TOOLS / MACHINING

Countersink depth & diameter calculator

Work out how deep to countersink so a flat-head screw sits flush, without drilling through a thin plate.

Depth from pilot surface4.00 mm
Theoretical point depth8.00 mm

At 90 degrees, sinking from a 8 mm pilot to a 16 mm top diameter needs 4.00 mm of depth. Do not drill through a thin plate: check remaining thickness against the drawing.

Common angles: 90 degrees gives depth 0.500 x top diameter from a point; 82 degrees gives about 0.575 x top diameter. Confirm the screw-head size and angle on the drawing.

Formula

Depth from the pilot surface = (top diameter - pilot diameter) / (2 x tan(angle / 2)). With no pilot, this is (top diameter / 2) / tan(angle / 2). At 90 degrees, tan(45 degrees) = 1, so depth = (top diameter - pilot diameter) / 2.

Standard / source

Machinist's Handbook countersink geometry, as summarised in the MetalCalz tool-expansion research (2026-10-27): 90 degrees = 0.500 x diameter, 82 degrees = about 0.575 x diameter from a point. No code table is reproduced here.

Worked example

Top diameter 16 mm, pilot 8 mm, angle 90 degrees: depth = (16 - 8) / (2 x 1) = 4.00 mm. Theoretical point depth = 16 / 2 / 1 = 8.00 mm. These are the calculator defaults, so the page loads showing 4.00 mm depth and 8.00 mm point depth.

Safety note

Planning estimate for drilling. Check remaining plate thickness after countersinking and confirm the screw-head diameter and angle on the drawing. For a structural fastener seat, a qualified person must verify the drawing requirements.