Solid lubricant materials have become necessary in applications for which traditional lubrication approaches become impractical. These materials are often mated against a harder metallic counter surface (counterface) of higher surface energy. During sliding, wear fragments from the solid lubricant transfer to the counterface to form a protective barrier known as the transfer film. Historically, the coverage attributes of these transfer films have correlated strongly to the tribological performance of the solid lubricant. Although transfer film quality is often identified as a critical contributor to the success of a candidate solid lubricant, the community lacks a quantitative means to measure quality. Transfer film cohesion and adhesion are likely very important but they are also difficult to measure and not necessarily related to the visual features that have motivated the use of adjectives like ‘quality’, ‘thin’, ‘uniform’, and ‘tenacious’. Area fraction and film thickness are more easily quantified, but to date, they have not proven to be robust predictors of tribological success. A recent visual study of transfer film evolution for a successful alumina–PTFE nanocomposite suggests that the characteristic size of domains of exposed counterface may correlate more closely with wear performance. This paper presents a method for quantifying transfer film quality based on this metric, which we call the free-space length (L f ). To illustrate the application of the method, we study the connection between the wear rate and free-space length for the transfer film of a well-studied alumina–PTFE system. The correlation to wear was best for the free-space length and worst for area fraction.