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Metric Clearance Hole Sizes Chart (ISO 273): M1.6 to M24
A clearance hole lets a bolt or screw pass through a part without touching the thread. ISO 273 gives three sizes for each metric thread: fine, medium and coarse. For most work, use the medium series: M3 → Ø3.4, M4 → Ø4.5, M5 → Ø5.5, M6 → Ø6.6, M8 → Ø9, M10 → Ø11.
Clearance hole chart, M1.6 to M24
All sizes in millimetres, from ISO 273:1979 (EN 20273). The "Try it" link opens a mounting plate with four holes of that size in the generator, with the medium hole already set.
| Thread | Fine (H12) | Medium (H13) | Coarse (H14) | Try it |
|---|---|---|---|---|
| M1.6 | 1.7 | 1.8 | 2 | Make a plate |
| M2 | 2.2 | 2.4 | 2.6 | Make a plate |
| M2.5 | 2.7 | 2.9 | 3.1 | Make a plate |
| M3 | 3.2 | 3.4 | 3.6 | Make a plate |
| M3.5 | 3.7 | 3.9 | 4.2 | Make a plate |
| M4 | 4.3 | 4.5 | 4.8 | Make a plate |
| M5 | 5.3 | 5.5 | 5.8 | Make a plate |
| M6 | 6.4 | 6.6 | 7 | Make a plate |
| M8 | 8.4 | 9 | 10 | Make a plate |
| M10 | 10.5 | 11 | 12 | Make a plate |
| M12 | 13 | 13.5 | 14.5 | Make a plate |
| M14 | 15 | 15.5 | 16.5 | Make a plate |
| M16 | 17 | 17.5 | 18.5 | Make a plate |
| M18 | 19 | 20 | 21 | Make a plate |
| M20 | 21 | 22 | 24 | Make a plate |
| M22 | 23 | 24 | 26 | Make a plate |
| M24 | 25 | 26 | 28 | Make a plate |
Tolerance classes H12, H13 and H14 are given in ISO 273 for information, where a tolerance needs to be specified.
Fine, medium or coarse: which to use
Medium (H13): the default
Medium holes suit most bolted joints in general engineering. There is enough slack to assemble parts with ordinary positional accuracy, and the hole is still well covered by a standard screw head or washer. Mechform's generators use the medium series when you type a screw size.
Fine (H12): when the screw locates the part
Fine holes leave little play, so the parts can only shift slightly before the screw shank touches the hole. Use them when hole positions are accurately controlled (CNC drilled or reamed) and you want parts to line up closely without dowels. Tighter holes need tighter positional tolerances on both parts, or the screws won't go in.
Coarse (H14): when things may not line up
Coarse holes give the most positional slack. They suit large, welded or fabricated assemblies, sheet metal and parts with several fixings whose positions may drift, at the cost of less bearing area under the head. A washer helps here.
What the tolerance means
An H tolerance means the hole may be larger than the nominal size, never smaller. The tolerance band widens with hole size and from H12 to H14. For example, from ISO 286-1:
| Hole size | H12 | H13 | H14 |
|---|---|---|---|
| up to 3 mm | +0.10 | +0.14 | +0.25 |
| over 3 to 6 mm | +0.12 | +0.18 | +0.30 |
| over 6 to 10 mm | +0.15 | +0.22 | +0.36 |
| over 10 to 18 mm | +0.18 | +0.27 | +0.43 |
| over 18 to 30 mm | +0.21 | +0.33 | +0.52 |
So an M5 medium hole, Ø5.5 H13, is acceptable anywhere from 5.50 to 5.68 mm. In practice, a drilled clearance hole is rarely toleranced on its own: the drawing's general tolerance (for example ISO 2768-m) or a note like "Ø5.5 THRU, 4 HOLES" is usually enough.
Making the holes
- Drilling: pick the drill for the series you want (for example a 5.5 mm drill for an M5 medium hole). Drilled holes usually come out slightly oversize, which is fine for a clearance hole.
- Laser and waterjet cutting: cut the hole at the nominal size from the DXF. Small holes in thick plate may taper, so check the fit on the exit side.
- 3D printing: FDM holes usually print undersize. Add 0.1 to 0.3 mm to the diameter, or print undersize and drill to size.
- Countersunk and counterbored holes are not covered by ISO 273; they have their own standards for the head recess.
Clearance holes in Mechform
Type a screw size and the hole is set for you: "plate 120x80x6 with 4 M5 holes" gives Ø5.5 holes (medium series). To use the fine or coarse series, type the diameter, for example "hole diameter 5.3" (fine) or "hole diameter 5.8" (coarse), or change Hole Ø in the Adjust panel. The drawing calls out the hole size and count.
Generate it now: mounting plate with M5 holes · block with M6 holes · M8 spacer
FAQ
What size clearance hole do I need for an M3 screw?
To ISO 273, an M3 clearance hole is 3.4 mm in the medium series. Use 3.2 mm (fine) for a closer fit or 3.6 mm (coarse) for more positional slack.
What size clearance hole for M4, M5 and M6?
Medium series (ISO 273): M4 4.5 mm, M5 5.5 mm, M6 6.6 mm. Fine series: 4.3, 5.3 and 6.4 mm. Coarse series: 4.8, 5.8 and 7 mm.
What size clearance hole for M8, M10 and M12?
Medium series (ISO 273): M8 9 mm, M10 11 mm, M12 13.5 mm. Fine series: 8.4, 10.5 and 13 mm. Coarse series: 10, 12 and 14.5 mm.
Which series should I use: fine, medium or coarse?
Medium is the usual default for general engineering. Choose fine when the screw also has to locate the parts closely, and coarse when hole positions or parts may be out of line, for example on large or fabricated assemblies.
What tolerance applies to a clearance hole?
ISO 273 gives H12 for fine, H13 for medium and H14 for coarse holes as informative tolerance classes. The hole can be larger than nominal, never smaller: an M5 medium hole (5.5 mm, H13) may measure 5.50 to 5.68 mm.
Is ISO 273 still current?
Yes. ISO 273:1979 is the current edition and is published in Europe as EN 20273:1991 (for example DIN EN 20273), with the same hole sizes.
Sources
- ISO 273:1979, Fasteners: Clearance holes for bolts and screws (identical to EN 20273:1991)
- ISO 286-1:2010, ISO code system for tolerances on linear sizes, table of standard tolerance grades
Check critical dimensions against the current standard before manufacture.