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. The drill radius is the opposite side, the cone depth is the adjacent side, and the half-angle is the angle at the tip:

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

A larger included angle produces a flatter, shorter cone. This is why 135° split-point drills break through cleanly at a shallower depth than standard 118° drills ground to the same diameter.

Worked example

For a 0.5 inch drill at 118°:

half angle = 59°
cone depth = 0.25 / tan(59°) = 0.25 / 1.6643 ≈ 0.150 inch

That 0.150 inch figure matches the machinist rule of thumb: cone depth ≈ 0.3× diameter for 118° drills.

To drill a clean through hole in a 0.75 inch plate, set the depth stop to:

0.75 + 0.150 = 0.900 inch

If you stop at exactly 0.75 inch, the tip is through the plate but the full cutting diameter has not yet cleared the back face — the hole is still conical at the exit. The cone depth closes that gap.

Cone depth for common angles

Practical guidance

Flat-bottomed holes: The cone depth tells you how much of the hole’s depth is conical. If you need a truly flat bottom, drill to full depth first, then follow up with a flat-bottom end mill or spotting drill. The cone depth is the minimum follow-up depth needed to flatten that apex.

Drilling through thin sheet: For thin stock, the cone breaks through before the full diameter engages, causing the drill to grab. Reduce feed as the tip exits and clamp the work firmly. Knowing the cone depth in advance lets you plan the feed reduction point.

Countersinks and special point angles: The same formula works for any conical tool — 90° countersinks, 82° fastener-head profiles, and custom grinds. For sizing a countersink to a specific screw head, use the screw-head diameter rather than the countersink tool diameter in the calculation.

Why the point angle matters for material selection

Different materials need different point geometries:

• 118° (standard) — the default for steel, aluminium, and general-purpose work. The longer, sharper point centres itself in the material before the cutting edges engage, reducing wander.
• 135° (split point) — designed for hard materials and production drilling. The split web reduces the dead-zone at the centre, lowers thrust force, and eliminates the need for a centre punch on most materials. The shorter cone also means it breaks through a through hole about a third sooner than a 118° drill of the same diameter.
• 90° (spotting / countersink) — creates a shallow starter cone that perfectly locates a 118° or 135° drill on the next pass, or finishes a countersunk fastener seat.

Edge case: drilling through stacked materials

When two workpieces are clamped together, the cone must fully clear the first layer before full-diameter engagement begins in the second layer. For a 0.5 inch 118° drill (cone depth ≈ 0.150 inch), if the first sheet is only 0.100 inch thick, the tip exits before the full diameter has cut — the drill may chatter or grab at that transition. Either reduce feed rate aggressively at breakthrough or use a split-point drill (shorter cone) to minimise the uncut zone.