Countersink guide
A countersink is a cone-shaped recess drilled at the top of a screw hole so a flat-head screw sits flush with the surface, or just below it. Two numbers matter: the angle of the cone, which has to match the screw head, and how wide (or how deep) to cut it.
The angle: 82° for inch wood screws
Flat and oval head wood screws made to the ANSI B18.6.1 standard have an 82° head, so they take an 82° countersink. A bit of the wrong angle leaves the head bearing on a thin ring instead of its whole face. Check the angle marked on the bit against the screw before you drill a whole project.
Only two sources publish this: Aspen Fasteners and AmesWeb
How wide and how deep, by screw
Cut the countersink as wide as the head: the largest a flat head may be under the standard. With a plain 82° bit that comes at this depth, measured from the surface to the cone’s point:
| Screw | Width (head) | Depth at 82° |
|---|---|---|
| #4 | 0.225 in (5.72 mm) | 0.129 in (3.29 mm) |
| #6 | 0.279 in (7.09 mm) | 0.16 in (4.08 mm) |
| #8 | 0.332 in (8.43 mm) | 0.191 in (4.85 mm) |
| #10 | 0.385 in (9.78 mm) | 0.221 in (5.62 mm) |
| #12 | 0.438 in (11.13 mm) | 0.252 in (6.4 mm) |
| #14 | 0.507 in (12.88 mm) | 0.292 in (7.41 mm) |
Combination bits and depth stops
A combination countersink bit has a pilot drill running through the cone, so it drills the pilot hole and the countersink in one pass. Because the cone starts at the drill’s width instead of a point, it reaches the head’s width at a shallower depth than a plain bit; the countersink depth calculator works that out from the drill size. A stop collar on the bit sets the depth so every hole matches.
To hide a screw under a wooden plug or filler, sink the head below the surface by the plug’s depth: the calculator adds that too. The pilot and clearance holes underneath are on the pilot hole chart.
Sources
- 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
- 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