A Dictionary of Units

This page provides a summary of most of the units of measurement to be found in use around the world today (and a few of historical interest), together with the appropriate conversion factors needed to change them into a 'standard' unit of the S I.

The units and conversion factors may be found either by looking under the category in which they are used [such as length, mass, density, energy etc.], or else by picking one unit from an alphabetically ordered list of units.

There is an outline of the SI; a list of its basic defining standards and also some of its derived units; then another list of all the SI prefixes and some notes on conventions of usage.

There is a short historical note on measures generally; descriptions of the Metric system, the U S system, the U K (Imperial) system and comments on the implementation of 'metrication' in the U K.

Finally there is a list of other sources concerned with the topic of measures and units.

The System International [SI]

Le System international d'Unites officially came into being in October 1960 and has been adopted by nearly all countries, though the amount of actual usage varies considerably.

The SI system is based on seven principal units of measure - See the Definitions section for more about these basic units.  Each of these units may also take a prefix. From these basic units many other units are derived and named.

¹ ``Weight'' in common parlance is often used to mean ``mass.''
² In addition to the thermodynamic temperature (symbol T) expressed in kelvins (see Table 1a), use is also made of Celsius temperature (symbol t) defined by the equation t = T-To where To=273.15 K by definition. To express Celsius temperature, the unit ``degree Celsius'' which is equal to the unit ``kelvin'' is used; in this case, ``degree Celsius'' is a special name used in place of ``kelvin.'' An interval or difference of Celsius temperature can, however, be expressed in kelvins as well as in degrees Celsius.

Other Sources

On the World Wide Web

Fundamental Physical Constants - National Institute of Standards and Technology Physics Laboratory

Online Reference Databases - National Institute of Standards and Technology Physics Laboratory

A set of units conversion calculators do the necessary calculations in order to change the value in one type of unit to its equivalent value in another type of unit.

International Standards Organization [I S O]

The organization (based in Switzerland) is responsible for the world-wide publication of standards for just about anything for which standards can be set. Many references to other organizations concerned with standards.

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Notes

Errors

Every care has been taken in the compilation of this document, and many checks have been carried out, the possibility of an error is always present in a work like this and that must be borne in mind by all users.

Accuracy

In a document like this it is impossible to know just what accuracy is needed by any particular user. Where the given value is an exact one then it has been indicated. In most cases other values are accurate to at least the number of significant figures shown. In some cases it might be more than that as trailing zeros have not been included.

Presentation

The conversion factors have mainly been presented as multipliers, but exceptions to that have been made for two reasons. First, it is easier to convey the exact value 'divide by 60' rather than the approximation 'multiply by 0.0166667' and it is more likely to be keyed in without errors if a calculator is being used. Second, most calculators accept only 8 digits, which means that 'multiply by 0.000 084 666' will become '0.000 0846' (3 significant figures) whereas 'divide by 11 811' will give the result to 6 significant figures. The appearance of a '1' needs no operator but shows that the named unit is exactly equivalent to the standard unit.

Inverse usage

In nearly all cases the conversion factors have been given to change 'non-standard' units into standard units of the SI. For those cases where it is necessary to do a conversion the other way it is only a matter of reversing the operation. For example to convert feet into metres you multiply by 0.3048 so, to convert metres into feet you divide by 0.3048. Following on from this it can be seen how conversions can be made between non-standard units, changing first into the standard unit and then back into the required unit.

Symbols

In a few places it will be noticed that some symbols have not been displayed. This a limitation imposed by the current state of HTML and/or the browsers being used to read this document. This also applies to notation such as the superscripts 2 and 3 in matters of area and volume. Further developments (in both the language and the browsers) should allow this situation to be improved in later editions.

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