3/8 Bolt Clearance Hole Size: Standards, Guidelines, and Practical Tips

When engineers, fabricators, and installers refer to a 3/8 bolt clearance hole size, they are typically specifying a hole that allows a 3/8-inch bolt to pass cleanly through a workpiece while accommodating alignment, load, and material constraints. There is no single universal number; instead, recommended clearance hole sizes vary by material, bolt head type, and whether the joint is meant to be slotted for adjustment or pinned for shear. In structural steel and general construction, a clearance hole for a 3/8-inch bolt is commonly drilled at approximately 13/16 inch (about 20.6 mm) or slightly larger, whereas in wood, a 25/32 inch (about 19.8 mm) hole is often used to allow for shrinkage and installation tolerance. For through holes in steel designed to carry tension, a 13/16-inch (about 21 mm) hole is a common baseline, while pinned connections or clevis applications may specify a 7/8-inch (about 22.2 mm) hole to allow shear pins or cotter pins and enough lateral movement for alignment. In wood, a 25/32-inch hole is a typical starter point, with countersinking added to accommodate bolt heads and nuts. These clearance hole size recommendations balance fastener grip, clamp load, and fabrication practicality across common materials and applications. The following sections break down definitions, standards, application guidance, and practical installation tips to help you choose the right clearance hole for 3/8-inch bolts in steel, wood, and aluminum.