Flat-head (countersunk) screws break the usual rule — because the head sinks into the material instead of sitting on top.
A flat-head (countersunk) screw is built to sit flush — the cone-shaped head drops into a matching recess so the top ends up level with the surface. Because the head is buried in the work, it counts toward the length. This is the opposite of a hex bolt, and it’s the most common length mistake we see.
Measure the overall length — from the top of the head to the tip. The head sinks flush into the material, so it’s part of the length.
Head sits on top of the surface → measure under the head to the tip. Head not counted.
Head sits flush in the surface → measure overall, top of head to tip. Head counted.
The test that always works: measure whatever ends up buried in the material. For a flat head that’s the head and the shank — so it’s the whole screw.
Other countersunk styles — like an oval (raised countersunk) head — sink into the same recess, so they follow the same overall-length idea. If a head sits flush or partly flush, measure overall and tell us the head style; we’ll confirm it. Diameter and thread are measured the same as always — see the measuring basics.
If the head sits on top, or there’s no head at all, the length is taken differently. Jump to the right guide.
