Countersink Depth Calculator (depth from diameter and angle)

Understanding the countersink depth calculator — concept and principle

A countersink is a conical recess that lets a flat-head screw sit flush with the surface. This calculator finds how deep to cut for a given countersink diameter, angle and hole size, and the depth to the cone's tip for setting a tool.

With a 90° cone the depth equals the radius difference: (12 − 6.6) ÷ 2 = 2.7 mm.

How it is calculated

Depth below the surface = (Dc − d) ÷ (2 tan(θ ÷ 2)), where Dc is the countersink diameter at the surface, d the hole diameter and θ the included angle.
Depth to the cone tip = Dc ÷ (2 tan(θ ÷ 2)).
Width of the conical ring = (Dc − d) ÷ 2.
For 90°, tan 45° = 1, so the depth equals half the diameter difference.

The principle

A countersink is a cone. For each unit of depth, its radius shrinks by tan(θ ÷ 2). Starting from the surface diameter and going down until the cone meets the hole gives the depth. Tools are often set from their tip, which is the imaginary point of the full cone, so the tip depth is also given. The angle must match the screw head: 90° for ISO metric flat-head screws and 82° for most US inch screws.

Worked example

Default inputs: countersink diameter 12 mm, angle 90°, hole 6.6 mm.

tan(90° ÷ 2) = 1.
Depth below the surface = (12 − 6.6) ÷ 2 = 2.7 mm (0.1063 in).
Depth to the cone tip = 12 ÷ 2 = 6 mm.
Width of the conical ring = 2.7 mm; countersink diameter = 0.4724 in; angle 90°.

How to read the result

Make the surface diameter slightly larger than the screw head so the head sits just below the surface. A 6.6 mm hole is a typical clearance hole for an M6 screw. Check a test piece, because tool wear and material springback change the result.

Common mistakes

Using an 82° countersink for 90° metric screws, or vice versa.
Measuring depth from the tool tip when the stop is set from the surface.
Forgetting the clearance hole diameter.
Cutting too deep, which weakens thin material.

Keep in mind: Results are geometric. Follow screw standards and machining practice, and check on a test piece before production.

Frequently asked questions

What countersink angle do metric screws use?

90° for ISO metric flat (countersunk) head screws.

What angle do US screws use?

Usually 82°.

How deep should a 90° countersink be?

Half the difference between the countersink diameter and the hole diameter.

Sources: ISO 15065, Countersinks for countersunk head screws with head configuration in accordance with ISO 7721 · Machinery's Handbook, Industrial Press (countersinks)

Checked 9 Nov 2026. Cone geometry depth = (D_c − d)/(2 tan(θ/2)): standard formula. Usual included angles (90° metric, 82° inch, 100° aircraft): standard values.

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