What size is a 4.0 screw

A 4.0 screw typically refers to a metric M4 screw with a 4.0 mm nominal major diameter (the outer diameter of the thread). This is one of the most common small fastener sizes used in electronics, appliances, furniture assembly, and light mechanical applications. The "M4" designation in the ISO metric screw thread standard indicates the 4 mm diameter, with the thread pitch (distance between threads) typically being 0.7 mm for standard M4 fasteners and 0.5 mm for fine-thread variants (designated M4×0.5).

The 4.0 mm measurement refers only to the thread's major diameter—the screw's total dimensions include head style (pan head, flat head, socket head, etc.), head diameter, and length, all of which vary based on application. For pilot hole drilling when installing M4 screws in materials, use approximately 3.3-3.5 mm drill bits for clearance holes in wood or plastic, or 3.3 mm for tapping threads directly in metal. M4 screws accept 3 mm hex keys for socket head variants and are distinct from the similarly sized #8 imperial screw (approximately 4.2 mm diameter), though the two are not directly interchangeable due to different thread forms. When specifying fasteners, always confirm whether measurements are metric or imperial, as confusion between M4 (4.0 mm) and #4 gauge (2.8 mm) screws leads to incorrect fastener selection.