## Predefined Units

Since the exact definitions of many kinds of units have evolved over the years, and since certain countries sometimes have local differences in their definitions, it is a good idea to examine Calc's definition of a unit before depending on its exact value. For example, there are three different units for gallons, corresponding to the US (`gal`), Canadian (`galC`), and British (`galUK`) definitions. Also, note that `oz` is a standard ounce of mass, `ozt` is a Troy ounce, and `ozfl` is a fluid ounce.

The temperature units corresponding to degrees Kelvin and Centigrade (Celsius) are the same in this table, since most units commands treat temperatures as being relative. The `calc-convert-temperature` command has special rules for handling the different absolute magnitudes of the various temperature scales.

The unit of volume "liters" can be referred to by either the lower-case `l` or the upper-case `L`.

The unit `A` stands for Amperes; the name `Ang` is used

The unit `pt` stands for pints; the name `point` stands for a typographical point, defined by `72 point = 1 in'. There is also `tpt`, which stands for a printer's point as defined by the TeX typesetting system: `72.27 tpt = 1 in'.

The unit `e` stands for the elementary (electron) unit of charge; because algebra command could mistake this for the special constant e, Calc provides the alternate unit name `ech` which is preferable to `e`.

The name `g` stands for one gram of mass; there is also `gf`, one gram of force. (Likewise for lb, pounds, and lbf.) Meanwhile, one "g" of acceleration is denoted `ga`.

The unit `ton` is a U.S. ton of `2000 lb', and `t` is a metric ton of `1000 kg'.

The names `s` (or `sec`) and `min` refer to units of time; `arcsec` and `arcmin` are units of angle.

Some "units" are really physical constants; for example, `c` represents the speed of light, and `h` represents Planck's constant. You can use these just like other units: converting `.5 c' to `m/s' expresses one-half the speed of light in meters per second. You can also use this merely as a handy reference; the u g command gets the definition of one of these constants in its normal terms, and u b expresses the definition in base units.

Two units, `pi` and `fsc` (the fine structure constant, approximately 1/137) are dimensionless. The units simplification commands simply treat these names as equivalent to their corresponding values. However you can, for example, use u c to convert a pure number into multiples of the fine structure constant, or u b to convert this back into a pure number. (When u c prompts for the "old units," just enter a blank line to signify that the value really is unitless.)