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Conversion of units refers to conversion factors between units of measurement. Techniques The simplest way to convert from one unit to another is to carry through the units themselves in the mathematical operation. To illustrate this process, consider the following examples. You would like to convert 6 feet into metres. Consulting the table below and finding that one foot is exactly 0.3048 metre, you can now perform the mathematical conversion: 6 ft × 0.3048 m/ft = 1.8288 m Notice that the "foot" units canceled out, leaving only metres, the desired result. (Since 0.3048 metre per foot have infinite precision, the precision of the answer is determined by the precision of the 6 ft figure; if, for example, you are defining the fathom, expressing it with 5 significant figures is correct. But if the 6 ft figure is a measurement, the result needs to be rounded appropriately.) Say your height is 183 centimetres, and you wish to convert this into inches: 183 cm / (2.54 cm/in) = 72.0 in To check our answer, we convert this result back into feet: 72 in / (12 in/ft) = 6.0 feet which confirms the earlier result. Multiple units can be manipulated in the same fashion: 7 mi/s × 1.609344 km/mi × 3600 s/h = 40,000 km/h Thus, Earth escape velocity is about 7 miles per second, or 40,000 kilometres per hour. Notice that since the calculation started with one significant figure (the 7), the answer also has one significant figure (the 4 in 40,000). Deciding whether to multiply or divide is determined by looking at the units and deciding which ones you want to "get rid" of. In the conversion just above, if we had divided by 3600 s/h instead of multiplying, the result would have come out in kilometre-hours per square second, clearly an incorrect and meaningless result. Rounding of results An important thing to remember is that the process of making a conversion cannot give you any more precise results than what you started with. While many of the conversion factors given in the tables below are exact, and others while not exact contain many significant digits, all the numbers you get after performing calculations on a calculator or with pencil and paper are not meaningful. After using these conversion factors, be sure to round off the results appropriately. Tables of conversion factors Legend Length | class="wikitable" |+ Length, l |----- !Name of unit !Symbol !Definition !Relation to SI units |----- | angstrom || Å | | ≡ 1.000 m = 0.1 nm |----- | astronomical unit || AU | | = 149 597 870.691 ± 0.030 km |----- | atomic unit of length || au | ≡ a0 | ≈ 5.291 772 083 ± 19 m |----- | barley corn || | ≡ 1/3 in | ≈ 8.466 667 mm |----- | Bohr radius || a0; b | ≡ α/(4π''R''∞) | ≈ 5.291 772 083 ± 19 m |----- | cable length (Imperial) || | ≡ 608 ft | = 185.3184 m |----- | cable length (International) || | ≡ 1/10 NM | = 185.2 m |----- | cable length (U.S.) || | ≡ 720 ft | = 219.456 m |----- | calibre || cal | ≡ 1 in | = 25.4 mm |----- | chain (Gunter's; Surveyor's) || ch | ≡ 66 ft | = 20.1168 m |----- | chain (Ramsden's; Engineer's) || ch | ≡ 100 ft | = 30.48 m |----- | cubit || | ≡ 18 in | = 0.4572 m |----- | ell || ell | ≡ 45 in | = 1.143 m |----- |rowspan="2"| fathom ||rowspan="2"| fm | ≡ 6 ft | = 1.8288 m |- | ≈ 1/1000 NM | = 1.852 m |----- | fermi || fm | ≡ 1.000 m | = 1.000 m |----- | finger || | ≡ 7/8 in | = 22.225 mm |----- | finger (cloth) || | ≡ 4 ½ in | = 0.1143 m |----- | foot (Benoît) || ft (Ben) | | ≈ 0.304 799 735 m |----- | foot (Clarke's; Cape) || ft (Cla) | | ≈ 0.304 797 265 4 m |----- | foot (Indian) || ft Ind | | ≈ 0.304 799 514 m |----- | foot (International) || ft | ≡ 1/3 yard | ≈ 0.304 800 610 m |----- | furlong || fur | ≡ 660 ft | = 201.168 m |----- | geographical mile || mi | ≡ 6082 ft | = 1853.7936 m |----- | hand || | ≡ 4 in | = 0.1016 m |----- | inch || in | ≡ 1/36 yd | = 25.4 mm |----- | league || lea | ≡ 3 mi | = 4828.032 m |----- | light-day || | ≡ 24 light-hours | = 2.590 206 837 12 m |----- | light-hour || | ≡ 60 light-minutes | = 1.079 252 848 8 m |----- | light-minute || | ≡ 60 light-seconds | = 1.798 754 748 m |----- | light-second || | | ≡ 2.997 924 58 m |----- | light-year || l.y. | ≡ c0 × 86 400 × 365.25 | = 9.460 730 472 580 8 m |----- | line || ln | ≡ 1/12 in (Klein 1988, 63) | ≈ 2.116 667 mm |----- | link (Gunter's; Surveyor's) || lnk | ≡ 1/100 ch | = 0.201 168 m |----- | link (Ramsden's; Engineer's) || lnk | ≡ 1 ft | = 0.3048 m |----- | metre (SI base unit) || m | ≡ 1 m | = 1 m |----- | mickey || | ≡ 1/200 in | = 1.27 m |----- | micron|| µ | | ≡ 1.000 m |----- | mil; thou || mil | ≡ 1.000 in | = 2.54 m |----- | mile || mi | ≡ 1760 yd = 5280 ft | = 1609.344 m |----- | mile (U.S. Survey) || mi | ≡ 5280 ft (US) | = 5280 × 1200/3937 m ≈ 1609.347 219 m |----- | nail (cloth) || | ≡ 2 ¼ in | = 57.15 mm |----- | nautical league || NL; nl | ≡ 3 NM | = 5556 m |----- | nautical mile || NM; nm | | ≡ 1852 m |----- | nautical mile (Admiralty) || NM (Adm); nm (Adm) | ≡ 6080 ft | = 1853.184 m |----- | pace || | ≡ 2.5 ft | = 0.762 m |----- | palm || | ≡ 3 in | = 76.2 mm |----- | parsec || pc | ≈ 180 × 60 × 60/π AU ≈ 206 264.806 25 AU 3.261 563 776 9 ± 6 {{e|−10}} light-years | = 3.085 677 581 3 ± 6 m |----- | point (ATA) || pt | ≡ 0.013837 in | = 0.351 459 8 mm |----- | point (Didot; European) || pt | | ≡ 0.376 065 mm |----- | point (metric) || pt | ≡ 3/8 mm | = 0.375 mm |----- | point (PostScript)|| pt | ≡ 1/72 in | ≈ 0.352 778 mm |----- | quarter || | ≡ ¼ yd | = 0.2286 m |----- | rod; pole; perch || rd | ≡ 16 ½ ft | = 5.0292 m |----- | rope || rope | ≡ 20 ft | = 6.096 m |----- | span || | ≡ 6 in | = 0.1524 m |----- | span (cloth) || | ≡ 9 in | = 0.2286 m |----- | spat || S | ≡ 1012 m | = 1 Tm |----- | stick || | ≡ 2 in | = 50.8 mm |----- | stigma; pm|| | ≡ 1.000 m | ≡ 1.000 m |----- | telegraph mile || mi | ≡ 6087 ft | = 1855.3176 m |----- | twip || twp | ≡ 1/1440 in | ≈ 1.763 889 m |----- | x unit; siegbahn || xu | | ≈ 1.0021 m |----- | yard (International) || yd | ≡3 ft ≡ 0.9144 m | = 0.9144 m |} Area Volume Angle Solid angle Mass 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". Time Speed Acceleration Force Pressure or mechanical stress Energy, work, or heat Power Angular momentum Electric current Electric charge Electromotive force Electrical resistance Dynamic viscosity Kinematic viscosity Information entropy Temperature Software tools Units and measurements can be easily converted using software tools, but only if the units are explicitly defined and the conversion is compatible (e.g., cmHg to kPa). One popular command-line utility is units. Another utility and programming library is UDUNITS. See also | |||||||
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