Estimate how much disk a game library needs once install sizes, updates and shader caches are counted.
A game library takes more disk than the sum of its store-page install sizes. Patches often keep the superseded files, launchers stage a download before applying it, and shader caches, saves and captures accumulate outside the game folders entirely. Adding a patch overhead and a cache allowance to the raw install total, then leaving free-space headroom, gives a drive size that will not need a game uninstalled every time a large update lands.
Game Download Storage
Library = games x average install x (1 + patch overhead) + cache and saves
Drive sizing
Recommended drive = library x (1 + free space headroom)
Install sizes are totalled, a patch and update overhead is added as a percentage, shader cache and save data are added as a fixed block, and free-space headroom is applied to give the drive capacity to buy.
SSDs slow down noticeably when nearly full, and launchers need room to stage an update before applying it — a 60 GB patch can require 120 GB free. Sizing with headroom avoids both problems.
Around 10-15% of capacity. Beyond that, write amplification and slower garbage collection start to affect load times, and large staged updates simply fail for lack of space.