Countersink depth calculator
How deep to drill a countersink so a flat-head screw finishes flush, or a little below the surface for a plug or filler. Pick a standard wood screw or enter the head you have, then the angle of your bit.
The largest size across the head. Measure your screw if it isn’t a standard wood screw.
0 for a plain countersink; the drill size for a combination bit.
0 for flush; more to leave room for a plug or filler.
Drill the countersink this deep
0.191 in
≈ 3/16 in, 4.85 mm
Measured from the surface to where the cone reaches the pilot drill or its point.
Arithmetic: exact by definition: depth = (head − pilot) ÷ 2 ÷ tan(angle ÷ 2).
How the depth is worked out
A countersink is a cone. For every bit of depth it widens by twice the tangent of half its angle, so the depth that reaches a given width is half that width divided by the tangent of half the angle. An 82° bit reaches 0.332 inch, the head of a #8 wood screw, at 0.191 inch deep; a 90° bit reaches any width at half that width.
A combination bit has a pilot drill running through the middle of the cone, so the cone starts at the drill’s width, not at a point, and the depth to the same head is shallower. Enter the drill size to allow for it. The depth is measured from the surface down to where the cone meets the drill.
Match the angle to the screw. Inch flat-head wood screws have an 82° head under the ANSI B18.6.1 standard; a bit of a different angle seats the head on its edge or its neck instead of its whole face. Measure a screw that isn’t a standard wood screw before you trust a chart’s head size.
The countersink guide covers bits and sizes; the pilot hole calculator gives the holes under the countersink.
Sources
- Engineers Edge, ANSI flat, pan and oval head wood screws per ANSI B18.6.1, size table; pilot hole drill sizes for wood screws, read 2026-11-07
- Engineering ToolBox, US wood screws: pilot holes, shank clearance holes and head diameters, read 2026-11-07
- Aspen Fasteners, Wood screws spec sheet: standard wood screw dimensions; 82° head angle, read 2026-11-07
- AmesWeb, Flat head wood screw sizes (ANSI B18.6.1-1981): conical head angle of approximately 82°, read 2026-11-07