Counterbore Dimensions for Socket Head Cap Screws
Counterbore diameter and depth for standard socket head cap screws, with the head size and the clearance hole underneath: inch #2 to 1 in (ASME B18.3 heads) and metric M3 to M24 (ISO 4762 heads, DIN 974-1 counterbores). Jump to inch or metric.
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Inch socket head cap screws
| Screw | Head dia (max) | Head height (max) | Counterbore dia | Depth, flush head | Clearance hole, close / free |
|---|---|---|---|---|---|
| #2 | 0.1403.56 mm | 0.0862.18 mm | 3/16" (0.188)4.76 mm | 0.0862.18 mm | #43 (0.0890)#41 (0.0960) |
| #3 | 0.1614.09 mm | 0.0992.51 mm | 7/32" (0.219)5.56 mm | 0.0992.51 mm | #37 (0.1040)#35 (0.1100) |
| #4 | 0.1834.65 mm | 0.1122.84 mm | 7/32" (0.219)5.56 mm | 0.1122.84 mm | #32 (0.1160)#30 (0.1285) |
| #5 | 0.2055.21 mm | 0.1253.18 mm | 1/4" (0.250)6.35 mm | 0.1253.18 mm | #30 (0.1285)#29 (0.1360) |
| #6 | 0.2265.74 mm | 0.1383.51 mm | 9/32" (0.281)7.14 mm | 0.1383.51 mm | #27 (0.1440)#25 (0.1495) |
| #8 | 0.2706.86 mm | 0.1644.17 mm | 5/16" (0.313)7.94 mm | 0.1644.17 mm | #18 (0.1695)#16 (0.1770) |
| #10 | 0.3127.92 mm | 0.1904.83 mm | 3/8" (0.375)9.53 mm | 0.1904.83 mm | #9 (0.1960)#7 (0.2010) |
| 1/4 | 0.3759.53 mm | 0.2506.35 mm | 7/16" (0.438)11.11 mm | 0.2506.35 mm | F (0.2570)H (0.2660) |
| 5/16 | 0.46911.91 mm | 0.3127.92 mm | 17/32" (0.531)13.49 mm | 0.3127.92 mm | P (0.3230)Q (0.3320) |
| 3/8 | 0.56214.27 mm | 0.3759.53 mm | 5/8" (0.625)15.88 mm | 0.3759.53 mm | W (0.3860)X (0.3970) |
| 7/16 | 0.65616.66 mm | 0.43811.13 mm | 23/32" (0.719)18.26 mm | 0.43811.13 mm | 29/64" (0.4531)15/32" (0.4688) |
| 1/2 | 0.75019.05 mm | 0.50012.70 mm | 13/16" (0.813)20.64 mm | 0.50012.70 mm | 33/64" (0.5156)17/32" (0.5313) |
| 5/8 | 0.93823.83 mm | 0.62515.88 mm | 1" (1.000)25.40 mm | 0.62515.88 mm | 41/64" (0.6406)43/64" (0.6719) |
| 3/4 | 1.12528.58 mm | 0.75019.05 mm | 1-3/16" (1.188)30.16 mm | 0.75019.05 mm | 49/64" (0.7656)51/64" (0.7969) |
| 7/8 | 1.31233.32 mm | 0.87522.23 mm | 1-3/8" (1.375)34.93 mm | 0.87522.23 mm | 57/64" (0.8906)59/64" (0.9219) |
| 1 | 1.50038.10 mm | 1.00025.40 mm | 1-5/8" (1.625)41.28 mm | 1.00025.40 mm | 1-1/64" (1.0156)1-3/64" (1.0469) |
Metric socket head cap screws (ISO 4762)
| Screw | Head dia dk (max) | Head height k (max) | Counterbore dia | Depth, flush head | Clearance hole, fine / medium |
|---|---|---|---|---|---|
| M3 | 5.50.217 in | 3.00.118 in | 6.50.256 in | 3.00.118 in | 3.2 / 3.4 |
| M4 | 7.00.276 in | 4.00.157 in | 8.00.315 in | 4.00.157 in | 4.3 / 4.5 |
| M5 | 8.50.335 in | 5.00.197 in | 10.00.394 in | 5.00.197 in | 5.3 / 5.5 |
| M6 | 10.00.394 in | 6.00.236 in | 11.00.433 in | 6.00.236 in | 6.4 / 6.6 |
| M8 | 13.00.512 in | 8.00.315 in | 15.00.591 in | 8.00.315 in | 8.4 / 9.0 |
| M10 | 16.00.630 in | 10.00.394 in | 18.00.709 in | 10.00.394 in | 10.5 / 11.0 |
| M12 | 18.00.709 in | 12.00.472 in | 20.00.787 in | 12.00.472 in | 13.0 / 13.5 |
| M16 | 24.00.945 in | 16.00.630 in | 26.01.024 in | 16.00.630 in | 17.0 / 17.5 |
| M20 | 30.01.181 in | 20.00.787 in | 33.01.299 in | 20.00.787 in | 21.0 / 22.0 |
| M24 | 36.01.417 in | 24.00.945 in | 40.01.575 in | 24.00.945 in | 25.0 / 26.0 |
Head sizes are the maximums for plain heads (ASME B18.3, ISO 4762); knurled heads run slightly larger. Inch counterbore diameters are the sizes socket screw makers publish; metric ones follow DIN 974-1 for ISO 4762 screws without washers. Flush depth equals the maximum head height. Clearance holes are from the clearance hole chart. M2, M2.5 and M14 are not listed.
Setting the depth
The table depth puts a maximum-height head exactly flush. Most drawings go a little deeper, 0.010 to 0.020 in or about 0.3 to 0.5 mm, so the head never stands proud from tolerance, a burr or a chip under it. Add the washer thickness if one goes under the head, and open the counterbore to clear the washer's outside diameter. Low-head socket screws (DIN 7984 and the inch low-head series) are much shorter, so size the depth to the screw actually on the bill of materials.
A typical inch callout for a 1/4-20 screw reads ⌀.266 THRU, CBORE ⌀.438 x .260 DEEP, or the same with the ASME counterbore and depth symbols. Shops cut these with a piloted counterbore sized to the screw, or by circular interpolation with an end mill, which makes any diameter without a special tool.
Frequently asked questions
- What size counterbore for a 1/4-20 socket head cap screw?
- A 7/16 in (0.438 in) counterbore, 0.250 in deep for a flush head, over an F (0.257 in) close-fit or H (0.266 in) free-fit hole. The screw head is 0.375 in across and 0.250 in tall, so most drawings go 0.010 to 0.020 in deeper to keep the head just below the surface.
- What size counterbore for an M6 socket head cap screw?
- An 11 mm counterbore over a 6.4 mm (fine) or 6.6 mm (medium) clearance hole. The ISO 4762 head is 10 mm across and 6 mm tall, so the depth for a flush head is 6 mm; 6.3 to 6.5 mm sinks it slightly.
- Why is the counterbore larger than the screw head?
- To clear the head with room for position error between the counterbore and the tapped hole below, for the hex key, and for the radius under the head. Too tight and the head rubs the wall or the screw cocks; too loose and the bearing ring under the head shrinks.
- What is the difference between a counterbore and a countersink?
- A counterbore is a flat-bottomed step for a cylindrical head such as a socket head cap screw. A countersink is a cone for a flat (countersunk) head screw, 82 degrees for inch screws and 90 degrees for metric.
