Emerging products for an ultra-energy-efficient economy demand new amorphous and nanocrystalline thin metallic glass (TMG) alloys. Planar flow casting (PFC) is the rapid solidification process of choice for forming TMGs. In this paper, TMG and crystalline alloys are demonstrated to process similarly by PFC despite their well-known different solid-state atomic structures. The high-fragility of the TMG alloy is argued to be responsible. A simple scaling law for ribbon thickness is reported. Thickness depends on the thermal properties of the alloy and heat transfer characteristic of the PFC machine. Perhaps surprisingly, explicit knowledge of the viscosity of the glass is not needed. Dimensional reasoning, mathematical modeling and experiment for two different alloys on two different PFC machines are the methods employed. (C) 2018 Elsevier Ltd. All rights reserved.