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Conversion of units is the conversion between different units of measurement for the same quantity, typically through multiplicative conversion factors.

Techniques

Process

The process of conversion depends on the specific situation and the intended purpose. This may be governed by regulation, contract, technical specifications or other published standards. Engineering judgment may include such factors as:

• The precision and accuracy of measurement and the associated uncertainty of measurement.
• The statistical confidence interval or tolerance interval of the initial measurement.
• The number of significant figures of the measurement.
• The intended use of the measurement including the engineering tolerances.
• Historical definitions of the units and their derivatives used in old measurements; e.g., international foot vs. US survey foot.

Some conversions from one system of units to another need to be exact, without increasing or decreasing the precision of the first measurement. This is sometimes called soft conversion. It does not involve changing the physical configuration of the item being measured.

By contrast, a hard conversion or an adaptive conversion may not be exactly equivalent. It changes the measurement to convenient and workable numbers and units in the new system. It sometimes involves a slightly different configuration, or size substitution, of the item. Nominal values are sometimes allowed and used.

Multiplication factors

Conversion between units in the metric system can be discerned by their prefixes (for example, 1 kilogram = 1000 grams, 1 milligram = 0.001 grams) and are thus not listed in this article. Exceptions are made if the unit is commonly known by another name (for example, 1 micron = 10 metre).

Table ordering

Within each table, the units are listed alphabetically, and the SI units (base or derived) are highlighted.

Tables of conversion factors

This article gives lists of conversion factors for each of a number of physical quantities, which are listed in the index. For each physical quantity, a number of different units (some only of historical interest) are shown and expressed in terms of the corresponding SI unit.

Length

Area

Volume

Plane angle

Solid angle

Mass

Notes:

• See Weight for detail of mass/weight distinction and conversion.
• Avoirdupois is a system of mass based on a pound of 16 ounces, while Troy weight is the system of mass where 12 troy ounces equals one troy pound.
• In this table, the unit gee is used to denote standard gravity in order to avoid confusion with the "g" symbol for grams.
• In physics, the pound of mass is sometimes written lbm to distinguish it from the pound-force (lbf). It should not be read as the mongrel unit "pound metre".

Density

Time

Frequency

Speed or velocity

A velocity consists of a speed combined with a direction; the speed part of the velocity takes units of speed.

Flow (volume)

Acceleration

Force

See also: Conversion between weight (force) and mass

Pressure or mechanical stress

Pressure

Name of unit	Symbol	Definition	Relation to SI units
atmosphere (standard)	atm		≡ 101 325 Pa
atmosphere (technical)	at	≡ 1 kgf/cm	= 9.806 65×10 Pa
bar	bar		≡ 10 Pa
barye (cgs unit)		≡ 1 dyn/cm	= 0.1 Pa
centimetre of mercury	cmHg	≡ 13 595.1 kg/m × 1 cm × g	≈ 1.333 22×10 Pa
centimetre of water (4 °C)	cmH2O	≈ 999.972 kg/m × 1 cm × g	≈ 98.063 8 Pa
foot of mercury (conventional)	ftHg	≡ 13 595.1 kg/m × 1 ft × g	≈ 40.636 66×10 Pa
foot of water (39.2 °F)	ftH2O	≈ 999.972 kg/m × 1 ft × g	≈ 2.988 98×10 Pa
inch of mercury (conventional)	inHg	≡ 13 595.1 kg/m × 1 in × g	≈ 3.386 389×10 Pa
inch of water (39.2 °F)	inH2O	≈ 999.972 kg/m × 1 in × g	≈ 249.082 Pa
kilogram-force per square millimetre	kgf/mm	≡ 1 kgf/mm	= 9.806 65×10 Pa
kip per square inch	ksi	≡ 1 kipf/sq in	≈ 6.894 757×10 Pa
micron (micrometre) of mercury	${\displaystyle \mu }$mHg	≡ 13 595.1 kg/m × 1 ${\displaystyle \mu }$m × g ≈ 0.001 torr	≈ 0.133 322 4 Pa
millimetre of mercury	mmHg	≡ 13 595.1 kg/m × 1 mm × g ≈ 1 torr	≈ 133.3224 Pa
millimetre of water (3.98 °C)	mmH2O	≈ 999.972 kg/m × 1 mm × g = 0.999 972 kgf/m	= 9.806 38 Pa
pascal (SI unit)	Pa	≡ N/m = kg/(m·s)	= 1 Pa
pièze (mts unit)	pz	≡ 1 000 kg/m·s	= 1×10 Pa = 1 kPa
pound per square foot	psf	≡ 1 lbf/ft	≈ 47.880 26 Pa
pound per square inch	psi	≡ 1 lbf/in	≈ 6.894 757×10 Pa
poundal per square foot	pdl/sq ft	≡ 1 pdl/sq ft	≈ 1.488 164 Pa
short ton per square foot		≡ 1 sh tn × g / 1 sq ft	≈ 95.760 518×10 Pa
torr	torr	≡ 101 325/760 Pa	≈ 133.322 4 Pa

Torque or moment of force

Energy

Power or heat flow rate

Action

Dynamic viscosity

Kinematic viscosity

Electric current

Electric charge

Electric dipole

Electromotive force, electric potential difference

Electrical resistance

Capacitance

Magnetic flux

Magnetic flux density

Inductance

Temperature

Information entropy

Often, information entropy is measured in shannons, whereas the (discrete) storage space of digital devices is measured in bits. Thus, uncompressed redundant data occupy more than one bit of storage per shannon of information entropy. The multiples of a bit listed above are usually used with this meaning. Other times the bit is used as a measure of information entropy and is thus a synonym of shannon.

Luminous intensity

The candela is the preferred nomenclature for the SI unit.

Luminance

Luminous flux

Illuminance

Radiation - source activity

Please note that although becquerel (Bq) and hertz (Hz) both ultimately refer to the same SI base unit (s), Hz is used only for periodic phenomena, and Bq is only used for stochastic processes associated with radioactivity.

Radiation - exposure

The roentgen is not an SI unit and the NIST strongly discourages its continued use.

Radiation - absorbed dose

Radiation - equivalent dose

Although the definitions for sievert (Sv) and gray (Gy) would seem to indicate that they measure the same quantities, this is not the case. The effect of receiving a certain dose of radiation (given as Gy) is variable and depends on many factors, thus a new unit was needed to denote the biological effectiveness of that dose on the body; this is known as the equivalent dose and is shown in Sv. The general relationship between absorbed dose and equivalent dose can be represented as

H = Q · D

where H is the equivalent dose, D is the absorbed dose, and Q is a dimensionless quality factor. Thus, for any quantity of D measured in Gy, the numerical value for H measured in Sv may be different.

Software tools

Home and office computers come with converters in bundled spreadsheet applications or can access free converters via the Internet. Units and measurements can be easily converted using these tools, but only if the units are explicitly defined and the conversion is compatible (e.g., cmHg to kPa).

General commercial sources of converters

• Advanced electronic calculators have unit-conversion functionality.
• Spreadsheet programs usually provide conversion functions or formulas or the user can write their own.
• Commercial mathematical, scientific and technical applications often include converters.

See also

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• Accuracy and precision
• Conversion of units of temperature
• English units
• False precision
• Imperial units
• International System of Units
• Mesures usuelles
• Metric prefix (e.g. "kilo-" prefix)

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• Metric system
• Natural units
• Rounding
• Significant figures
• United States customary units
• Units (software)
• Units conversion by factor-label
• Units of measurement

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Notes and references

1. jobs (2012-09-14). "The astronomical unit gets fixed : Nature News & Comment". Nature.com. doi:10.1038/nature.2012.11416. Retrieved 2013-08-31.
2. "NIST Reference on Constants, Units, and Uncertainty."(2010). National Institute of Standards and Technology. Retrieved October 17, 2014.
3. ^ U.S. National Institute of Standards and Technology Handbook 44 Appendix C - General Tables of Units of Measurement 2013 Edition
4. ^ Lide, D. (Ed.). (1990). Handbook of Chemistry and Physics (71st ed). Boca Raton, FL: CRC Press. Section 1.
5. ^ National Bureau of Standards. (June 30, 1959). Refinement of values for the yard and the pound. Federal Register, viewed September 20, 2006 at National Geodetic Survey web site.
6. The International Astronomical Union and Astronomical Units
7. Klein, Herbert Arthur. (1988). The Science of Measurement: a Historical Survey. Mineola, NY: Dover Publications 0-4862-5839-4.
8. ^ The International System of Units, Section 2.1 (8 ed.), Bureau International des Poids et Mesures, 2006, retrieved August 26, 2009
9. International System of Units, 8th ed. (2006), Bureau International des Poids et Mesures, Section 4.1 Table 8.
10. P. Kenneth Seidelmann, Ed. (1992). Explanatory Supplement to the Astronomical Almanac. Sausalito, CA: University Science Books. p. 716 and s.v. parsec in Glossary.
11. ^ Whitelaw, Ian. (2007). A Measure of All Things: The Story of Man and Measurement. New York: Macmillan 0-312-37026-1. p. 152.
12. ^ De Vinne, Theodore Low (1900). The practice of typography: a treatise on the processes of type-making, the point system, the names, sizes, styles and prices of plain printing types 2nd ed. New York: The Century Co. p. 142–150.
13. Pasko, Wesley Washington (1894). American dictionary of printing and bookmaking. (1894). New York: Howard Lockwood. p. 521.
14. ^ Rowlett, Russ (2005), How Many? A Dictionary of Units of Measurement
15. Thompson, A. and Taylor, B.N. (2008). Guide for the Use of the International System of Units (SI). National Institute of Standards and Technology Special Publication 811. p. 57.
16. ^ US Code of Federal Regulations, Title 21, Section 101.9, Paragraph (b)(5)(viii), retrieved August 29, 2009
17. Barry N. Taylor, Ed.,NIST Special Publication 330: The International System of Units (SI) (2001 Edition), Washington: US Government Printing Office, 43,"The 12th Conference Generale des Poids et Mesures (CGPM)…declares that the word “litre” may be employed as a special name for the cubic decimetre".
18. CODATA Value: atomic unit of mass. (2006). National Institute of Standards and Technology. Retrieved 16 September 2008.
19. The Swiss Federal Office for Metrology gives Zentner on a German language web page and quintal on the English translation of that page ; the unit is marked "spécifiquement suisse !"
20. ^ Pedersen O. (1983). "Glossary" in Coyne, G., Hoskin, M., and Pedersen, O. Gregorian Reform of the Calendar: Proceedings of the Vatican Conference to Commemorate its 400th Anniversary. Vatican Observatory. Available from Astrophysics Data System.
21. Richards, E.G. (1998), Mapping Time, Oxford University Press, pp. 94–95, ISBN 0-19-850413-6
22. Steel, Duncan (2000), Marking Time, John Wiley & Sons, p. 46, ISBN 0-471-29827-1
23. ^ Richards, E. G. (2013). "Calendars" in S. E. Urban & P. K. Seidelmann, eds. Explanatory Supplement to the Astronomical Almanac. Mill Valley, CA: University Science Books.
24. Richards, E. G. (2013). "Calendars" in S. E. Urban & P. K. Seidelmann, eds. Explanatory Supplement to the Astronomical Almanac. Mill Valley, CA: University Science Books. p. 587.
25. Tom Benson. (2010.) "Mach Number" in Beginner's Guide to Aeronautics. NASA.
26. CODATA Value: atomic unit of force. (2006). National Institute of Standards and Technology. Retrieved September 14, 2008.
27. ^ Comité International des Poids et Mesures, Resolution 2, 1946, retrieved August 26, 2009
28. ^ Barry N. Taylor, (April 1 995), Guide for the Use of the International System of Units (SI) (NIST Special Publication 811), Washington, DC: US Government Printing Office, pp. 57–68.
29. Barry N. Taylor, (April 1995), Guide for the Use of the International System of Units (SI) (NIST Special Publication 811), Washington, DC: US Government Printing Office, p. 5.
30. International System of Units, 8th ed. (2006), Bureau International des Poids et Mesures, Section 4.1 Table 7.
31. The NIST Reference on Constants, Units, and Uncertainty, 2006, retrieved August 26, 2009
32. Robert G. Mortimer Physical chemistry,Academic Press, 2000 ISBN 0-12-508345-9, page 677
33. ^ NIST Guide to SI Units, Appendix B.9, retrieved August 27, 2009
34. Standard for the Use of the International System of Units (SI): The Modern Metric System IEEE/ASTM SI 10-1997. (1997). New York and West Conshohocken, PA: Institute of Electrical and Electronics Engineers and American Society for Testing and Materials. Tables A.1 through A.5.
35. The NIST Reference on Constants, Units, and Uncertainty, retrieved August 28, 2009
36. Ambler Thompson & Barry N. Taylor. (2008). Guide for the Use of the International System of Units (SI). Special Publication 811. Gaithersburg, MD: National Institute of Standards and Technology. p. 10.
37. ^ The International System of Units, Section 2.2.2., Table 3 (8 ed.), Bureau International des Poids et Mesures, 2006, retrieved August 27, 2009
38. The NIST Guide to the SI (Special Publication 811), section 5.2, 2008, retrieved August 27, 2009
39. Ambler Thompson & Barry N. Taylor. (2008). Guide for the Use of the International System of Units (SI). Special Publication 811. Gaithersburg, MD: National Institute of Standards and Technology. p. 5.
40. Comité international des poids et mesures, 2002, Recommendation 2, retrieved August 27, 2009

Notes

1. The technical definition of tropical year is the period of time for the ecliptic longitude of the Sun to increase 360 degrees. (Urban & Seidelmann 2013, Glossary, s.v. year, tropical)

External links

• Statutory Instrument 1995 No. 1804 Units of measurement regulations 1995 From legislation.gov.uk
• How Many? A dictionary of units of measurement
• Template:PDFlink
• NIST Guide to SI Units Many conversion factors listed.
• The Unified Code for Units of Measure
• Units, Symbols, and Conversions XML Dictionary
• Template:Dmoz
• Template:Dmoz

• Units of measurement
• Metrication
• Conversion of units of measurement