  We present the calibration and background model for the Large Area X-ray
Proportional Counter (LAXPC) detectors on-board AstroSat. LAXPC instrument has
three nominally identical detectors to achieve large collecting area. These
detectors are independent of each other and in the event analysis mode, they
record the arrival time and energy of each photon that is detected. The
detectors have a time-resolution of 10 $\mu$s and a dead-time of about 42
$\mu$s. This makes LAXPC ideal for timing studies. The energy resolution and
peak channel to energy mapping were obtained from calibration on ground using
radioactive sources coupled with GEANT4 simulations of the detectors. The
response matrix was further refined from observations of the Crab X-ray source
after launch. At around 20 keV the energy resolution of detector is about
10--15\%, while the combined effective area of the 3 detectors is about 6000
cm$^2$.

Instrumentation and Methods for Astrophysics