Through-hole diameters for metric bolts, M1 to M64, in the three ISO 273 series.
A clearance hole lets a bolt pass through a part without touching the thread. ISO 273 sets three series for each bolt size: fine, medium and coarse. An M10 bolt, for example, takes a 10.5, 11 or 12 mm hole. Medium is the normal choice in machine work. Fine holds parts closer in position. Coarse makes assembly easier but leaves more room to move. Find your bolt below, then check that the hole leaves enough room for position error between the two parts.
All diameters in mm, from ISO 273:1979. Tolerance classes: fine H12, medium H13, coarse H14.
| Bolt size | Fine (mm) | Medium (mm) | Coarse (mm) | Common drill (medium) |
|---|---|---|---|---|
| M1 | 1.1 | 1.2 | 1.3 | 1.2 mm |
| M1.2 | 1.3 | 1.4 | 1.5 | 1.4 mm |
| M1.4 | 1.5 | 1.6 | 1.8 | 1.6 mm |
| M1.6 | 1.7 | 1.8 | 2.0 | 1.8 mm |
| M2 | 2.2 | 2.4 | 2.6 | 2.4 mm |
| M2.5 | 2.7 | 2.9 | 3.1 | 2.9 mm |
| M3 | 3.2 | 3.4 | 3.6 | 3.4 mm |
| M3.5 | 3.7 | 3.9 | 4.2 | 3.9 mm |
| M4 | 4.3 | 4.5 | 4.8 | 4.5 mm |
| M5 | 5.3 | 5.5 | 5.8 | 5.5 mm |
| M6 | 6.4 | 6.6 | 7.0 | 6.6 mm |
| M7 | 7.4 | 7.6 | 8.0 | 7.6 mm |
| M8 | 8.4 | 9.0 | 10.0 | 9.0 mm |
| M10 | 10.5 | 11.0 | 12.0 | 11.0 mm |
| M12 | 13.0 | 13.5 | 14.5 | 13.5 mm |
| M14 | 15.0 | 15.5 | 16.5 | 15.5 mm |
| M16 | 17.0 | 17.5 | 18.5 | 17.5 mm |
| M18 | 19.0 | 20.0 | 21.0 | 20.0 mm |
| M20 | 21.0 | 22.0 | 24.0 | 22.0 mm |
| M22 | 23.0 | 24.0 | 26.0 | 24.0 mm |
| M24 | 25.0 | 26.0 | 28.0 | 26.0 mm |
| M27 | 28.0 | 30.0 | 32.0 | 30.0 mm |
| M30 | 31.0 | 33.0 | 35.0 | 33.0 mm |
| M33 | 34.0 | 36.0 | 38.0 | 36.0 mm |
| M36 | 37.0 | 39.0 | 42.0 | 39.0 mm |
| M39 | 40.0 | 42.0 | 45.0 | 42.0 mm |
| M42 | 43.0 | 45.0 | 48.0 | 45.0 mm |
| M45 | 46.0 | 48.0 | 52.0 | 48.0 mm |
| M48 | 50.0 | 52.0 | 56.0 | 52.0 mm |
| M52 | 54.0 | 56.0 | 62.0 | 56.0 mm |
| M56 | 58.0 | 62.0 | 66.0 | 62.0 mm |
| M60 | 62.0 | 66.0 | 70.0 | 66.0 mm |
| M64 | 66.0 | 70.0 | 74.0 | 70.0 mm |
33 rows
Take the M10 row: 10.5, 11.0 and 12.0 mm. The bolt is 10 mm, so the medium hole leaves 1 mm of diametral clearance, or 0.5 mm each side. That 0.5 mm is all the misalignment the joint can absorb. If you are about to tap the mating part instead, you need a different number from the tap drill size chart. Thread sizes and pitches are in the metric thread size chart.
Two parts with holes on the same pattern must line up, and the clearance is what pays for the error. With a bolt and nut, and holes in both parts, the position tolerance (as a diameter) is T = H − F, where H is the hole and F the bolt. With one part tapped, it is T = (H − F) ÷ 2, because the bolt cannot float in its thread. So a pattern that assembles with nuts can fail once one part is tapped. On a large pattern, thermal growth and flatness also use up this allowance.
The table gives nominal diameters. A drill rarely cuts exactly to size, and a hole under nominal may not clear. H12, H13 and H14 allow +0.18, +0.27 and +0.43 mm in the 10–18 mm range; the ISO fits and tolerances chart lists other ranges. Holes only go larger, never smaller, so a clearance hole does not need a reamer. The drill bit size chart shows which standard drills exist.
ISO 273 is a general fastener standard. Structural connections follow the structural code, not this table. The AISC standard hole is 18 mm for M16, 22 mm for M20 and 27 mm for M24, against 17.5, 22 and 26 mm in the ISO 273 medium series. In US work the standard hole is 1/16 in larger than the bolt, up to ⅞ in, so a ½ in bolt gets a 9/16 in (14.3 mm) hole. Oversize holes reduce the slip resistance of friction joints, so a larger hole is not a free choice there. Bolt capacity itself comes from the fastener strength calculator.
By O.E. — Partner, chairman of the board and CEO of a metal products manufacturer for 21 years; the company exports to 45 countries.
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Classification: Standard clearance-hole dimensions
Sources
ISO 273:1979 — Fasteners — Clearance holes for bolts and screws (confirmed 2024)
ISO
ANSI/AISC 360, Tables J3.3 and J3.3M — Nominal hole dimensions
American Institute of Steel Construction
Clearance hole diameters are standardised ISO 273 values for metric bolts M1–M64, in three series: fine, medium and coarse. The series is chosen by the required assembly tolerance, not by the bolt grade.
A clearance hole must be larger than the bolt to allow assembly. ISO 273 defines three series: fine for located assemblies, medium for general use, and coarse for through-holes where alignment is not critical. The hole diameter is tabulated per bolt size and series.
Classification: Stable (mature standard)
ISO 273 clearance classes have been stable for decades. Confirm the current edition for precision work.
Hole and bolt diameters in millimetres. Clearance is the difference between hole and bolt diameter.
Go deeper with FlowUtil guides that explain the concepts behind this reference table.
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