The Fluid Metrology Group developed a dual-purpose, automated apparatus to calibrate hydrometers via Cuckow’s method and to also measure the density of reference liquids via Archimedes’ principle. For Cuckow’s method, the hydrometer under test is suspended from a weigh scale while gradations on the hydrometer’s stem are aligned with the surface of a liquid of know density. (We use tridecane surrounded by a temperature-controlled bath.) The hydrometer calibrations have an uncertainty of 0.01 % or less at a 95 % confidence level. In the Archimedes-principle mode, a stainless steel sphere of know mass and volume is weighed while it is suspended by a thin wire in the liquid of unknown density. In this mode, the apparatus measures liquid densities at ambient pressure with uncertainties of 10 parts in 106. We measured the densities of iso-octane and toluene to produce standard reference materials so that these liquids could be used, together with air and pure water, to calibrate vibrating tube density meters. The Liquid Density Laboratory has a vibrating tube density meter and a falling ball viscometer that NIST uses to determine the properties of liquids used in flow calibration standards.