Installed base of a product is the number of units that are currently in use by customers. It provides a measurement of a company's existing customer base and the extent of their investment in a particular product or technology. In contrast to market share, which only reflects sales over a specific period and relative to the total market, the installed base represents the number of units currently in use. This information can be used to evaluate a company's customer base, investment, and market position.

The installed base plays a significant role in various industries, including computing, automotive, medical devices, and home appliances. It can impact the level of support, availability of replacement parts, and other factors. Some companies that have leveraged their installed base include Apple, Microsoft, Nintendo in the computing industry, and Tesla in the automotive industry.