Countersink & Counterbore Chart

Counterbore = cylindrical pocket for socket-head / button-head screws to sit flush. Countersink = conical pocket for flat-head screws (82° for standard UNC / UNF cap screws, 90° for metric ISO 10642 / DIN 7991, 100° for aerospace / 110° for some flat-head sheet-metal screws). Data reflects screw head nominal + typical clearance recommendation for class 1 fit.

Socket-head cap screw (ASME B18.3) — inch

Screw	Head Ø (max)	Head Height	Counterbore Ø	Counterbore Depth
#0 (0.060)	0.096	0.060	7/64 (0.109)	0.060
#2 (0.086)	0.140	0.086	5/32 (0.156)	0.086
#4 (0.112)	0.183	0.112	7/32 (0.219)	0.112
#6 (0.138)	0.226	0.138	1/4 (0.250)	0.138
#8 (0.164)	0.270	0.164	9/32 (0.281)	0.164
#10 (0.190)	0.312	0.190	5/16 (0.312)	0.190
1/4	0.375	0.250	13/32 (0.406)	0.250
5/16	0.469	0.312	1/2 (0.500)	0.312
3/8	0.563	0.375	19/32 (0.594)	0.375
7/16	0.656	0.438	11/16 (0.688)	0.438
1/2	0.750	0.500	25/32 (0.781)	0.500
5/8	0.938	0.625	31/32 (0.969)	0.625
3/4	1.125	0.750	1-5/32 (1.156)	0.750
7/8	1.313	0.875	1-11/32 (1.344)	0.875
1"	1.500	1.000	1-1/2 (1.500)	1.000

Socket-head cap screw (DIN 912 / ISO 4762) — metric

Screw	Head Ø	Head Height	Counterbore Ø	Counterbore Depth
M2	3.8 mm	2.0 mm	4.3 mm	2.4 mm
M2.5	4.5 mm	2.5 mm	5.0 mm	2.9 mm
M3	5.5 mm	3.0 mm	6.5 mm	3.4 mm
M4	7.0 mm	4.0 mm	8.0 mm	4.6 mm
M5	8.5 mm	5.0 mm	10.0 mm	5.7 mm
M6	10.0 mm	6.0 mm	11.0 mm	6.8 mm
M8	13.0 mm	8.0 mm	15.0 mm	9.0 mm
M10	16.0 mm	10.0 mm	18.0 mm	11.0 mm
M12	18.0 mm	12.0 mm	20.0 mm	13.0 mm
M14	21.0 mm	14.0 mm	24.0 mm	15.0 mm
M16	24.0 mm	16.0 mm	26.0 mm	17.5 mm
M20	30.0 mm	20.0 mm	33.0 mm	21.5 mm
M24	36.0 mm	24.0 mm	40.0 mm	25.5 mm

Flat-head cap screw (82° countersink, ASME B18.3.2)

Screw	Head Ø (max)	Countersink Ø	Countersink Depth
#4	0.225	0.240	0.081
#6	0.279	0.300	0.108
#8	0.332	0.360	0.130
#10	0.385	0.420	0.153
1/4	0.507	0.535	0.199
5/16	0.635	0.668	0.249
3/8	0.762	0.795	0.298
7/16	0.812	0.845	0.319
1/2	1.017	1.054	0.399
5/8	1.270	1.314	0.499
3/4	1.525	1.573	0.599
1"	2.030	2.090	0.800

Flat-head cap screw (90° countersink, DIN 7991 / ISO 10642) — metric

Screw	Head Ø	Countersink Ø	Countersink Depth
M2	3.8 mm	4.0 mm	1.2 mm
M2.5	4.7 mm	5.0 mm	1.5 mm
M3	6.0 mm	6.5 mm	1.7 mm
M4	8.0 mm	8.5 mm	2.3 mm
M5	10.0 mm	10.5 mm	2.8 mm
M6	12.0 mm	12.5 mm	3.3 mm
M8	16.0 mm	16.5 mm	4.4 mm
M10	20.0 mm	20.5 mm	5.5 mm
M12	24.0 mm	24.5 mm	6.5 mm
M16	30.0 mm	30.5 mm	7.5 mm
M20	36.0 mm	36.5 mm	8.5 mm

Button-head cap screw (ANSI B18.3.4) — inch

Screw	Head Ø	Head Height	Counterbore Ø
#0	0.114	0.033	0.130
#4	0.213	0.061	0.225
#6	0.262	0.075	0.275
#8	0.312	0.089	0.328
#10	0.361	0.103	0.375
1/4	0.437	0.140	0.453
5/16	0.547	0.176	0.562
3/8	0.656	0.211	0.672
1/2	0.875	0.281	0.891

Countersink angle reference

82°	Standard UNC / UNF flat-head cap screws (ASME B18.3.2). Most common in North America.
90°	Metric flat-head screws (DIN 7991, ISO 10642). Also used for some oversize imperial flat-heads.
100°	Aerospace flat-head (MS14218, NAS1351). Lower profile, better for high-strength thin-wall panels.
120°	Sheet-metal / wood screws.
60°	Centre-drill spotting; not for fastener countersinking.

Tips

For counterbores, drill the clearance hole first, then counterbore — the pilot on the counterbore tool rides in the hole.
Depth rule: counterbore depth = head height + 0.005 to 0.030" (thin panels use the low end so the screw grips through).
For structural joints, recess the head at least 0.010" below the surface to protect the head and improve gasket seating.
Countersink tools have a leading angle rating (e.g. 82° × 1/2"). Match the angle to the screw standard — using an 82° sink on a 90° metric screw (or vice versa) leaves line contact instead of full seat and the screw will never tighten correctly.
For production, combination drill-and-countersink tools ("screwmachine" style) do both operations in one pass.

Was this article helpful?