danst83,
for a rod magnet the magnetic field on the axis,
see for example following link:
http://www.ibsmagnet.com/fluxdensity/index.php
If the magentic field at off-axis location is of interest (and is no longer resticted to on-axis),
then formula are getting very complex (depending also on geometry of the magnet).
In general, one has to attribute for every elementary magnetic dipole
the magnet constists of, that is, one has to integrate over the
total magnetic volume the field contribution each magnetic dipole generates
at the point of interst (e.g. the location of your hall sensor).
That gives then a lot of mathematics - but thanks to
those ready-to-use online calculators, several most common
magnetic configurations can be investigated easily.