Drill Point & Countersink Depth Calculator

A twist drill does not cut a flat-bottomed hole — it leaves a cone, and the height of that cone is pure trigonometry from the point angle. The half-angle is half the included point angle, and the point length p = (D ÷ 2) ÷ tan(point angle ÷ 2). For the standard 118° drill that works out to 0.300 × D; for a 135° point it is 0.207 × D; for a 90° point it is exactly 0.5 × D. So a 10 mm standard drill adds about 3 mm of cone below the full-diameter part of the hole — which is why, if you need 25 mm of full-width hole, you must program the tip about 28 mm deep, not 25.

That distinction — full-diameter depth versus tip depth — is one of the most common causes of a hole that is right on the drawing but wrong on the part. The drawing usually calls the depth of the full-diameter portion (the usable hole); the machine cares where the tip goes. This tool works both ways: give it the full-diameter depth you need and it adds the point length to get the Z to program, or give it the Z you have and it tells you how much of that is actually full-diameter hole. On a blind hole with a flat-bottom requirement, it also reminds you that you will need a second, flat-bottomed tool — a drill alone cannot leave a flat.

A countersink is the same cone geometry read the other way. To open an existing hole of diameter d out to a top diameter D at an included angle θ, the tool must plunge Z = (D − d) ÷ 2 ÷ tan(θ ÷ 2) below the surface. That is the number you set on the machine, and it is surprisingly sensitive: because tan(θ/2) sits in the denominator, a small change in the target diameter is a smaller change in Z on an 82° or 90° tool, which is exactly why countersinks are easy to run too deep. Running the calculation the other way — diameter produced by a given Z — is how you predict where a witness cut will end up before you commit to it.

The common angles are worth knowing. 90° is the metric/DIN standard for countersunk screws and is the usual deburring angle; 82° is the imperial/UNC screw standard, and mixing the two up leaves a screw head either proud or bottomed on its edge. 100° appears on some aerospace fasteners, 120° is common for deburring and for rivets, and 60° is used for centre work and lathe centres. Setting an 82° tool to a 90° screw, or vice versa, is a classic way to get a countersink that looks right but seats the fastener wrongly — the angle matters as much as the diameter.

For seating a countersunk screw flush, the practical target is to open the countersink to the screw head diameter (plus a whisker so the head sits just below flush), at the screw head angle. Because the fastener standards fix those angles at 82° or 90°, matching the tool to the fastener is not optional. If a job depends on countersinks being consistent across a batch — a visible panel, a sealing face — that is a fixturing and inspection question as much as a geometry one, and it is the sort of thing worth flagging on the drawing so we set it up to gauge rather than to eye.