Drill Bit Size Chart

Understanding Drill Bit Sizes for Bolts and Screws

Types of Holes

Clearance Holes: These allow a bolt or screw to pass through freely without threading. The hole is slightly larger than the bolt diameter to allow easy insertion and adjustment. Standard clearance provides room for minor misalignment, while close fit offers minimal play for precision work.

Pilot Holes: Smaller holes drilled before driving a screw, especially in hard materials. They prevent splitting in wood and reduce the torque needed to drive the screw. For wood screws, pilot holes are typically 75-90% of the screw's core diameter.

Tap Drill Holes: Precise-sized holes for cutting internal threads with a tap. The hole must be smaller than the bolt diameter to leave material for the threads. Too small causes tap breakage; too large results in weak threads.

Clearance Hole Guidelines

• Standard clearance: Typically bolt diameter + 1-2mm (or +1/16") - allows for alignment adjustment
• Close fit: Bolt diameter + 0.5mm (or +1/32") - minimal play for precision assemblies
• Large clearance: For slotted holes or when significant movement is needed

Tap Drill Size Selection

Tap drill sizes are calculated to leave approximately 75% thread engagement, which provides optimal strength without excessive tap wear. This percentage balances thread strength with ease of tapping.

For metric threads: Tap drill ≈ Major diameter - Thread pitch (e.g., M8 x 1.25 uses 6.8mm drill)

For imperial threads: Consult standard tap drill charts as calculations vary with TPI

Material Considerations

Soft materials (wood, plastic):

• Can use slightly smaller clearance holes
• Pilot holes prevent splitting and cracking
• Self-tapping screws may not need pilot holes in very soft materials

Hard materials (metal, hardwood):

• Require precise clearance holes to prevent binding
• Always use pilot holes to prevent drill wander and reduce stress
• Tapping requires cutting fluid and precise drill size

Common Mistakes to Avoid

• Using clearance holes for tapping: Results in stripped or weak threads
• Oversized tap drill holes: Creates shallow threads with poor holding power
• No pilot hole in hardwood: Causes splitting and misalignment
• Wrong drill bit type: Use twist bits for metal, brad-point for wood, masonry bits for concrete
• Drilling without center punch: Causes drill wander, especially in metal

Drilling Best Practices

• Always mark and center-punch the drill location in metal
• Start with a smaller pilot drill for large holes (step drilling)
• Use cutting fluid when drilling or tapping metal
• Clamp workpieces securely to prevent movement
• Drill perpendicular to the surface for straight holes
• Clear chips frequently when drilling deep holes
• Use sharp drill bits - dull bits cause overheating and poor holes

Metric vs Imperial Drill Bits

Metric drill bits increase in 0.1mm increments and are labeled in millimetres (e.g., 3.0mm, 6.5mm, 10.2mm). Imperial drill bits use fractional inches (1/16", 1/8"), number sizes (#1-#80), and letter sizes (A-Z). A quality drill index should include both systems for versatility.

When to Use Each Hole Type

Use clearance holes when: Bolting two pieces together with nuts, allowing for adjustment, or when the fastener doesn't create its own threads

Use pilot holes when: Driving wood screws, working with brittle materials, or preventing splitting

Use tap drill holes when: Creating threaded holes in metal, making precision threaded assemblies, or repairing stripped threads