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.

What is the Countersink Depth Calculator?

Calculates the countersink plunge depth needed to seat a flat-head screw flush, from the screw head diameter, the countersink included angle of 82, 90, 100 or 120 degrees, and the existing pilot hole diameter. Gives a depth to set a drill stop or program on a CNC. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Countersink Depth Calculator

Name: Countersink Depth Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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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.

Angle selection guide

Choosing the wrong included angle means the screw head won’t seat correctly — it will either sit proud on a too-shallow cone or rattle in a too-wide one. Match the angle to the specific fastener type:

Angle	Common application
82°	US inch flat-head screws (ANSI/ASME)
90°	Metric flat-head screws (ISO, DIN 7991)
100°	Aerospace and aircraft fasteners (NAS/MS spec)
120°	Countersinks for rivets and certain large-head bolts

When in doubt, check the screw manufacturer’s datasheet — the head angle is always specified and it must match your cutter exactly.

Worked examples

Example 1 — inch fastener on a drill press: A #10 flat-head screw has a head diameter of 0.385 in. With an 82° countersink and a 0.196 in pilot hole:

plunge = (0.385 − 0.196) / (2 × tan(41°))
       = 0.189 / (2 × 0.8693)
       ≈ 0.109 in

Set your depth stop to 0.109 in. Add 0.003–0.005 in if you want the head to sit fractionally below flush.

Example 2 — metric screw on a CNC mill: An M5 flat-head screw has a head diameter of 9.3 mm. With a 90° countersink and a 5.5 mm clearance hole:

plunge = (9.3 − 5.5) / (2 × tan(45°))
       = 3.8 / (2 × 1.0)
       = 1.9 mm

Program the Z move to −1.9 mm from the surface. For a 90° cutter, the math simplifies to half the difference in diameters.

Practical guidance

Units: Enter diameters in the same unit you will use on the machine. The depth comes out in that same unit — no conversion needed.

Pilot hole matters: A larger pilot means less cone to cut, so a shallower plunge. Always enter the actual drilled pilot or clearance hole diameter, not zero, or you will over-plunge and the head will sink below flush.

Test on scrap first: Machine tolerances, tool runout, and workpiece variation all shift the final depth slightly. Run a test hole in scrap material, seat the screw, and adjust before cutting the real part.

Slightly deep is better than slightly shallow: A head that protrudes above the surface will catch on mating parts, interfere with clamps, and look unprofessional. Sitting a few thousandths below flush is almost always acceptable.

CNC Z programming: The plunge depth from this calculator is the distance from the surface. Set your work offset (G54 or equivalent) to the surface of the part, and program G1 Z-<depth> directly from the result.