Cone depth is the axial length of the conical tip of the drill — how much farther the very point reaches past where the full-diameter cutting edges begin. It matters whenever a hole must reach full diameter at a given depth.

Cone depth equals the drill radius divided by the tangent of half the point angle. For a 118 degree drill the half-angle is 59 degrees, giving a cone depth of about 0.3 times the diameter.

Why does a 135 degree drill have a shorter point?

A larger included angle makes a flatter, shorter cone. At 135 degrees the half-angle is 67.5 degrees, so the tangent is larger and the cone depth shrinks to roughly 0.2 times the diameter compared with 0.3 for a 118 degree drill.

How do I set a depth stop for a through hole?

Add the cone depth to the material thickness. The full diameter does not break through until the cylindrical cutting edges clear the back face, so the drill must travel the thickness plus the length of its own point.

Does this work for countersinks?

The same trigonometry applies to any conical point, so a 90 or 60 degree countersink cone depth can be read off too. For sizing a countersink to a fastener head, though, use the head diameter rather than the tool diameter.

Drill Point Angle Calculator

The tip of a twist drill is a cone, and its length quietly affects every depth you set and every flat-bottomed or through hole you cut. This calculator gives the cone depth for any drill diameter and point angle, plus the depth-stop setting that guarantees a through hole reaches full diameter.

How it works

The cone is a simple right-triangle problem built on the drill radius and the point half-angle:

half angle = point angle / 2
cone depth = (diameter / 2) / tan(half angle)
through-hole stop = material thickness + cone depth

A larger point angle produces a flatter, shorter cone, which is why split-point 135 degree drills reach full diameter sooner than standard 118 degree drills.

Example and tips

A 0.5 inch drill ground at 118 degrees has a cone depth of about 0.150 inch, the familiar rule-of-thumb of roughly 0.3 times the diameter. To drill cleanly through a 0.75 inch plate, set the depth stop to 0.75 plus 0.150, or 0.900 inch, so the full diameter breaks the back face. When a flat-bottomed hole is required the cone depth tells you how much remains conical and how far to follow up with an end mill or flat-bottom drill.