  We present the calibration of the Swift UVOT grisms, of which there are two,
providing low-resolution field spectroscopy in the ultraviolet and optical
bands respectively. The UV grism covers the range 1700-5000 Angstrom with a
spectral resolution of 75 at 2600 Angstrom for source magnitudes of u=10-16
mag, while the visible grism covers the range 2850-6600 Angstrom with a
spectral resolution of 100 at 4000 Angstrom for source magnitudes of b=12-17
mag. This calibration extends over all detector positions, for all modes used
during operations. The wavelength accuracy (1-sigma) is 9 Angstrom in the UV
grism clocked mode, 17 Angstrom in the UV grism nominal mode and 22 Angstrom in
the visible grism. The range below 2740 Angstrom in the UV grism and 5200
Angstrom in the visible grism never suffers from overlapping by higher spectral
orders. The flux calibration of the grisms includes a correction we developed
for coincidence loss in the detector. The error in the coincidence loss
correction is less than 20%. The position of the spectrum on the detector only
affects the effective area (sensitivity) by a few percent in the nominal modes,
but varies substantially in the clocked modes. The error in the effective area
is from 9% in the UV grism clocked mode to 15% in the visible grism clocked
mode .
