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Socket Head Cap Screw Dimensions

Guide · Updated October 28, 2026

Socket head cap screws are the standard screw for machine parts, fixtures and tooling. Their heads are sized so they can sit in a counterbore flush or below the surface. On inch screws from #10 up, the head diameter is about 1.5 times the screw diameter and the head height equals the screw diameter. Metric ISO 4762 screws follow a similar pattern. The values below are the maximum head dimensions from ASME B18.3 and ISO 4762.

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Step by step

Identify the screw size: Measure the thread and count TPI or pitch.
Read the head size: Use the chart for head diameter, head height and hex key.
Size the counterbore: Make the counterbore larger than the maximum head diameter so the head clears, and deep enough to sink the head the amount you want.
Pick the clearance drill: Use a close or normal fit clearance hole through the part.
Check the key: Use a full size hex key; worn or ball-end keys round out the socket under full torque.

Inch socket head cap screws (ASME B18.3)

#4: head 0.183, height 0.112, key 3/32
#6: head 0.226, height 0.138, key 7/64
#8: head 0.270, height 0.164, key 9/64
#10: head 0.312, height 0.190, key 5/32
1/4: head 0.375, height 0.250, key 3/16
5/16: head 0.469, height 0.312, key 1/4
3/8: head 0.562, height 0.375, key 5/16
7/16: head 0.656, height 0.438, key 3/8
1/2: head 0.750, height 0.500, key 3/8
5/8: head 0.938, height 0.625, key 1/2
3/4: head 1.125, height 0.750, key 5/8
7/8: head 1.312, height 0.875, key 3/4
1: head 1.500, height 1.000, key 3/4

Head diameters and heights are maximums. Note that 7/16 and 1/2 both use a 3/8 key, and 7/8 and 1 both use a 3/4 key.

Metric socket head cap screws (ISO 4762)

M3: head 5.5 mm, height 3 mm, key 2.5 mm
M4: 7 mm, 4 mm, key 3
M5: 8.5 mm, 5 mm, key 4
M6: 10 mm, 6 mm, key 5
M8: 13 mm, 8 mm, key 6
M10: 16 mm, 10 mm, key 8
M12: 18 mm, 12 mm, key 10
M16: 24 mm, 16 mm, key 14
M20: 30 mm, 20 mm, key 17
M24: 36 mm, 24 mm, key 19

Metric heads are also as tall as the nominal diameter. ISO 4762 and the older DIN 912 share these key sizes.

Counterbore sizing

A practical counterbore is about 1/32 to 1/16 in larger than the maximum head diameter on inch screws, which is why 1/4 screws commonly get a 7/16 counterbore and 3/8 screws get a 5/8 counterbore. Metric screws commonly get 1 to 2 mm over the head: 11 mm for M6, 15 mm for M8 and 18 mm for M10. Depth is the head height plus how far below the surface you want the head, often 0.010 to 0.030 in.

Using a piloted counterbore that matches the clearance hole keeps the counterbore concentric so the head seats evenly.

Worked example: 1/4-20 SHCS in a counterbore

Head 0.375 max, height 0.250. Counterbore 7/16 (0.4375) leaves 0.031 in clearance on each side. To sink the head 0.020 below the surface, counterbore 0.270 deep. Through hole: 9/32 (0.281) normal fit or 17/64 close fit.

Worked example: M8 SHCS

Head 13 mm max, height 8 mm, key 6 mm. Counterbore 14 or 15 mm diameter, depth 8.5 mm for a 0.5 mm sink. Clearance hole 9.0 mm medium fit. Check that the wall between neighboring counterbores is enough; on a 20 mm bolt spacing with 15 mm counterbores, only 5 mm of wall is left.

Common problems

Making counterbores to the nominal head size with no clearance, forgetting knurled heads can be slightly over the plain head size, under-depth counterbores that leave a head proud of a fixture surface, and using a worn hex key. Check counterbore depth with a depth mic or depth gage.

FAQ

What size Allen key for a 1/4-20 socket head cap screw?

3/16 in.

What size Allen key for an M6 socket head cap screw?

5 mm.

What size counterbore for 1/4 SHCS?

7/16 in diameter is common, with depth equal to head height plus any sink.

How do I check counterbore depth?

Check the gage itself before trusting a measurement: gage blocks set mics, calipers and indicators; build any stack free at gageblockcalc.com.

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General shop information only — follow your drawing, tool maker's data and your quality system.