The dynamic gravimetric standard is used to calibrate micro-flow meters and to conduct research on new flow sensors. This micro-flow standard measures mass flows dm/dt of water from 0.16 µg/s to 16 mg/s with uncertainty of 0.1 µg/s +0.002×dm/dt [µg/s], at a 95 % confidence level. The standard has been validated by informal comparisons with the national metrology institutes of Switzerland and Denmark. NIST’s micro-flow standard uses either an elevated reservoir or a syringe pump to generate the flow that passes through a meter under test and it is then collected in beaker resting on a (6 g full scale) micro-balance that weighs the beaker every 0.4 s. The time-rate-of-change of the buoyancy-corrected weight gives the mass flow. The water flows from the meter under test into a glass pipette that passes through the free surface of the collected water. Evaporation of water from the collection beaker is retarded by covering the free surface with a low-vapor-pressure oil layer.