Clearance Hole Chart
Clearance hole sizes for bolts and screws: inch sizes #2 to 1 in with close-fit and free-fit drills, and metric M1.6 to M36 in the ISO 273 fine, medium and coarse series. Jump to inch or metric.
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Inch screws: close and free fit
| Screw size | Nominal dia | Close fit drill | Free fit drill |
|---|---|---|---|
| #2 | 0.08602.18 mm | #43 (0.0890)2.26 mm | #41 (0.0960)2.44 mm |
| #3 | 0.09902.51 mm | #37 (0.1040)2.64 mm | #35 (0.1100)2.79 mm |
| #4 | 0.11202.84 mm | #32 (0.1160)2.95 mm | #30 (0.1285)3.26 mm |
| #5 | 0.12503.18 mm | #30 (0.1285)3.26 mm | #29 (0.1360)3.45 mm |
| #6 | 0.13803.51 mm | #27 (0.1440)3.66 mm | #25 (0.1495)3.80 mm |
| #8 | 0.16404.17 mm | #18 (0.1695)4.31 mm | #16 (0.1770)4.50 mm |
| #10 | 0.19004.83 mm | #9 (0.1960)4.98 mm | #7 (0.2010)5.11 mm |
| #12 | 0.21605.49 mm | #2 (0.2210)5.61 mm | #1 (0.2280)5.79 mm |
| 1/4 | 0.25006.35 mm | F (0.2570)6.53 mm | H (0.2660)6.76 mm |
| 5/16 | 0.31257.94 mm | P (0.3230)8.20 mm | Q (0.3320)8.43 mm |
| 3/8 | 0.37509.53 mm | W (0.3860)9.80 mm | X (0.3970)10.08 mm |
| 7/16 | 0.437511.11 mm | 29/64" (0.4531)11.51 mm | 15/32" (0.4688)11.91 mm |
| 1/2 | 0.500012.70 mm | 33/64" (0.5156)13.10 mm | 17/32" (0.5313)13.49 mm |
| 9/16 | 0.562514.29 mm | 37/64" (0.5781)14.68 mm | 39/64" (0.6094)15.48 mm |
| 5/8 | 0.625015.88 mm | 41/64" (0.6406)16.27 mm | 43/64" (0.6719)17.07 mm |
| 3/4 | 0.750019.05 mm | 49/64" (0.7656)19.45 mm | 51/64" (0.7969)20.24 mm |
| 7/8 | 0.875022.23 mm | 57/64" (0.8906)22.62 mm | 59/64" (0.9219)23.42 mm |
| 1 | 1.000025.40 mm | 1-1/64" (1.0156)25.80 mm | 1-3/64" (1.0469)26.59 mm |
The close and free columns found on most tap-and-clearance drill charts; common shop practice rather than one standard. When a print cites ASME B18.2.8, use its close, normal and loose series instead.
Metric screws: ISO 273
| Thread | Fine (H12), mm | Medium (H13), mm | Coarse (H14), mm |
|---|---|---|---|
| M1.6 | 1.70.0669 in | 1.80.0709 in | 2.00.0787 in |
| M2 | 2.20.0866 in | 2.40.0945 in | 2.60.1024 in |
| M2.5 | 2.70.1063 in | 2.90.1142 in | 3.10.1220 in |
| M3 | 3.20.1260 in | 3.40.1339 in | 3.60.1417 in |
| M3.5 | 3.70.1457 in | 3.90.1535 in | 4.20.1654 in |
| M4 | 4.30.1693 in | 4.50.1772 in | 4.80.1890 in |
| M5 | 5.30.2087 in | 5.50.2165 in | 5.80.2283 in |
| M6 | 6.40.2520 in | 6.60.2598 in | 7.00.2756 in |
| M8 | 8.40.3307 in | 9.00.3543 in | 10.00.3937 in |
| M10 | 10.50.4134 in | 11.00.4331 in | 12.00.4724 in |
| M12 | 13.00.5118 in | 13.50.5315 in | 14.50.5709 in |
| M14 | 15.00.5906 in | 15.50.6102 in | 16.50.6496 in |
| M16 | 17.00.6693 in | 17.50.6890 in | 18.50.7283 in |
| M18 | 19.00.7480 in | 20.00.7874 in | 21.00.8268 in |
| M20 | 21.00.8268 in | 22.00.8661 in | 24.00.9449 in |
| M22 | 23.00.9055 in | 24.00.9449 in | 26.01.0236 in |
| M24 | 25.00.9843 in | 26.01.0236 in | 28.01.1024 in |
| M27 | 28.01.1024 in | 30.01.1811 in | 32.01.2598 in |
| M30 | 31.01.2205 in | 33.01.2992 in | 35.01.3780 in |
| M33 | 34.01.3386 in | 36.01.4173 in | 38.01.4961 in |
| M36 | 37.01.4567 in | 39.01.5354 in | 42.01.6535 in |
ISO 273 hole diameters. Each series carries its own hole tolerance: H12 for fine, H13 for medium and H14 for coarse.
Choosing a fit
A clearance hole has two jobs: let the screw through, and absorb the position error between the holes in the two parts. The tighter the hole, the less error it forgives. Close fit, like ISO fine, suits parts drilled together, dowelled parts where the screws only clamp, and screws that do some locating. Free fit, like ISO medium, is the everyday default for machined and sheet metal parts. ISO coarse is for weldments, castings and structural work where the holes are punched, burned or laid out by hand.
Leave room for what else goes in the hole. A counterbored socket head needs the head to bear on material around the hole, and a flat head needs the countersink to be concentric with it. Heavy coatings such as powder coat or hot-dip galvanizing shrink the hole, so size it for the coated condition or mask it.
Frequently asked questions
- What size clearance hole for a 1/4-20 bolt?
- An F drill (0.257 in) for a close fit or an H drill (0.266 in) for a free fit. A 17/64 in drill (0.2656 in) is a common fractional stand-in for the free fit.
- What size clearance hole for an M6 screw?
- ISO 273 gives 6.4 mm (fine), 6.6 mm (medium) and 7.0 mm (coarse). Medium is the usual default for general machined and fabricated parts.
- When should I use a close fit instead of a free fit?
- Close fit when the screw helps locate the part or when both parts are drilled together in one setup. Free fit when the holes are made separately, in different parts or by different processes, so positional error in each part still lets the screws drop in.
- How does the clearance hole relate to the position tolerance?
- For a floating fastener (a bolt and nut through clearance holes in both parts) the position tolerance at maximum material condition can be the hole size minus the fastener size: T = H - F. A 1/4 in screw in an H-drilled hole (0.266 in) allows 0.016 in diameter. For a fixed fastener (screw into a tapped hole) split the difference between the two parts, commonly T = (H - F) / 2 each.
