British History Online is a leading digital library for British history, receiving over a million page views per month. This JISC-funded project will investigate the creation of a sustainability index which can be used to compare the performance of functionality of sub-site areas, a whole site, or even between separate sites. Because researchers' reasons for using a website can be a lot broader than the user experience, it is necessary to filter visits which include using or being exposed to the specific pieces of functionality enhancements proposed herewith. Once visits can be interrogated this way, it becomes possible to measure their effect across the user's visit as a whole. The project looks at amending discrete areas of content, leaving whole other areas untouched to allow contrast, and communicating results in a non-specialised language. In addition, an initial analysis will benchmark performance before any development has taken place enabling a longitudinal analysis of usage.