# Section21Units of Measure

Units of measure can be given xml treatment too with the `quantity` element. In LaTeX, the `siunitx` package is loaded to achive unit handling. Since that package only offers SI units, some other common units will be added by MBX in the preamble. In HTML, the capabilities of `siunitx` are simulated, weakly. Note that at present, you should not attempt to use the `quantity` element within a math environment.

The value of gravitational constant \(g\) is 9.8 ^{m}⁄_{s2}. Force is measured in ^{kg m}⁄_{s2}, also known as one N. A quantity with rather ridiculous units is 23 ^{µha23}⁄_{°C s2}. One Hz is the same as ^{1}⁄_{s}. You can have a unitless quantity, like 42, which may help with consistency between such numbers and units in the LaTeX output. Some non-SI units are available, such as the absurd ^{°F ft lb}⁄_{gal}. The LaTeX command `\pi` is recognized within `mag` in conversions to HTML, which is consistent with the behavior with a conversion to LaTeX, for example there are 2\(\pi\) rad in a full circle.

For a full list of the allowed units and prefixes, see `mathbook-units.xsl`. If you have a need for more units, they need to be added to `mathbook-units.xsl` in the section that deals with units which are not part of `siunitx` by default. Note that the `mag` element should come first, followed by the `unit` element, followed by the `per` element.