  We test the accuracy of ALMA flux density calibration by comparing ALMA flux
density measurements of extragalactic sources to measurements made by the
Planck mission; Planck is absolutely calibrated to sub-percent precision using
the dipole signal induced by the satellite's orbit around the solar system
barycenter. Planck observations ended before ALMA began systematic
observations, however, and many of the sources are variable, so we employ
measurements by the Atacama Cosmology Telescope (ACT) to bridge the two epochs.
We find the ALMA flux density scale (based on observations of Uranus) is
consistent with Planck; for instance ALMA flux densities in Band 3 ($\sim$100
GHz) average $0.99 \pm 0.02$ times those measured by Planck. We also test the
absolute calibration of both ACT and the South Pole Telescope (SPT), again with
reference to Planck, as well as the internal consistency of Planck, ACT and SPT
measurements of compact sources.
