Measure the available width and height, required shelf depth, doorways and access points, obstacles such as electrical boards or pipes, and the aisles or working space you need.
For long shelving rows, remember that the total installed width may not simply equal the nominal bay width multiplied by the number of bays because uprights and frames form part of the complete run.
Related: Plan your layout with StorageFlow | Shelving & Racking Buyers Guide