A container can have enough internal length, width and height for a piece of cargo and still be impossible to load through the doorway. Door clearance is a separate geometric constraint.
Check the cross-section
When a rectangular crate passes through a doorway, one cargo dimension points into the container and the other two dimensions must fit the door width and height. The Container Door Fit Calculator tests all three possible through-directions and rotates the cross-section.
Nominal dimensions are only planning references
Actual door openings vary by container manufacturer and unit. Use the equipment specification supplied by the operator whenever a small clearance decides whether the cargo can be loaded.
A doorway pass is not the end
After the item enters, it may still need room to turn, move on handling equipment, clear the roof or fit beside dunnage and securing systems. A geometric door check should therefore be followed by an execution-level loading review for oversized cargo.