The available data on the microwave spectrum of formic acid are critically reviewed for information applicable to radioastronomy. Molecular data such as the derived rotational constants, centrifugal distortion constants and electric dipole moment are tabulated. The observed rotational transitions are presented for the astronomically interesting isotopic forms. Calculated rotational transitions up to 300 GHz are presented for the ground vibrational state of H12C16O16OH, H12C16O16OD, D12C16O16OH and H13C16O16OH. Some observed transitions are also listed for H12C16O18OH and H12C18O16OH. Estimated error limits have been reported for all measured transitions.