Countersink Depth Calculator
Calculate how deep to countersink a hole so a flat-head screw or bolt sits flush with the surface.
What This Calculator Does
A countersunk hole has a cone-shaped recess so a flat (countersunk) screw head sits flush with or below the surface. This calculator finds the depth of that cone based on the screw head size and the countersink tool’s angle.
The Formula
The cone’s depth is found from the difference between the head diameter and the pilot hole diameter, divided by two, then divided by the tangent of half the included angle: depth = ((head diameter − hole diameter) / 2) / tan(angle / 2). Common included angles are 82° (standard in the US for wood and machine screws), 90° (common metric/ISO standard), and 100° (used in aviation for flush riveting).
Always test the countersink depth on scrap material first, since drill bit wear and material hardness can shift the actual cutting depth slightly from the calculated value.