100 kg against standard gravity with a velocity of 1 m/s giving a value of 980.665 W. However, many other European countries defined their units of power (the Pferdestärke in Germany, the paardekracht in the Netherlands and the cavallo vapore in Italy) using 75 kg rather than 100 kg, which gave a value of 735.49875 W (about 0.985 HP). Eventually, the poncelet was replaced with the cheval vapeur, which was identical to equivalent units of measure in neighbouring countries. In 1977, these units, along with the stere and the livre (and amongst others, the German pfund) were proscribed by EEC Directive 71/354/EEC  which required EU member states to standardise on the International System of Units (SI) and therefore to use the watt and its multiples.