The normal clearance hole for an M6 screw is 6.6 mm and for a 1/4 in screw it is an H drill (0.2660 in). This page gives close, normal and loose clearance holes for inch screws from #2 to 1 in and metric screws from M1.6 to M36, then counterbore sizes for socket head cap screws and countersink diameters for flat head screws.
A clearance hole lets the screw pass through one part and thread into the next. A smaller (close) hole locates the parts more accurately but needs accurately placed holes; a larger (loose) hole tolerates more position error. Each table lists inch and millimetre values together. Use the search box to filter every table at once, for example M8 or 3/8.
Inch Clearance Holes (#2 to 1 in)
| Screw | Nominal dia | Close fit | Normal (free) fit |
|---|---|---|---|
| #2 | 0.0860 in2.18 mm | #43 0.0890 in2.26 mm | #41 0.0960 in2.44 mm |
| #3 | 0.0990 in2.51 mm | #37 0.1040 in2.64 mm | #35 0.1100 in2.79 mm |
| #4 | 0.1120 in2.84 mm | #32 0.1160 in2.95 mm | #31 0.1200 in3.05 mm |
| #5 | 0.1250 in3.17 mm | #30 0.1285 in3.26 mm | #29 0.1360 in3.45 mm |
| #6 | 0.1380 in3.51 mm | #27 0.1440 in3.66 mm | #25 0.1495 in3.80 mm |
| #8 | 0.1640 in4.17 mm | #18 0.1695 in4.31 mm | #16 0.1770 in4.50 mm |
| #10 | 0.1900 in4.83 mm | #9 0.1960 in4.98 mm | #7 0.2010 in5.11 mm |
| #12 | 0.2160 in5.49 mm | #2 0.2210 in5.61 mm | #1 0.2280 in5.79 mm |
| 1/4 | 0.2500 in6.35 mm | F 0.2570 in6.53 mm | H 0.2660 in6.76 mm |
| 5/16 | 0.3125 in7.94 mm | P 0.3230 in8.20 mm | Q 0.3320 in8.43 mm |
| 3/8 | 0.3750 in9.52 mm | W 0.3860 in9.80 mm | X 0.3970 in10.08 mm |
| 7/16 | 0.4375 in11.11 mm | 29/64 0.4531 in11.51 mm | 15/32 0.4688 in11.91 mm |
| 1/2 | 0.5000 in12.70 mm | 33/64 0.5156 in13.10 mm | 17/32 0.5312 in13.49 mm |
| 9/16 | 0.5625 in14.29 mm | 37/64 0.5781 in14.68 mm | 39/64 0.6094 in15.48 mm |
| 5/8 | 0.6250 in15.88 mm | 41/64 0.6406 in16.27 mm | 21/32 0.6562 in16.67 mm |
| 3/4 | 0.7500 in19.05 mm | 49/64 0.7656 in19.45 mm | 25/32 0.7812 in19.84 mm |
| 7/8 | 0.8750 in22.22 mm | 57/64 0.8906 in22.62 mm | 59/64 0.9219 in23.42 mm |
| 1" | 1.0000 in25.40 mm | 1-1/64 1.0156 in25.80 mm | 1-5/64 1.0781 in27.38 mm |
Close and normal (free) fit drills are the common inch clearance-drill chart. Inch loose fit is not given here; where you need one, the ISO 273 coarse value for the nearest metric size or the next fractional drill up works well.
Metric Clearance Holes, ISO 273 (M1.6 to M36)
| Screw | Close (fine) | Normal (medium) | Loose (coarse) |
|---|---|---|---|
| M1.6 | 1.7 mm0.0669 in | 1.8 mm0.0709 in | 2.0 mm0.0787 in |
| M2 | 2.2 mm0.0866 in | 2.4 mm0.0945 in | 2.6 mm0.1024 in |
| M2.5 | 2.7 mm0.1063 in | 2.9 mm0.1142 in | 3.1 mm0.1220 in |
| M3 | 3.2 mm0.1260 in | 3.4 mm0.1339 in | 3.6 mm0.1417 in |
| M4 | 4.3 mm0.1693 in | 4.5 mm0.1772 in | 4.8 mm0.1890 in |
| M5 | 5.3 mm0.2087 in | 5.5 mm0.2165 in | 5.8 mm0.2283 in |
| M6 | 6.4 mm0.2520 in | 6.6 mm0.2598 in | 7.0 mm0.2756 in |
| M8 | 8.4 mm0.3307 in | 9.0 mm0.3543 in | 10.0 mm0.3937 in |
| M10 | 10.5 mm0.4134 in | 11.0 mm0.4331 in | 12.0 mm0.4724 in |
| M12 | 13.0 mm0.5118 in | 13.5 mm0.5315 in | 14.5 mm0.5709 in |
| M14 | 15.0 mm0.5906 in | 15.5 mm0.6102 in | 16.5 mm0.6496 in |
| M16 | 17.0 mm0.6693 in | 17.5 mm0.6890 in | 18.5 mm0.7283 in |
| M18 | 19.0 mm0.7480 in | 20.0 mm0.7874 in | 21.0 mm0.8268 in |
| M20 | 21.0 mm0.8268 in | 22.0 mm0.8661 in | 24.0 mm0.9449 in |
| M22 | 23.0 mm0.9055 in | 24.0 mm0.9449 in | 26.0 mm1.0236 in |
| M24 | 25.0 mm0.9843 in | 26.0 mm1.0236 in | 28.0 mm1.1024 in |
| M27 | 28.0 mm1.1024 in | 30.0 mm1.1811 in | 32.0 mm1.2598 in |
| M30 | 31.0 mm1.2205 in | 33.0 mm1.2992 in | 35.0 mm1.3780 in |
| M33 | 34.0 mm1.3386 in | 36.0 mm1.4173 in | 38.0 mm1.4961 in |
| M36 | 37.0 mm1.4567 in | 39.0 mm1.5354 in | 42.0 mm1.6535 in |
Socket Head Cap Screw Counterbores
Inch socket head cap screws (ASME B18.3)
| Screw | Head dia (max) | Counterbore dia | Counterbore depth (min) | Clearance hole |
|---|---|---|---|---|
| #2 | 0.1400 in3.56 mm | 3/16 0.1875 in4.76 mm | 0.0860 in2.18 mm | #41 0.0960 in2.44 mm |
| #4 | 0.1830 in4.65 mm | 7/32 0.2188 in5.56 mm | 0.1120 in2.84 mm | #31 0.1200 in3.05 mm |
| #6 | 0.2260 in5.74 mm | 9/32 0.2812 in7.14 mm | 0.1380 in3.51 mm | #25 0.1495 in3.80 mm |
| #8 | 0.2700 in6.86 mm | 5/16 0.3125 in7.94 mm | 0.1640 in4.17 mm | #16 0.1770 in4.50 mm |
| #10 | 0.3120 in7.92 mm | 3/8 0.3750 in9.52 mm | 0.1900 in4.83 mm | #7 0.2010 in5.11 mm |
| 1/4 | 0.3750 in9.52 mm | 7/16 0.4375 in11.11 mm | 0.2500 in6.35 mm | H 0.2660 in6.76 mm |
| 5/16 | 0.4690 in11.91 mm | 17/32 0.5312 in13.49 mm | 0.3125 in7.94 mm | Q 0.3320 in8.43 mm |
| 3/8 | 0.5625 in14.29 mm | 5/8 0.6250 in15.88 mm | 0.3750 in9.52 mm | X 0.3970 in10.08 mm |
| 7/16 | 0.6560 in16.66 mm | 3/4 0.7500 in19.05 mm | 0.4375 in11.11 mm | 15/32 0.4688 in11.91 mm |
| 1/2 | 0.7500 in19.05 mm | 13/16 0.8125 in20.64 mm | 0.5000 in12.70 mm | 17/32 0.5312 in13.49 mm |
| 5/8 | 0.9380 in23.83 mm | 1" 1.0000 in25.40 mm | 0.6250 in15.88 mm | 21/32 0.6562 in16.67 mm |
| 3/4 | 1.1250 in28.57 mm | 1-3/16 1.1875 in30.16 mm | 0.7500 in19.05 mm | 25/32 0.7812 in19.84 mm |
| 7/8 | 1.3120 in33.32 mm | 1-3/8 1.3750 in34.92 mm | 0.8750 in22.22 mm | 59/64 0.9219 in23.42 mm |
| 1" | 1.5000 in38.10 mm | 1-5/8 1.6250 in41.27 mm | 1.0000 in25.40 mm | 1-5/64 1.0781 in27.38 mm |
Metric socket head cap screws (ISO 4762)
| Screw | Head dia dk (max) | Counterbore dia | Counterbore depth (min) | Clearance hole |
|---|---|---|---|---|
| M1.6 | 3.0 mm0.1181 in | 3.5 mm0.1378 in | 1.6 mm0.0630 in | 1.8 mm0.0709 in |
| M2 | 3.8 mm0.1496 in | 4.4 mm0.1732 in | 2.0 mm0.0787 in | 2.4 mm0.0945 in |
| M2.5 | 4.5 mm0.1772 in | 5.5 mm0.2165 in | 2.5 mm0.0984 in | 2.9 mm0.1142 in |
| M3 | 5.5 mm0.2165 in | 6.0 mm0.2362 in | 3.0 mm0.1181 in | 3.4 mm0.1339 in |
| M4 | 7.0 mm0.2756 in | 8.0 mm0.3150 in | 4.0 mm0.1575 in | 4.5 mm0.1772 in |
| M5 | 8.5 mm0.3346 in | 10.0 mm0.3937 in | 5.0 mm0.1969 in | 5.5 mm0.2165 in |
| M6 | 10.0 mm0.3937 in | 11.0 mm0.4331 in | 6.0 mm0.2362 in | 6.6 mm0.2598 in |
| M8 | 13.0 mm0.5118 in | 15.0 mm0.5906 in | 8.0 mm0.3150 in | 9.0 mm0.3543 in |
| M10 | 16.0 mm0.6299 in | 18.0 mm0.7087 in | 10.0 mm0.3937 in | 11.0 mm0.4331 in |
| M12 | 18.0 mm0.7087 in | 20.0 mm0.7874 in | 12.0 mm0.4724 in | 13.5 mm0.5315 in |
| M14 | 21.0 mm0.8268 in | 24.0 mm0.9449 in | 14.0 mm0.5512 in | 15.5 mm0.6102 in |
| M16 | 24.0 mm0.9449 in | 26.0 mm1.0236 in | 16.0 mm0.6299 in | 17.5 mm0.6890 in |
| M20 | 30.0 mm1.1811 in | 33.0 mm1.2992 in | 20.0 mm0.7874 in | 22.0 mm0.8661 in |
| M24 | 36.0 mm1.4173 in | 40.0 mm1.5748 in | 24.0 mm0.9449 in | 26.0 mm1.0236 in |
Head diameter is the maximum from ASME B18.3 (inch) and ISO 4762 (metric). Counterbore depth equals the maximum head height (= nominal diameter), which leaves the head flush at worst; add 0.010-0.020 in or 0.3-0.5 mm to sink it below the surface. Counterbore diameters are common practice (DIN 974-2-style for metric) and leave room for head and burr clearance.
Flat Head Countersink Diameters
Inch flat head screws, 82° countersink
| Screw | Machine screw head (B18.6.3) | Socket flat head (B18.3) | Clearance hole |
|---|---|---|---|
| #2 | 0.1720 in4.37 mm | — | #41 0.0960 in2.44 mm |
| #3 | 0.1990 in5.05 mm | — | #35 0.1100 in2.79 mm |
| #4 | 0.2250 in5.71 mm | 0.2550 in6.48 mm | #31 0.1200 in3.05 mm |
| #5 | 0.2520 in6.40 mm | — | #29 0.1360 in3.45 mm |
| #6 | 0.2790 in7.09 mm | 0.3070 in7.80 mm | #25 0.1495 in3.80 mm |
| #8 | 0.3320 in8.43 mm | 0.3590 in9.12 mm | #16 0.1770 in4.50 mm |
| #10 | 0.3850 in9.78 mm | 0.4110 in10.44 mm | #7 0.2010 in5.11 mm |
| #12 | 0.4380 in11.13 mm | — | #1 0.2280 in5.79 mm |
| 1/4 | 0.5070 in12.88 mm | 0.5310 in13.49 mm | H 0.2660 in6.76 mm |
| 5/16 | 0.6350 in16.13 mm | 0.6560 in16.66 mm | Q 0.3320 in8.43 mm |
| 3/8 | 0.7620 in19.35 mm | 0.7810 in19.84 mm | X 0.3970 in10.08 mm |
| 7/16 | 0.8120 in20.62 mm | — | 15/32 0.4688 in11.91 mm |
| 1/2 | 0.8750 in22.22 mm | — | 17/32 0.5312 in13.49 mm |
| 9/16 | 1.0000 in25.40 mm | — | 39/64 0.6094 in15.48 mm |
| 5/8 | 1.1250 in28.57 mm | — | 21/32 0.6562 in16.67 mm |
| 3/4 | 1.3750 in34.92 mm | — | 25/32 0.7812 in19.84 mm |
Metric countersunk screws, 90° countersink
| Screw | Socket countersunk (ISO 10642) | Cross recess (ISO 7046) | Clearance hole |
|---|---|---|---|
| M1.6 | — | 3.6 mm0.1417 in | 1.8 mm0.0709 in |
| M2 | — | 4.4 mm0.1732 in | 2.4 mm0.0945 in |
| M2.5 | — | 5.5 mm0.2165 in | 2.9 mm0.1142 in |
| M3 | 6.72 mm0.2646 in | 6.3 mm0.2480 in | 3.4 mm0.1339 in |
| M4 | 8.96 mm0.3528 in | 9.4 mm0.3701 in | 4.5 mm0.1772 in |
| M5 | 11.20 mm0.4409 in | 10.4 mm0.4094 in | 5.5 mm0.2165 in |
| M6 | 13.44 mm0.5291 in | 12.6 mm0.4961 in | 6.6 mm0.2598 in |
| M8 | 17.92 mm0.7055 in | 17.3 mm0.6811 in | 9.0 mm0.3543 in |
| M10 | 22.40 mm0.8819 in | 20.0 mm0.7874 in | 11.0 mm0.4331 in |
| M12 | 26.88 mm1.0583 in | — | 13.5 mm0.5315 in |
| M16 | 33.60 mm1.3228 in | — | 17.5 mm0.6890 in |
| M20 | 40.32 mm1.5874 in | — | 22.0 mm0.8661 in |
Head diameters are the theoretical sharp maximum: ASME B18.6.3 flat head machine screws and ASME B18.3 flat socket head cap screws (82°), ISO 10642 hexagon socket and ISO 7046 cross-recessed countersunk screws (90°). — = size not covered by that standard in this table. Countersinking to this diameter sets a perfect head flush; go a few thou (0.1-0.2 mm) larger to make sure it sits just below.
How to Use These Charts
- Pick the fit first. Start with normal. Choose close for doweled or accurately positioned parts, loose for sheet metal, weldments and assemblies with many holes.
- Use the clearance to set position tolerance. For a floating fastener (clearance holes in both parts and a nut), each hole's position tolerance can be up to the clearance at maximum material. For a fixed fastener (one part tapped), split it between the two parts.
- Counterbore = diameter, depth and pilot. Drill the clearance hole first, then counterbore with a piloted tool or interpolate with an end mill. A flat bottom matters more than the exact diameter.
- Countersink to diameter, not depth. Measure the countersink diameter at the surface, or use the Countersink & Chamfer Depth Calculator to convert diameter to a Z depth for your tool.
Worked example: a 5/16-18 socket head cap screw, sunk flush. Clearance hole: Q drill (0.3320 in, normal fit). Counterbore: 17/32 in (0.5312 in) diameter, 0.3125 in deep minimum (0.325 in to sit just below). Metric equivalent M8: 9.0 mm hole, 15 mm counterbore, 8.0 mm deep minimum (about 8.4 mm to sit below).
Shop tips
- For close-fit holes, drill slightly under and ream or interpolate if location matters more than speed.
- Break the edge of the clearance hole lightly so the fillet under the screw head does not ride on a sharp corner.
- Hex head bolts need a larger counterbore than socket heads to leave room for a socket; check the Socket Head Reference Guide and your socket OD.
Frequently Asked Questions
Drill 6.6 mm for a normal fit. ISO 273 gives 6.4 mm for a close (fine) fit, 6.6 mm for a normal (medium) fit and 7.0 mm for a loose (coarse) fit.
Use an H drill (0.2660 in) for a normal or free fit, or an F drill (0.2570 in) for a close fit. A 17/64 in drill (0.2656 in) is almost identical to H.
At least the head height, which for a standard socket head cap screw equals the nominal diameter: 0.250 in for 1/4 in and 6 mm for M6. Add about 0.010 to 0.020 in (0.3 to 0.5 mm) if the head must sit below the surface or a washer is used.
Inch flat head screws use an 82 degree countersink. Metric countersunk screws such as ISO 10642 and ISO 7046 use 90 degrees. Using the wrong angle makes the head touch only on its outer edge or only near the shank, so it does not seat properly.
Use close fit where the screw helps locate the parts and the holes are accurately positioned, such as on a CNC mill. Use normal fit for most work, and loose fit for sheet metal, weldments or long hole patterns where position error is larger.
Related Calculators and Charts
Metric clearance holes: ISO 273 fine, medium and coarse series. Inch clearance holes: common close/free-fit drill practice (drill sizes per ASME B94.11M). Socket head dimensions: ASME B18.3 and ISO 4762; counterbore diameters are common shop practice. Countersink head diameters: ASME B18.6.3, ASME B18.3, ISO 10642 and ISO 7046. Check critical values against the standard or the fastener manufacturer.