Countersink Depth Calculator

To determine how deep to sink a conical cutter, the calculation treats the countersink profile as a right triangle. The core mathematical formula is derived using trigonometry: depth = (diameter / 2) / tan(angle / 2). Here, the diameter represents the top opening of the conical hole, while the angle refers to the total included angle of the countersink bit (commonly 82, 90, or 100 degrees). Because trigonometry functions rely on right-angle triangles, the total diameter and angle are both halved for the calculation.

Imagine you are installing a flat-head screw that requires a top hole diameter (diameter) of 12 millimeters, and your countersink bit has an included angle of 90 degrees. First, divide the diameter by 2, which gives 6 millimeters. Next, divide the angle by 2 to get a 45-degree angle. Calculate the tangent of 45 degrees, which equals 1. Finally, divide 6 by 1, resulting in a required countersink depth of exactly 6 millimeters. This ensures the screw head will sit completely flush with the workpiece surface.

Always measure your fastener head before drilling to verify the target top diameter, as manufacturer tolerances can vary slightly. Test your setup on scrap material of the exact same species or metal type to confirm the depth stop or drill press setting. Finally, match your tool's angle precisely to the screw head angle—mixing an 82-degree screw with a 90-degree countersink creates a weak, single-point contact zone.