Scalability describes a desirable characteristic of a system that has come to the forefront with the emergence of the Internet as a platform for business systems. Scalability broadly defined as the ability of a system to adapt to change while maintaining an acceptable level of performance is a critical characteristic any system dealing with the Internet must possess. Most all software vendors, system architectures, and system designers claim to have a scalable system. This article develops a mathematical foundation on which scalability measurement of enterprise information systems can be based. The mathematical foundation is based on measurement theory and this paper shows how this can be applied to measure scalability of enterprise systems in the context of business-to-business commerce in supply chain.