SI units in LaTeX

The siunitx package takes care of the space between a number and its unit, the upright font of units and the decimal marker. You always type a point in the source; the output-decimal-marker option switches the printed marker if a journal or course asks for a comma.

Preamble

\usepackage{siunitx}
\sisetup{separate-uncertainty=true}

Number with a unit

\SI{9.81}{\metre\per\second\squared}

Result with uncertainty

\SI{2.45 +- 0.03}{\ohm}

Number only

\num{6.022e23}

Common mistakes

A comma as the decimal separator in math mode: $1,5$ prints as "1, 5" with a space after the comma.
Italic units: $9.81 m/s^2$ sets m and s as variables.