A clearance hole lets a bolt or screw pass through a part without engaging threads. Close-fit holes locate parts accurately, normal fit is the default for general assembly, and loose fit allows for misalignment between several parts. Standards list the diameters so parts made in different shops fit together.
How to use the Clearance Hole Calculator
- System
- Metric size
- Inch size
- Fit
Results update as you type, and every field accepts the units shown in its menu.
How it works
Choose metric or inch, the fastener size and the fit class. The calculator looks up the hole diameter from the standard table.
Metric clearance holes (ISO 273, mm)
| Size | Close | Normal | Loose |
|---|---|---|---|
| M1.6 | 1.7 | 1.8 | 2 |
| M2 | 2.2 | 2.4 | 2.6 |
| M2.5 | 2.7 | 2.9 | 3.1 |
| 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 |
| M8 | 8.4 | 9 | 10 |
| M10 | 10.5 | 11 | 12 |
| M12 | 13 | 13.5 | 14.5 |
| M14 | 15 | 15.5 | 16.5 |
| M16 | 17 | 17.5 | 18.5 |
| M20 | 21 | 22 | 24 |
| M24 | 25 | 26 | 28 |
| M30 | 31 | 33 | 35 |
| M36 | 37 | 39 | 42 |
Inch clearance holes (in)
| Size | Close | Normal | Loose |
|---|---|---|---|
| #2 | 0.094 | 0.096 | 0.106 |
| #4 | 0.116 | 0.125 | 0.141 |
| #6 | 0.144 | 0.156 | 0.172 |
| #8 | 0.177 | 0.188 | 0.203 |
| #10 | 0.201 | 0.219 | 0.234 |
| ¼ | 0.266 | 0.281 | 0.297 |
| 5/16 | 0.328 | 0.344 | 0.359 |
| ⅜ | 0.391 | 0.406 | 0.422 |
| 7/16 | 0.453 | 0.469 | 0.484 |
| ½ | 0.516 | 0.531 | 0.562 |
| ⅝ | 0.656 | 0.688 | 0.719 |
| ¾ | 0.781 | 0.812 | 0.844 |
| ⅞ | 0.906 | 0.938 | 0.969 |
| 1 | 1.031 | 1.062 | 1.094 |
Inputs explained
| Field | What to enter |
|---|---|
| System | Choose one: Metric (ISO 273), Inch (ASME B18.2.8) |
| Metric size | Choose one: M1.6, M2, M2.5, M3 … |
| Inch size | Choose one: #2, #4, #6, #8 … |
| Fit | Choose one: Close, Normal, Loose |
Formula
Hole diameter from ISO 273 (metric) or ASME B18.2.8 (inch) tables for the chosen fit
Worked example
Inputs: System: Metric (ISO 273); Metric size: M6; Fit: Normal.
Results: Clearance hole diameter = 6.6 mm; Nearest fraction = 17/64.
Frequently asked questions
What clearance hole for an M6 bolt?
6.4 mm close, 6.6 mm normal and 7.0 mm loose fit (ISO 273).
What clearance hole for a ¼ in bolt?
0.266 in (H drill) close, 0.281 in (9/32) normal and 0.297 in (19/64) loose.
Why not just drill the bolt size?
Without clearance the bolt binds; small differences in hole position then make assembly impossible.