Countersink Depth Calculator

A countersink is a conical recess cut around a drilled hole so the underside of a flat-head screw can sit level with, or slightly below, the work surface. This countersink depth calculator determines the axial depth of that recess from the screw diameter, head diameter, and included angle. It is useful when setting a drill press stop, programming a CNC operation, checking a drawing, or selecting a combined drill and countersink bit. Accurate depth improves appearance, prevents proud screw heads, and avoids removing more material than the joint requires. The calculation uses the geometry of a truncated cone. Half of the difference between the head diameter and screw diameter is the radial change. Dividing that value by the tangent of half the included angle gives the required vertical depth. A larger head increases depth, while a wider included angle makes the recess shallower. Common hardware may use 82-degree, 90-degree, or 100-degree heads, so measuring or checking the fastener specification matters. The opening diameter shown in the result equals the entered head diameter because the calculator assumes a flush fit. Use dimensions in millimeters and keep all measurements in the same unit if you adapt the result. Real work also needs practical allowances. Coatings, burrs, wood compression, bit runout, and head tolerances can change the final seating position. Make a test cut in scrap from the same material, then adjust the stop in small increments. In thin stock, compare the calculated depth with material thickness before drilling because an oversized countersink can weaken the surface or break through it. For structural, aerospace, or code-controlled work, use the fastener drawing and applicable machining standard rather than relying only on a nominal measurement. This tool reports the theoretical geometry rather than recommending a specific bit or feed rate. Choose a cutter whose included angle matches the screw, keep it concentric with the pilot hole, and deburr metal carefully. In wood, a sharp cutter and moderate speed reduce chatter and tear-out. The result offers a reliable setup starting point while leaving the operator responsible for material behavior and the desired flushness.