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 |
Tips for an accurate estimate
- Take measurements at floor level and at the ceiling – walls and rooms are often slightly out of square.
- Use the manufacturer’s coverage, spacing and installation data when it differs from the typical values here.
- Keep a little extra of every finish material for touch-ups and repairs later.
- Check local building codes and permits before structural, electrical, plumbing or gas work.
Common mistakes to avoid
- Sizing heating or cooling equipment by area alone when insulation, windows and climate make a big difference.
- Buying too little paint, stain or flooring and ending up with mismatched batches.
- Overlooking the extra material for steps, railings, gables and trim.
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 fitWorked example
Inputs — System: Metric (ISO 273); Metric size: M6; Inch size: ¼; 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.
What formula does the Clearance Hole Calculator use?
Hole diameter from ISO 273 (metric) or ASME B18.2.8 (inch) tables for the chosen fit.