Inch Countersink Size Chart

ASME B18.6.3 includes multiple 82-degree and 100-degree flat countersunk machine-screw head forms. Verify the individual fastener table before selecting the major countersink diameter.

Metric Countersink Size Chart

ISO 15065 specifies matched countersink dimensions for countersunk-head screws whose head configuration follows ISO 7721.

Countersink Angle Quick Reference

Typical purposes include flush fastening, reduced snagging, smooth finished surfaces, aerodynamic surfaces, and improved assembly clearance.

A chamfer may simply break a sharp edge. A countersink is dimensioned specifically by angle and major diameter to receive a flat-head fastener. This distinction matters in drawings and CNC programming.

The countersink major diameter normally relates closely to the flat-head screw’s maximum head diameter, depending on whether the head must sit flush, slightly recessed, or slightly proud, and on the drawing’s depth tolerance. You cannot choose the correct countersink diameter from nominal screw diameter alone; use the actual head geometry from the fastener standard or manufacturer data.

90° Countersink

Since tan(45°) = 1, a 90° countersink simplifies to h = (D – d) / 2.

ISO 3294 standardizes countersink tools with included angles of 60°, 90°, and 120°. Other standards and commercial cutters cover 82° and 100° forms for the fasteners that require them.

ASME B18.6.3 includes multiple 82° flat countersunk-head forms, including undercut forms. Do not replace head-specific ASME dimensions with a generic diameter-to-countersink ratio.

Use the Clearance Hole Size Chart to select the smaller pass-through hole below the countersink.

ISO 15065 specifies countersink dimensions for countersunk-head screws whose head configuration follows ISO 7721. Use the specific applicable product and fastener standard for actual values.

ASME B18.6.3 includes 100° flat countersunk-head machine screws. Active NASM24693 and NASM24694 cover 100° countersunk machine and structural screw forms. Angle matching remains mandatory.

If angles do not match, contact can occur only near the inner or outer edge of the cone, bearing stress can increase, the head may not sit flush, fastener alignment can suffer, and thin material can deform.

For screw diameter and thread identification, see the Machine Screw Size Chart and Thread Size Chart.

ISO 10642:2026 covers hexagon socket countersunk-head screws with metric coarse pitch threads from M2 through M20, in steel and stainless steel, product grade A. ISO 10642 explicitly notes reduced loadability associated with countersunk-head/socket geometry, so countersunk screw selection must not be treated as identical to a full-strength head form.

ISO 7721 remains current after confirmation in 2024 and covers countersunk-head screw configuration and gauging for thread sizes M1.6 through M20. ISO 7721 defines the head configuration, while ISO 15065 defines matching countersink dimensions for many screw types.

ISO 15065:2005 remains current after confirmation in 2023 and specifies countersinks matching ISO 7721 head geometry. It applies to multiple screw-product standards, including ISO 2009, 2010, 7046, 7047, 7050, and 7051. Use it with the exact applicable screw product and drawing.

ISO 10642:2026 is the current edition covering hexagon socket countersunk-head screws with coarse-pitch metric threads from M2 through M20, in steel and stainless steel, product grade A. Its stated scope warns that countersunk-head/socket geometry produces reduced loadability compared with full-strength head forms, which is an important safety and design caveat.

Do not let “countersink size” become confused with drill diameter. Use the Clearance Hole Size Chart for the smaller pass-through hole beneath the countersink.

A countersunk screw can be installed into a clearance hole plus nut, a clearance hole plus threaded second member, or a tapped hole directly. Countersink geometry is related to the head, while tap drill is related to thread formation. A pilot hole controls thread engagement in materials such as wood, while a countersink controls head seating. See the Tap Drill Chart and Pilot Hole Size Chart.

Do not use D = 2d or another universal ratio. The exact diameter must come from the fastener-head standard, manufacturer data, or engineering drawing.

Depths are calculated from h = (D-d)/[2tan(θ/2)]. They are geometry outputs only, not head-dimension claims or allowable production tolerances.

Thin material may not have enough thickness for a full countersink depth. Potential issues include knife-edge holes, deformation, and reduced bearing area. Possible engineered solutions include a dimpled countersink, an undercut fastener, or a different head type. Do not countersink thin sheet blindly.

ASME B18.6.3 includes undercut 82° flat countersunk-head screws. They provide countersunk head geometry with reduced head height, making them useful where material thickness limits a full standard head recess. Exact dimensions must be taken from the applicable standard table or manufacturer product data.

In thick plate, a standard machined countersink is generally more straightforward because adequate material thickness can support a full bearing surface. Still check concentricity, target seating, actual head geometry, and drawing tolerance.

ISO 3294 covers tooling at 60°, 90°, and 120°, while other tooling standards and products cover additional angles. Do not assume any general countersink bit is correct for every fastener.

Common countersink tool styles include single-flute, multi-flute, and zero-flute or deburring tools. Chatter, finish, material type, and machine rigidity all affect tool selection. A countersink-style tool may be used for deburring, but deburring usually creates only a small edge break; a true countersink is dimensioned for fastener seating.

Aerospace rivets often use 100° countersunk heads. Active standards such as NASM20426 and NASM20427 cover 100° countersunk rivet forms. Do not apply screw-head countersink dimensions automatically to rivets, even when the included angle appears similar.