What countersink angle do I need for my screws?

US inch flat-head screws use an 82 degree included angle, while most metric and DIN flat-head screws use 90 degrees. Aircraft and aerospace fasteners often use 100 degrees. Match the countersink angle to the screw head angle or the head will not seat flush.

How is countersink depth calculated?

The countersink is a cone, so the depth to reach a given diameter equals that diameter divided by twice the tangent of half the included angle. Subtracting the pilot hole diameter first gives the actual plunge depth from the existing hole, which is what you program.

Should the screw head sit flush or below the surface?

For most assemblies the head should sit just flush or a few thousandths below the surface so nothing protrudes. Going slightly deeper is safer than leaving the head proud, so add a small amount to the calculated depth and test on scrap first.

What is the difference between a countersink and a counterbore?

A countersink is a tapered cone that seats a conical flat-head screw, while a counterbore is a flat-bottomed cylindrical recess that seats a flat-bottomed socket-head or hex-head screw. This tool covers the angled countersink case.

Does the pilot hole size matter?

Yes. The countersink only needs to cut from the existing pilot diameter up to the head diameter, so a larger pilot means a shallower plunge. Entering the real pilot or clearance diameter gives the correct machine depth rather than the depth from a sharp point.

Countersink Depth Calculator

A countersink lets a flat-head screw sit flush by cutting a cone that matches the underside of the head. This calculator works out how deep to plunge that cone so the resulting top diameter equals the screw head diameter, using the countersink angle and the pilot hole you are starting from.

How it works

A countersink of included angle A is a cone. The vertical depth needed to open the cone to a given diameter is that diameter divided by twice the tangent of half the angle. Because the tool starts from an existing pilot hole rather than a point, you subtract the pilot diameter first:

depth = (head diameter − pilot diameter) / (2 × tan(angle / 2))
full cone = head diameter / (2 × tan(angle / 2))

The full-cone figure is the depth measured from a sharp point, useful when a combined drill-and-countersink leaves no separate pilot. The plunge depth is what you set on a drill-press stop or program as a Z move.

Example and notes

A number 10 flat-head screw with a 0.385 inch head, an 82 degree countersink, and a 0.196 inch pilot hole needs about 0.109 inch of plunge depth. Keep the units consistent — if you enter millimetres for the diameters, the depth comes out in millimetres. Cut slightly deeper rather than shallower so the head finishes flush or just below the surface, and always confirm on a scrap piece before running the real part.