metre.info

Modern weights and measures – A complete reference.

 

Table of contents – Sitemap

 

(SI, short for “International System of Units”, is the technical name for the modern metric system.)

This website consists of five parts:

Introduction etc.

Tables of units

Rules of the metric system

Explanations for every measure

Table of obsolete metric units

 

The longest and probably most important part of the dictionary is the unit encyclopaedias. Every weight and measure of the metric system is listed. Each article usually contains the official definition, an explanation of what the unit measures and how it is used, some facts about the history of the unit and some reference points. This exhaustive list of all metric units is organised alphabetically, but split into the three sections of base units, derived units and additional units. It is usually fastest to look up a unit in the tables of units and click on it, rather than scrolling down the page to find it. First time visitors should probably start with the tables of units.

The tables of units at the beginning are an exhaustive list of all metric units. They also include some formulas that show the relation to other units. The tables are great for anyone who has to calculate with units or just wants a handy, concise and exhaustive reference of all measurement units.

Please note that some of the reference points in the unit explanations have been rounded or are approximate values only. They are certainly not more accurate than the number of decimal places would suggest.

This compendium lists only modern weights and measures. One way to help understanding archaic units is to convert them into the respective current unit and then to look up the result here. Excellent websites for unit conversions are given in the links section.

The table of obsolete metric units provides a list with conversion values and a brief comment for some former metric units.

 

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The modern metric system has been a great success story of the last 200 years. It greatly simplified life, helped the understanding of our world and has reduced costly and dangerous mistakes and misunderstandings.

Nearly all countries are now using metric units exclusively. Most developed countries had already switched to the metric system in the 1800s. Only a handful of countries are still caught in the cumbersome and costly transition phase, which involves so much confusion, converting units back and forth and makes the understanding of physics and our world in general by the general public so much more difficult.

This encyclopaedia of the metric system is intended both for experienced users of metric units and for newcomers. It tries to explain all official weights and measures in simple language, gives examples and also provides definitions and formulas for people working and calculating with measurement units.

For most units, reference points are also given. This is particularly useful if a unit is unfamiliar to the reader. Scientific definitions and most formulas are not much help in daily life, but knowing two reference points for a unit will be, in most cases, sufficient for understanding a measure.

This website gives an exhaustive list and provides explanations and backgrounds to all current metric units. If a unit is not listed, it is not an official measurement unit within the modern metric system.

This website is not intended to be a tool or a help for the conversion of non-metric units. There are already a number of excellent websites available (see links section), which provide this assistance much better than I could have done. All non-metric units, archaic or fairly recent, are defined in terms of the official metric units. All users of non-metric units use, knowingly or unknowingly, the metric system and benefit from its coherence and stability. Non-metric standards and definitions for weights and measures have altogether disappeared many years ago. This means that, in order to understand the precise definition of non-metric units, the underlying metric units must be understood also.

In order to work or to calculate with non-metric units, it is usually easiest to convert them into metric first, before starting any calculation or project. Metric units do not involve any conversion factors; measurements and units can be put straight into formulas and the result can often be calculated without so much as the help of a calculator. Even converting between different non-metric units is often made easier by converting into metric first and then into the second non-metric unit.

The beauty of the metric system is its logic, coherence and stability (the magnitude of units does not change over time). Another big advantage is that only very few units need to be known to understand every possible weight and measure.

 

“If you can measure that of which you speak,
and you can express it by number,
you know something of your subject;
but if you cannot measure it,
your knowledge is meagre and unsatisfactory.”

Lord Kelvin

 

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The metric system originated in France during the 1790s. At that time, weights and measures consisted pretty much exclusively of measurements for length, weight, area, volume and time. Later, with the advancement of science and discoveries such as electricity and radioactivity, the number of different measures grew. Today, the metric system covers every conceivable measurable quantity.

 

Situation before the creation of metric units

Historically, not only had most countries and regions their own, unique weights and measures, but also many trades. Usually, the magnitude of these units, while they often kept their original name, changed over time. These weights and measures were not part of an orderly system; they just developed and changed arbitrarily. Reasons for changing old or introducing new measures could be as simple as attempts at hiding increases in taxation or price.

This is still a problem when converting archaic units or when non-metric units are used.

Before the introduction of the metric system, a bewildering array of measures was used. Some countries, such as France, had up to 800 different measures by name. More confusing still, even measures with the same name could describe very different quantities. The tiny duchy of Baden in Germany had 112 different ells.

Non-metric measures suffered from the following inconsistencies:

§         Measures varied by region (a London pound was different from a Newcastle pound)

§         Measures varied by object measured (a gallon of beer was different from a gallon of wine)

§         Measures varied by trade or context (the nautical mile was different from the geographical mile)

§         Measures varied by the origin of the measure (Britain used at least three different sets of measures: avoirdupois, troy and apothecary)

§         Measures changed over time (The pint of 1850 was different from the pint of 1800)

This chaos was the norm in most countries.

 

Fundamentals of the metric system

To improve on this confusing situation which meant that a farmer did not understand nautical measures and a merchant was not familiar with weights and measures used by doctors, the fathers of the metric system intended to create a system for all people, for all time.

One of the fundamental ideas behind the metric system was the use of only one measure per physically measurable quantity. To avoid having to use very small or large numbers with these measures, prefixes were invented. Prefixes (such as milli- or kilo-) are qualifiers that change the magnitude of a measurement unit when put in front of the unit name. By using this system of units and prefixes, the number of units with different names can be greatly reduced – weights and measures become simpler.

The prefixes were designed to work nicely with the decimal system; that is our system of writing numbers. The decimal system had been used for thousands of years and was the only numbering system in use in recent history. Prefixes can be changed by simply moving the decimal marker – no conversions are necessary.

Another idea was to define every measurement unit in terms of base units. The metric system today has seven such base units; all other units can be converted into base units. For these unit conversions, conversion factors are not needed. The conversion factor is usually 1, thus eliminating mistakes and inaccuracy through rounding.

The metric system created, for the first time, a system that allowed weights and measures to be understood by anybody.

 

History and current situation

The metric system had enjoyed considerable success and spread to many European countries throughout the 1800s. During the 1870s, the French government decided to make the metric system truly international by installing an international body to oversee weights and measures.

On 20 May 1875, the Treaty of the Meter was signed by 17 countries. This established the International Bureau of Weights and Measures (BIPM), which is still in charge of our weights and measures. It receives its instructions from the International Committee for Weights and Measures (CIPM), which in turn is authorized by the General Conference on Weights and Measures (CGPM). Delegates from all member states of the Treaty of the Meter attend the CGPM meetings. At present, the CGPM meets every four years – this is when major decisions affecting our weights and measures are being taken.

This international and democratic nature of the metric system led to it being adopted in all countries. The units listed on this website are legal measures in all countries around the globe. Magnitudes and definitions of units, symbols and rules are identical in the whole world.

Some countries still permit the use of some non-metric units, but even these units have been defined in terms of metric units for a very long time. The metric system thus is the basis for all measuring world-wide.

 

The metric system is not static; it evolves along with all science. Since the 11th meeting of the CGPM in 1960, the metric system is called SI (International System of Units). In this document, the terms metric system, modern metric system and SI are used interchangeably.

Evolution of the metric system does not mean that the magnitudes of units are changing. A metric unit of a certain name always has meant and will mean exactly the same thing, no matter where, or when, or how used. This is one of the great achievements of metric: stability, reliability and reduction of error, confusion and misunderstandings.

 

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About:

This is a private website, sharing one of my interests with other people. I was always interested in physics and became more involved with weights and measures when I had to communicate with countries and people that were using different units from the ones I was accustomed to. I started a collection of notes and other material about measurement units to help me with that task. Over time, my collection grew and I thought it might be helpful or interesting to other people as well. That’s why I created this website.

Sources:

I used a large variety of sources to write this dictionary (encyclopaedias, dictionaries, books about physics, weight and measures, mathematics, history and also many websites). Probably the most important web-based sources are listed in the links section.

I believe all of the information used for and contained in this website is in the public domain.

Copyright:

This website is intended to be a free source for information for anybody interested and is not part of any organization. The contents of this website can be freely used and copied (please mention the source and include a link to this website), but not with the intent of making money. If a commercial organization wants to use content from this website, permission from the author is required.

Obviously, mistakes do happen – the author can not be held responsible for any errors, mistakes, inaccuracies or misunderstandings on this website or any other issues arising from it.

Links to metre.info

For website owners: I do appreciate links to metre.info. Please read this before creating the link.

Updates:

First placed online:       2002-07-18

Last update:                  2005-10-09

 

 

Web counter from www.digits.com 

 

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For website owners: I do appreciate links to metre.info. Please read this before creating the link.

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Obviously, symbols for measurement units and their prefixes have been used throughout this document. It is easiest to look up those in the tables of units.

The only other abbreviations used are:

 

BA: British Association for the Advancement of Science. Organization that played a significant role in developing the metric system in the second half of the 19^th century.

BAAS: Former name for the BA (British Association for the Advancement of Science).

BIPM: International Bureau of Weights and Measures (Bureau International des Poids et Mesures). This international institution was set up by the Convention of the Metre in 1875. It is financed and run jointly by the member states of the convention. Its mandate is to provide the basis for a single, coherent system of measurements throughout the world, traceable to the International System of Units (SI). The headquarters of the BIPM are located on international territory in Sèvres, near Paris in France. The BIPM is managed and administered by the CIPM.

The BIPM is the heart and the guardian of the metric system. It currently has about 70 staff and a budget of about ten million euros.

CGPM: General Conference on Weights and Measures (Conférence Générale des Poids et Mesures). This conference is a council set up by the Convention of the Metre in 1875. It is made up of representatives of every member state of the convention and meets every four yours. This conference discusses the report from the CIPM and decides on all major issues relating to the metric system and the BIPM. The CGPM is the highest institution of the Convention of the Metre. Its meetings are thus the most significant events in the evolvement and improvement of our system of weights and measures. The first CGPM met in 1899.

CGS: Measure or system of measures defined in terms of the centimeter, gram and second. CGS-measures are now obsolete and have been replaced with SI units. More information about CGS-units can be found on the page about obsolete units.

CIE: International Commission on Illumination (Commission Internationale de l'Eclairage). International organisation devoted to the cooperation and exchange of information among its member countries on all matters relating to the science and art of lighting. The CIE is independent of other institutions such as the BIPM, ISO or IEC and is recognised as an international standardisation body. It currently has about 40 member countries. The CIE was established on 1913 and has its headquarters in Wien, Austria.

CIPM: International Committee for Weights and Measures (Comité International des Poids et Mesures). The CIPM was established by the Convention of the Metre in 1875. This committee consists of 18 representatives, elected by the CGPM. Its principal task is to ensure world-wide uniformity of measures. It meets annually at the BIPM and discusses reports presented to it by the so-called Consultative Committees (expert working groups set up by the CIPM that cover specified, individual areas of metrology). The CIPM supervises the BIPM and the affairs of the Convention of the Metre. The CIPM submits proposals to the meetings of the CGPM.

Convention of the Metre: This convention is a diplomatic treaty between nations which gives authority to the CGPM, the CIPM and the BIPM to act in matters of world metrology. It is the basis of all international agreement on units of measurement. The convention was first signed in 1875 by 17 nations. By 2003, a total of 51 nations, including all major industrialised countries, had become members of the convention. Eleven additional countries are associates of the CGPM.

GMT: Greenwich Mean Time; formerly the universal world time, now replaced by the Coordinated Universal Time (UTC). GMT was based on the mean solar day, the newer UTC is based on the current definition for the second, measured by atomic clocks and adjusted to match the solar day.

IEC: International Electrotechnical Commission. It is the leading global organization that prepares and publishes international standards for all electrical, electronic and related technologies. These serve as a basis for national standardization and as references. The IEC is, together with the ISO and ITU, one of the three principal organisations in international standardisation. It currently has about 60 member countries. The IEC was established 1906 and has its headquarters in Geneva, Switzerland.

ISO: International Organization for Standardization (the abbreviation ISO is derived from the Greek word “isos”, meaning “equal”). This organisation is a network of the national standards institutes of 146 countries. The ISO is, together with the IEC and ITU, one of the three principal organisations in international standardisation. The ISO was established in 1947 and has its headquarters in Geneva, Switzerland.

ITU: International Telecommunication Union. The Union was established as an impartial, international organization within which governments and the private sector could work together to coordinate the operation of telecommunication networks and services and advance the development of communications technology. The ITU is, together with the ISO and IEC, one of the three principal organisations in international standardisation. It currently has about 190 member countries. The ITU was established in 1865 and has its headquarters in Geneva, Switzerland.

Metre Convention: See under Convention of the Metre.

MKS: Measure or system of measures defined in terms of the meter, kilogram and second. The term MKS was used to differentiate measures based on the meter, kilogram and second from measures based on the centimeter, gram and second (CGS-measures). Both MKS and CGS-measures are metric and have been developed simultaneously.

In 1954, the 10th CGPM adopted a set of base units for the metric system, which included the meter, kilogram and second. This decision resolved the competition between MKS and CGS-units in favour of the MKS-units. As a result, all CGS-units became obsolete.

The term MKS, sometimes also expanded to MKSA (for meter, kilogram, second, and ampere), is now rarely used. The modern metric system has seven base units, with the meter, kilogram and second being just three of the seven. That is why “MKS-units” is an inaccurate description of modern units. It is better to use terms such as SI-units, modern metric units, official metric units, etc., to describe today’s weights and measures. MKS is therefore usually used to describe a certain set of metric units before 1954.

SI: International System of Units (Systemè International d’Unités); the name for the modern metric system. This name was adopted in 1960 by the 11th CGPM.

TAI: International Atomic Time. See under UTC.

UTC: Coordinated Universal Time (replaced the Greenwich Mean Time as the universal world time in 1926). The UTC is the basis for legal time world-wide. This time scale is kept by more than 200 high-precision atomic clocks in over 50 national time laboratories around the world. The BIPM uses the data from these clocks to compute the so-called International Atomic Time (TAI). The TAI is stable and uniform, based only on the definition of the second. TAI can be different from the solar time, which depends on the rotation of the earth. Since the earth’s rotation is not perfectly uniform, the TAI needs to be modified slightly to have a time standard that matches the solar time. This time standard that is close to the solar time, is UTC.

The UTC is identical to the TAI, except for an integral number of seconds. The UTC is kept in step with the slightly irregular rotation of the earth by inserting leap seconds when necessary. The difference between UTC and TAI is the sum of the leap seconds that have been inserted or deducted.

This ensures that UTC is never more than 0.9 seconds different from the solar time set by the earth. UTC is still based on the Greenwich Meridian, which means that the sun crosses the 0° meridian at 12:00 o’clock.

UTC is broadcast by radio signals in many countries and also by the satellites of the Global Positioning System (GPS). It is also published on the BIPM website.

 

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