Countersink Calculator: Diameter, Angle & Depth

Published September 22, 2026 · MfgWorkbench editorial reference

Find the axial depth of an ideal conical countersink from the opening diameter, pilot-hole diameter and included angle. The same geometry applies to a straight-edged conical chamfer.

Use the included angle, not the half angle

Depth h = (D − d) / [2 tan(θ/2)], where D is the large diameter, d is the small diameter and θ is the full included cone angle. Radial chamfer width is (D − d)/2. All lengths must use the same unit. A 90° countersink has a 45° half angle, so its axial depth equals its radial width.

The result measures from the top opening down to the intersection with the small-diameter cylindrical wall. It is not the distance from the tip of an actual countersink tool to the surface. A tool with a flat or truncated point needs an additional geometric offset.

Worked example and sensitivity

For D = 10 mm, d = 6 mm and θ = 90°, h = 2 mm. Keeping those diameters but changing to 82° increases h to about 2.3007 mm. A fastener designed for one included angle should not be assumed to seat correctly in the other.

At a 90° included angle, another 0.10 mm of axial depth increases opening diameter by 0.20 mm. This explains why a small Z-offset change can visibly alter a countersink. The relationship is ΔD = 2 tan(θ/2) × Δh.

Check seating, not only diameter

Confirm the head angle and dimensions from the fastener specification, including any head fillet and allowed protrusion. Check remaining wall thickness and edge distance. A counterbore has a flat-bottomed cylindrical recess and uses different geometry.

Measure the finished opening with a suitable countersink gauge or inspection method. Burrs, runout, tool wear and chatter can prevent proper seating even when the ideal dimensions are correct. Inputs describe ideal geometry; they do not establish an acceptable structural joint.