Measure width, depth and height
Record the smallest usable dimension where hinges, trim or shelf supports reduce space.
Check the path in
A container that fits once inside still has to pass through the opening.
Leave handling clearance
Packed bins need finger room and enough space to tilt or slide.
Keep the measurements
A simple phone note prevents buying by memory.
Practical boundary. Measure first, respect product load limits, and do not improvise wall, ceiling or overhead installations when the structure or appropriate hardware is uncertain.