Bolt Clearance Hole Sizes
When a bolt passes through a part without threading into it, the hole needs to be slightly larger than the bolt diameter. How much larger depends on how precisely the holes need to align.
US Bolt Clearance Holes
| Bolt Size | Close Fit (in) | Normal Fit (in) | Loose Fit (in) |
|---|---|---|---|
| #4 (0.112) | 0.120 | 0.128 | 0.136 |
| #6 (0.138) | 0.144 | 0.150 | 0.161 |
| #8 (0.164) | 0.170 | 0.177 | 0.189 |
| #10 (0.190) | 0.196 | 0.204 | 0.221 |
| 1/4″ | 0.266 | 0.281 | 0.312 |
| 5/16″ | 0.328 | 0.344 | 0.375 |
| 3/8″ | 0.391 | 0.406 | 0.438 |
| 7/16″ | 0.453 | 0.469 | 0.500 |
| 1/2″ | 0.516 | 0.531 | 0.562 |
| 5/8″ | 0.641 | 0.656 | 0.688 |
| 3/4″ | 0.766 | 0.781 | 0.812 |
| 7/8″ | 0.891 | 0.906 | 0.938 |
| 1″ | 1.016 | 1.031 | 1.062 |
Metric Bolt Clearance Holes (per ISO 273)
| Bolt Size | Close Fit (mm) | Normal Fit (mm) | Loose Fit (mm) |
|---|---|---|---|
| M3 | 3.2 | 3.4 | 3.6 |
| M4 | 4.3 | 4.5 | 4.8 |
| M5 | 5.3 | 5.5 | 5.8 |
| M6 | 6.4 | 6.6 | 7.0 |
| M8 | 8.4 | 9.0 | 10.0 |
| M10 | 10.5 | 11.0 | 12.0 |
| M12 | 13.0 | 13.5 | 14.5 |
| M14 | 15.0 | 15.5 | 16.5 |
| M16 | 17.0 | 17.5 | 18.5 |
| M20 | 21.0 | 22.0 | 24.0 |
| M24 | 25.0 | 26.0 | 28.0 |
When to Use Each Fit
- Close Fit: Precision alignment required. Parts locate off the bolts. Common in fixture and tooling design. Holes typically reamed.
- Normal Fit: Standard for most applications. Parts located by pins or features, bolts just clamp. Drilled holes.
- Loose Fit: Slotted holes, thermal expansion allowance, field assembly where holes might not line up perfectly. Also used for structural steel connections.
Pro Tips
- Don’t locate parts with clearance holes. If your assembly depends on bolts for alignment, use dowel pins or close-fit holes + shoulder bolts.
- Slotted holes: Use the loose-fit width and extend length as needed. Always orient the slot in the direction of expected movement.
- Countersunk holes: The clearance is at the shank, but the head angle (typically 82° or 90°) must match the screw.
Related: UNC Tap Drill Sizes | SHCS Dimensions & Counterbores | Hex Cap Bolt Dimensions