A 2x4 piece of lumber measures approximately 1 1/2 inches by 3 1/2 inches. Milling techniques and dryness of the material impact the actual, final measurements.
The nominal dimensions of lumber are the size of the lumber before drying and milling. Before those processes, a 2x4 measures 2 inches by 4 inches. The reduction in size happens because drying shrinks the lumber, and milling removes surface material to make the lumber cleaner and smoother but subsequently smaller. The length of a 2x4 is not impacted by these processes, so a 2x4 that is 96 inches in length actually measures 1 1/2 inches by 3 1/2 inches by 96 inches.