What is a Clearance Hole?
A clearance hole is a hole drilled through a material that is slightly larger than the fastener (bolt or screw) that will pass through it. The clearance allows the fastener to pass freely through the hole without engaging the material's threads, enabling proper clamping force when the nut is tightened on the other side.
The amount of clearance depends on the fit class required. A close fit provides minimal play for precise alignment, a normal fit is the standard for general construction, and a loose fit provides extra clearance for easy assembly or to accommodate thermal expansion and misalignment.
Clearance Hole Sizing
Inch Fastener Clearance Holes
| Bolt Size | Close Fit | Normal Fit | Loose Fit |
|---|---|---|---|
| 1/4" | 17/64" (0.266") | 9/32" (0.281") | 5/16" (0.313") |
| 5/16" | 21/64" (0.328") | 11/32" (0.344") | 3/8" (0.375") |
| 3/8" | 25/64" (0.391") | 13/32" (0.406") | 7/16" (0.438") |
| 1/2" | 33/64" (0.516") | 17/32" (0.531") | 9/16" (0.563") |
| 5/8" | 41/64" (0.641") | 21/32" (0.656") | 11/16" (0.688") |
| 3/4" | 49/64" (0.766") | 25/32" (0.781") | 13/16" (0.813") |
Metric Fastener Clearance Holes (ISO 273)
| Bolt Size | Fine (H12) | Medium (H13) | Coarse (H14) |
|---|---|---|---|
| M5 | 5.3 mm | 5.5 mm | 6.0 mm |
| M6 | 6.4 mm | 6.6 mm | 7.0 mm |
| M8 | 8.4 mm | 9.0 mm | 10.0 mm |
| M10 | 10.5 mm | 11.0 mm | 12.0 mm |
| M12 | 13.0 mm | 13.5 mm | 14.5 mm |
Frequently Asked Questions
When should I use a close fit vs. loose fit?
Use close fit when precise alignment between parts is critical (such as machine assemblies). Use normal fit for general construction and fabrication. Use loose fit when thermal expansion, field assembly, or manufacturing tolerances require extra clearance.
What happens if the clearance hole is too small?
If the bolt engages threads in the clearance hole, it will not develop proper clamping force, the joint will be weaker, and the bolt will be subjected to both tension and shear. Always ensure the bolt passes freely through the clearance hole.
Do I need a washer with a clearance hole?
Washers are recommended, especially with loose-fit clearance holes, to distribute the clamping force over a larger area. Without a washer, the bolt head may pull through the oversized hole under load, particularly in soft materials like wood or thin sheet metal.