Metric system in the context of "Drill and tap size chart"

The International System of Units, internationally known by the abbreviation SI (from French Système international d'unités), is the modern form of the metric system and the world's most widely used system of measurement. It is the only system of measurement with official status in nearly every country in the world, employed in science, technology, industry, and everyday commerce. The SI system is coordinated by the International Bureau of Weights and Measures, which is abbreviated BIPM from French: Bureau international des poids et mesures.

The SI comprises a coherent system of units of measurement starting with seven base units, which are the second (symbol s, the unit of time), metre (m, length), kilogram (kg, mass), ampere (A, electric current), kelvin (K, thermodynamic temperature), mole (mol, amount of substance), and candela (cd, luminous intensity). The system can accommodate coherent units for an unlimited number of additional quantities. These are called coherent derived units, which can always be represented as products of powers of the base units. Twenty-two coherent derived units have been provided with special names and symbols.

The inch (symbol: in or ″) is a unit of length in the British Imperial and the United States customary systems of measurement. It is equal to ⁠1/36⁠ yard or ⁠1/12⁠ of a foot. Derived from the Roman uncia ("twelfth"), the word inch is also sometimes used to translate similar units in other measurement systems, usually understood as deriving from the width of the human thumb.

Standards for the exact length of an inch have varied in the past, but since the adoption of the international yard during the 1950s and 1960s the inch has been based on the metric system and defined as exactly 25.4 mm.

Fuel efficiency (or fuel economy) is a form of thermal efficiency, meaning the ratio of effort to result of a process that converts chemical potential energy contained in a carrier (fuel) into kinetic energy or work. Overall fuel efficiency may vary per device, which in turn may vary per application, and this spectrum of variance is often illustrated as a continuous energy profile. Non-transportation applications, such as industry, benefit from increased fuel efficiency, especially fossil fuel power plants or industries dealing with combustion, such as ammonia production during the Haber process.

In the context of transport, fuel economy is the energy efficiency of a particular vehicle, given as a ratio of distance traveled per unit of fuel consumed. It is dependent on several factors including engine efficiency, transmission design, and tire design. In most countries, using the metric system, fuel economy is stated as "fuel consumption" in liters per 100 kilometers (L/100 km) or kilometers per liter (km/L or kmpl). In a number of countries still using other systems, fuel economy is expressed in miles per gallon (mpg), for example in the US and usually also in the UK (imperial gallon); there is sometimes confusion as the imperial gallon is 20% larger than the US gallon so that mpg values are not directly comparable. Traditionally, litres per mil were used in Norway and Sweden, but both have aligned to the EU standard of L/100 km.

Metrication or metrification is the act or process of converting to the metric system of measurement. All over the world, countries have transitioned from local and traditional units of measurement to the metric system. This process began in France during the 1790s, and has persistently advanced over two centuries, accumulating into 95% of the world officially exclusively using the modern metric system. Nonetheless, this also highlights that certain countries and sectors are either still transitioning or have chosen not to fully adopt the metric system.

Hecto (symbol: h) is a decimal unit prefix in the metric system denoting a factor of one hundred. It was adopted as a multiplier in 1795, and comes from the Greek ἑκατόν hekatón, meaning "hundred". In 19th century English it was sometimes spelled "hecato", in line with a puristic opinion by Thomas Young. Its unit symbol as an SI prefix in the International System of Units (SI) is the lower case letter h.

The prefix is rarely used in general, but has certain specific applications:

A unit of length refers to any arbitrarily chosen and accepted reference standard for measurement of length. The most common units in modern use are the metric units, used in every country globally. In the United States the U.S. customary units are also in use. British Imperial units are still used for some purposes in the United Kingdom and some other countries. The metric system is sub-divided into SI and non-SI units.

United States customary units form a system of measurement units commonly used in the United States and most U.S. territories since being standardized and adopted in 1832. The United States customary system developed from English units that were in use in the British Empire before the U.S. became an independent country. The United Kingdom's system of measures evolved by 1824 to create the imperial system, with imperial units, which was officially adopted in 1826, changing the definitions of some of its units. Consequently, while many U.S. units are essentially similar to their imperial counterparts, there are noticeable differences between the systems.

The majority of U.S. customary units were redefined in terms of the meter and kilogram with the Mendenhall Order of 1893 and, in practice, for many years before. These definitions were refined by the international yard and pound agreement of 1959.

Metrology is the scientific study of measurement. It establishes a common understanding of units, crucial in linking human activities. Modern metrology has its roots in the French Revolution's political motivation to standardise units in France when a length standard taken from a natural source was proposed. This led to the creation of the decimal-based metric system in 1795, establishing a set of standards for other types of measurements. Several other countries adopted the metric system between 1795 and 1875; to ensure conformity between the countries, the Bureau International des Poids et Mesures (BIPM) was established by the Metre Convention. This has evolved into the International System of Units (SI) as a result of a resolution at the 11th General Conference on Weights and Measures (CGPM) in 1960.

Metrology is divided into three basic overlapping activities:

Chinese units of measurement, known in Chinese as the shìzhì ("market system"), are the traditional units of measurement of the Han Chinese. Although Chinese numerals have been decimal (base-10) since the Shang, several Chinese measures use hexadecimal (base-16). Local applications have varied, but the Chinese dynasties usually proclaimed standard measurements and recorded their predecessor's systems in their histories.

In the present day, the People's Republic of China maintains some customary units based upon the market units but standardized to round values in the metric system, for example the common jin or catty of exactly 500 g. The Chinese name for most metric units is based on that of the closest traditional unit; when confusion might arise, the word "market" (市, shì) is used to specify the traditional unit and "common" or "public" (公, gōng) is used for the metric value. Taiwan, like Korea, saw its traditional units standardized to Japanese values and their conversion to a metric basis, such as the Taiwanese ping of about 3.306 m based on the square ken. The Hong Kong SAR continues to use its traditional units, now legally defined based on a local equation with metric units. For instance, the Hong Kong catty is precisely 604.78982 g.

The traditional French units of measurement prior to metrication were established under Charlemagne during the Carolingian Renaissance. Based on contemporary Byzantine and ancient Roman measures, the system established some consistency across his empire but, after his death, the empire fragmented and subsequent rulers and various localities introduced their own variants. Some of Charlemagne's units, such as the king's foot (French: pied du Roi) remained virtually unchanged for about a thousand years, while others important to commerce—such as the French ell (aune) used for cloth and the French pound (livre) used for amounts—varied dramatically from locality to locality. By the 18th century, the number of units of measure had grown to the extent that it was almost impossible to keep track of them and one of the major legacies of the French Revolution was the dramatic rationalization of measures by the new metric system. The change was extremely unpopular, however, and a metricized version of the traditional units—the mesures usuelles—had to be brought back into use for several decades.

The litre (Commonwealth spelling) or liter (American spelling) (SI symbols L and l, other symbol used: ℓ) is a metric unit of volume. It is equal to 1 cubic decimetre (dm), 1000 cubic centimetres (cm) or 0.001 cubic metres (m). A cubic decimetre (or litre) occupies a volume of 10 cm × 10 cm × 10 cm (see figure) and is thus equal to one-thousandth of a cubic metre.

The original French metric system used the litre as a base unit. The word litre is derived from an older French unit, the litron, whose name came from Byzantine Greek—where it was a unit of weight, not volume—via Late Medieval Latin, and which equalled approximately 0.831 litres. The litre was also used in several subsequent versions of the metric system and is accepted for use with the SI, despite it not being an SI unit. The SI unit of volume is the cubic metre (m). The spelling used by the International Bureau of Weights and Measures is "litre", a spelling which is shared by most English-speaking countries. The spelling "liter" is predominantly used in American English.

A graduation is a marking used to indicate points on a visual scale, which can be present on a container, a measuring device, or the axes of a line plot, usually one of many along a line or curve, each in the form of short line segments perpendicular to the line or curve. Often, some of these line segments are longer and marked with a numeral, such as every fifth or tenth graduation. The scale itself can be linear (the graduations are spaced at a constant distance apart) or nonlinear.

Linear graduation of a scale occurs mainly (but not exclusively) on straight measuring devices, such as a rule or measuring tape, using units such as inches or millimetres.

A system of units of measurement, also known as a system of units or system of measurement, is a collection of units of measurement and rules relating them to each other. Systems of measurement have historically been important, regulated and defined for the purposes of science and commerce. Instances in use include the International System of Units or SI (the modern form of the metric system), the British imperial system, and the United States customary system.

Avoirdupois (/ˌævərdəˈpɔɪz, ˌævwɑːrdjuːˈpwɑː/; abbreviated avdp.) is a measurement system of weights that uses pounds and ounces as units. It was first commonly used in the 13th century AD and was updated in 1959.

In 1959, by international agreement among countries that used the pound as a unit of mass, the International Avoirdupois Pound was fixed at the modern definition of exactly 0.45359237 kilograms.. It remains the everyday system of weights used in the United States, and is still used, in varying degrees, in everyday life in the United Kingdom, Canada, Australia, and some other former British colonies, despite their official adoption of the metric system.

The pound or pound-mass is a unit of mass used in both the British imperial and United States customary systems of measurement. Various definitions have been used; the most common today is the international avoirdupois pound, which is legally defined as exactly 0.45359237 kilograms, and which is divided into 16 avoirdupois ounces. The international standard symbol for the avoirdupois pound is lb; an alternative symbol (when there might otherwise be a risk of confusion with the pound-force) is lb_m (for most pound definitions), # (chiefly in the U.S.), and ℔ or ″̶ (specifically for the apothecaries' pound).

The unit is descended from the Roman libra (hence the symbol lb, descended from the scribal abbreviation, ℔). The English word pound comes from the Roman libra pondo ('the weight measured in libra'), and is cognate with, among others, German Pfund, Dutch pond, and Swedish pund. These units are now designated as historical and are no longer in common usage, being replaced by the metric system.