The Fluid Flow Group of the National Institute of Standards and Technology in Gaithersburg, Maryland offers calibration services for flow meters used in gas, water, and liquid hydrocarbon. Gas flow meters are calibrated using PVTt systems, piston provers, or bell provers for flows ranging from 0.04 L/min to 78,000 L/min. A critical nozzle based gas flow standard allows performance testing of flow meters in gas mixtures at temperatures up to 700 K for flows ranging from 60 L/min to 6,200 L/min. The water flow standard is a static gravimetric system that handles flows up to 38,000 L/min. A dynamic gravimetric flow standard provides hydrocarbon liquid flow calibrations between 0.04 L/min and 1,500 L/min. Further details of these calibration services are documented including the principle of operation and measurement uncertainties.