Drill depth calculator
A drill’s point adds L = (d/2) / tan(θ/2) to the depth you program. A 10 mm drill ground to 118° has a 3.004 mm point, so 20 mm of full diameter takes a programmed depth of 23.004 mm to the tip. Enter the drill and the depth on the drawing to get the depth to program, or the depth you programmed to see how much full diameter it leaves.
The same calculator gives a countersink’s depth for its diameter (or the reverse) with a sharp, flat or piloted tool, the chamfer on a hole already drilled, and a counterbore’s depth to seat a head.
- Total depth Z (program)
- Point length L
- Full-diameter depth F
- Point length per Ø (L/d)
How it’s calculated
Point length L = (d/2) ÷ tan(θ/2), so 0.3004·d at 118° and d/2 for a 90° spot drill; a flat bottom has none. Total depth = full-diameter depth + L. A total shorter than L leaves the hole 2·Z·tan(θ/2) across at the face.
Why does the point angle change the depth?
A drill cuts a cone at the bottom of the hole. The drawing’s depth is usually to where the hole stops being full diameter, but the machine is programmed to the tip. The cone’s height is half the diameter over the tangent of half the point angle: a 118° point is about 0.3 × the diameter, a 140° point about 0.18 ×, a 90° spot drill exactly half the diameter, and a flat-bottom drill nothing.
A countersink works the same way upside down. Its diameter at the face is twice the depth times the tangent of half its angle, less what a flat end or a pilot takes off the tip; on a hole already drilled, the chamfer only starts where the cone meets the hole, so the hole’s diameter sets how deep the chamfer runs.