Countersink Depth Calculator

A flat-head screw only sits flush if the countersink is cut to the right depth, and that depth depends on the head size and the angle of the cone. Too shallow and the head stands proud; too deep and it sinks below the surface. This gives you the plunge depth to cut so the head lands flush, from the head diameter, the hole it passes through and the countersink angle.

The cone has to match the head

A flat-head screw is a cone, and the countersink is a matching cone cut into the work. For the head to sit flush, two things have to line up: the angle and the depth. The angle of the bit must equal the angle of the screw, or the head touches only at an edge and rocks. Inch flat-head screws are 82 degrees, metric ones 90, and flush aircraft fasteners 100, so pick the bit that matches your screw before you worry about depth.

Depth from the head and the hole

Once the angles match, the depth sets how wide the cone opens at the surface, and you want it to open to exactly the head diameter. The cone starts at the clearance hole the screw passes through, not at the center, so the depth is half the head diameter minus the hole, divided by the tangent of half the angle. A bigger head or a smaller hole means a deeper countersink. This is the number you dial into a CNC or set a depth stop to.

Creep up on it by hand

Cutting a countersink by hand or on a drill press, sneak up on the depth rather than plunging to the number cold. Take a little, drop a screw in, and look: proud means go deeper, below flush means you have gone too far and there is no undo. Because the cone widens fast, a small change in depth is a visible change in how the head sits, so the last few thousandths matter. A depth stop makes it repeatable once you have it dialed in.

When sources and screws vary

Head diameters vary a little between screw standards and makers, and a painted or plated surface changes what flush looks like, so treat the depth as a close starting point and confirm with the actual screw. A combination countersink that drills the pilot and cuts the cone in one pass keeps the two aligned, which matters because an off-center countersink leaves the head sitting crooked no matter how right the depth is.

More MakerKiln tools

• pilot holePilot Hole Size CalculatorPilot, clearance and countersink drill for a wood screw, by gauge and wood hardness.→
• tap drillTap Drill Size CalculatorMetric tap drill at the percent of thread you want, plus the clearance hole.→
• bolt circleBolt Circle CalculatorX and Y coordinates of evenly spaced holes on a bolt circle.→

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