  We present the stellar calibrator sample and the conversion from instrumental
to physical units for the 24 micron channel of the Multiband Imaging Photometer
for Spitzer (MIPS). The primary calibrators are A stars, and the calibration
factor based on those stars is 4.54*10^{-2} MJy sr^{-1} (DN/s)^{-1}, with a
nominal uncertainty of 2%. We discuss the data-reduction procedures required to
attain this accuracy; without these procdures, the calibration factor obtained
using the automated pipeline at the Spitzer Science Center is 1.6% +/- 0.6%
lower. We extend this work to predict 24 micron flux densities for a sample of
238 stars which covers a larger range of flux densities and spectral types. We
present a total of 348 measurements of 141 stars at 24 micron. This sample
covers a factor of ~460 in 24 micron flux density, from 8.6 mJy up to 4.0 Jy.
We show that the calibration is linear over that range with respect to target
flux and background level. The calibration is based on observations made using
3-second exposures; a preliminary analysis shows that the calibration factor
may be 1% and 2% lower for 10- and 30-second exposures, respectively. We also
demonstrate that the calibration is very stable: over the course of the
mission, repeated measurements of our routine calibrator, HD 159330, show a
root-mean-square scatter of only 0.4%. Finally, we show that the point spread
function (PSF) is well measured and allows us to calibrate extended sources
accurately; Infrared Astronomy Satellite (IRAS) and MIPS measurements of a
sample of nearby galaxies are identical within the uncertainties.
