How Do I Use Units in Mathematica?

In order to make use of units in Mathematica, it is useful to load the Miscellaneous`Units` package.

Needs["Miscellaneous`Units`"]

When this package is loaded, 270 units of measure and five functions used for conversion are defined. The conversion functions are Convert, ConvertTemperature, SI, CGS, and MKS. Units are treated as any other symbol in Mathematica, so expressing a number with a unit is like multiplying a number by a symbol.

Most units defined in this package are singular, not plural, so always use the singular form to be safe. For example, this package uses 3 Acre, not 3 Acres. There are some exceptions to this rule. For example, you can use Feet or Foot. A complete list of symbols can be found below.

Numbers with units can be multiplied.

12 Foot * 16 Foot

192 Foot2

More complicated units can also be used.

85 Meter/Second * 10 Second

850 Meter

Conversions between units can be carried out.

Convert[78 Pole, Foot]

1287. Foot

Convert[9.8 Meter/Second, Foot/Second]

(32.15223097112862` 
Foot)/Second

Converting between temperature units is handled differently. For this reason, a separate function is defined to handle temperature conversions.

ConvertTemperature[212, Fahrenheit, Celsius]

100

Here is a list of all of the symbols defined when the Miscellaneous`Units` package is loaded.

Flatten[Names/@{"Miscellaneous`Units`*",
        "Miscellaneous`SIUnits`*"}]

{Abampere, Abcoulomb, Abfarad, Abhenry, Abmho, Abohm, Abvolt, Acre, AMU, Angstrom, Apostilb, ArcMinute, ArcSecond, Are, AssayTon, AstronomicalUnit, Atmosphere, AtomicMassUnit, Atto, AU, AvoirdupoisOunce, AvoirdupoisPound, Bag, BakersDozen, Bale, Bar, Barn, Barrel, Barye, Baud, Biot, Bit, BoardFoot, BohrMagneton, Bolt, BritishThermalUnit, BTU, Bucket, Bushel, Butt, Cable, Caliber, Calorie, Candle, Carat, Celsius, Cental, Centi, Centigrade, Centimeter, Century, CGS, Chain, ChevalVapeur, Cicero, Convert, ConvertTemperature, Cord, Cubit, Cup, Curie, Dalton, Day, Deca, Decade, Deci, Denier, Didot, DidotPoint, Diopter, Dozen, Drachma, Dyne, ElectronVolt, Ell, Ephah, Erg, Exa, Fahrenheit, Fathom, Feet, Femto, Fermi, Fifth, Firkin, FluidDram, FluidOunce, Foot, FootCandle, Fortnight, Furlong, Gal, Gallon, Gauss, Geepound, Giga, Gilbert, Gill, Grade, Grain, Gram, GramWeight, Gravity, Gross, GrossHundredweight, Hand, Hectare, Hecto, Hefner, Hogshead, Horsepower, Hour, Hundredweight, ImperialGallon, I mperialPint, Inch, InchMercury, Jeroboam, Jigger, Kayser, Kilo, KilogramForce, KilogramWeight, Knot, Lambert, League, Libra, LightYear, Link, Liter, Lumerg, Magnum, Maxwell, Mega, MetricTon, Mho, Micro, Micron, Mil, Mile, Millennium, Milli, MillimeterMercury, Mina, Minim, Minute, MKS, Month, Nano, NauticalMile, NetHundredweight, Nibble, Nit, Noggin, NuclearMagneton, Obolos, Oersted, Omer, Ounce, Parsec, Peck, Pennyweight, Percent, Perch, Peta, Phot, Pica, Pico, Pint, Poise, Pole, Pondus, Pony, Pound, Poundal, PoundForce, PoundsPerSquareInch, PoundWeight, PrintersPoint, PSI, Puncheon, Quadrant, Quart, Quintal, Rad, Radian, Rankine, RegisterTon, Reyn, Rhes, RightAngle, Rod, Roentgen, Rontgen, Rood, Rope, Rutherford, Rydberg, Seam, Section, Shekel, ShortHundredweight, ShortTon, Shot, SI, SiderealSecond, SiderealYear, Skein, Slug, SolarMass, Span, Stadion, Stadium, Statampere, Statcoulomb, Statfarad, Stathenry, Statohm, StatuteMile, Statvolt, Steradian, Stere, Stilb, Stokes, Stone, SurveyMile, Tablespoon, Talbot, Talent, Teaspoon, Tera, Therm, Ton, TonForce, Tonne, Torr, Township, TropicalYear, TroyOunce, Tun, UKGallon, UKPint, Week, Wey, WineBottle, XUnit, Yard, Year, Yocto, Yotta, Zepto, Zetta, Amp, Ampere, Becquerel, Candela, Coulomb, Farad, Gray, Henry, Hertz, Joule, Kelvin, Kilogram, Lumen, Lux, Meter, Mole, Ohm, Pascal, Siemens, Tesla, Volt, Watt, Weber}

For more information on units and their use in Mathematica, refer to the Documentation Center and The Beginner's Guide to Mathematica Version 4 by Jerry Glynn and Theodore Gray.