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Countersink depth formula

A countersink creates a conical recess for a flat-head fastener to sit flush with or below the surface. The depth depends on the head diameter of the screw and the countersink angle (typically 82 deg for inch fasteners, 90 deg for metric).

• Countersink depth = (diameter / 2) / tan(angle / 2)
• Pilot drill depth ≈ countersink depth × 0.3 -- for the minor diameter clearance
• Common angles: 82 deg (ANSI/UN flat-head screws), 90 deg (ISO metric flat-head), 100 deg (aircraft rivets)

How to use this Countersink Depth Calculator

Enter the countersink diameter (screw head diameter) and the countersink angle. The calculator gives the depth to feed the countersink tool. Add 0.05-0.1 mm for flush seating; add 0.2-0.3 mm for below-flush (for filling or covering).