Metric Bolt Clearance Hole Sizes
Fine, medium and coarse through-hole sizes for M3 to M36 bolts (ISO 273), with the tap drill.
A clearance hole is the hole a bolt passes through, drilled a little larger than the thread so the parts can be assembled even when the holes are not perfectly aligned. ISO 273 gives three series: fine (close fit, for accurately placed holes), medium (the normal choice) and coarse (loose, for rough work such as structural steel).
The table
| Thread | Fine (close) hole (mm) | Medium (normal) hole (mm) | Coarse (loose) hole (mm) | Clearance in the medium hole (mm) | Tap drill for the thread (mm) |
|---|---|---|---|---|---|
| M3 | 3.2 | 3.4 | 3.6 | 0.4 | 2.5 |
| M4 | 4.3 | 4.5 | 4.8 | 0.5 | 3.3 |
| M5 | 5.3 | 5.5 | 5.8 | 0.5 | 4.2 |
| M6 | 6.4 | 6.6 | 7 | 0.6 | 5 |
| M8 | 8.4 | 9 | 10 | 1 | 6.75 |
| M10 | 10.5 | 11 | 12 | 1 | 8.5 |
| M12 | 13 | 13.5 | 14.5 | 1.5 | 10.25 |
| M14 | 15 | 15.5 | 16.5 | 1.5 | 12 |
| M16 | 17 | 17.5 | 18.5 | 1.5 | 14 |
| M20 | 21 | 22 | 24 | 2 | 17.5 |
| M24 | 25 | 26 | 28 | 2 | 21 |
| M30 | 31 | 33 | 35 | 3 | 26.5 |
| M36 | 37 | 39 | 42 | 3 | 32 |
Source and notes: ISO 273, through holes for bolts and screws: fine series (close fit, accurate positions), medium series (normal fit, the usual choice) and coarse series (loose fit, rough work). The tap drill is the diameter minus the coarse pitch (about 75 percent thread).
How to use this table
- Choose the series: medium unless you have a reason.
- Drill or specify the hole size listed for the bolt’s thread.
- For a tapped hole in the other part, use the tap drill in the last column (diameter minus pitch).
Worked example: an M10 bolt through two plates (floating fastener)
- Medium hole 11.0 mm against a 10 mm bolt: 1.0 mm of total clearance.
- The two parts share that clearance: split evenly, each part gets a positional tolerance zone of ⌀0.50 mm, and the bolt will still go through even when both holes sit at the extremes.
- With the fine hole of 10.5 mm the clearance is only 0.5 mm, which allows a zone of only ⌀0.25 mm in each part.
Notes and cautions
- The clearance is the difference between the hole and the bolt. It is what lets the parts be moved to line up before the bolt is tightened.
- A larger hole means less bearing area under the head or washer. For a very coarse hole, use a larger washer.
- Holes for slip-critical joints, dowels and precision locating use different rules.
Related tools and guides
- Position and Location (GD&T guide)
- Tap Drill Size and Thread Pitch Lookup
- Metric Hex Bolt, Nut and Washer Dimensions
- Metric Thread Table (Coarse)
- All reference tables
FAQ
What size hole for an M8 bolt?
9 mm for a normal (medium) clearance hole, 8.4 mm for a close fit and 10 mm for a loose fit.
How do I tolerance holes for bolts?
For a floating fastener, the sum of the positional tolerances of the two holes equals the hole size minus the bolt size (at maximum material). The GD&T guide shows how.
Sources
- ISO 273, Fasteners: clearance holes for bolts and screws.
- ISO 273, through holes for bolts and screws: fine series (close fit, accurate positions), medium series (normal fit, the usual choice) and coarse series (loose fit, rough work). The tap drill is the diameter minus the coarse pitch (about 75 percent thread).