Drill Tip Length Calculator

A twist drill does not reach full diameter the moment its point touches the work. The conical point has to travel its own length first, so a hole programmed to an exact shoulder depth ends up shallow at full diameter unless you add the tip length. This calculator works out that point length from drill diameter and point angle, in millimetres and inches, and adds the cutting edge length machinists need for load and regrind checks.

Quick Answer

How to Use the Drill Tip Length Calculator

Enter the drill diameter in millimetres, pick the point angle, and select Calculate. The tip length, its inch equivalent, the cutting edge length and material guidance appear together.

1. Enter the diameter. Use the nominal drill size. If you work in inches, multiply by 25.4 first: a 3/8 in drill is 9.525 mm.
2. Pick the point angle. Use the preset that matches your drill, or choose Custom angle for reground and special points and type the measured angle.
3. Optionally pick the material. It does not change the geometry. It selects the drilling guidance printed with the result.
4. Add the tip to your depth. Programmed depth to the shoulder plus tip length equals the total travel needed for full-diameter depth. The depth section below works through blind and through holes.

The same cone geometry shows up anywhere a rotating cutter meets a face at an angle. Crews sizing earth augers run into the parallel problem of point and flight length, which the auger length calculator covers for boring work.

The Drill Point Formula, Derived From the Cone

Tip length equals half the diameter divided by the tangent of half the point angle. Picture the point as a cone: the radius is the run, the tip length is the rise, and half the included angle sits between the cone wall and the drill axis.

Tip length L = (D / 2) / tan(point angle / 2)

Cutting edge length = (D / 2) / sin(point angle / 2)

For the standard 118 degree point, half the angle is 59 degrees and tan(59 degrees) is about 1.6643, which collapses the formula to the shop rule L is about 0.300 times D. That rule is printed in machining references including Machinery’s Handbook, and production drilling guidance from tooling makers such as Sandvik Coromant’s drilling knowledge pages uses the same cone model when setting hole depth. If you want the angle geometry itself, the central angle calculator works through the same tangent and sine relationships for circles and sectors.

One honest limitation: the formula treats the point as a perfect cone meeting at a sharp centre. Real drills have a chisel edge, a flat across the middle left by the web, so the true point is a fraction shorter than the cone figure. That is why critical blind holes are still finished with a test cut and a depth micrometer reading.

118 Degree vs 135 Degree Points, Side by Side

The point angle is the biggest practical choice in this calculation, because it changes both the tip length and how the drill behaves in the cut.

Angle recommendations follow common tooling guidance; always confirm against the drill maker’s sheet for coated and geometry-specific drills, for example the Dormer Pramet drilling technical resources or the Kennametal drilling technical resources.

Worked Example: A 10 mm Drill in Mild Steel

A fixture plate needs a 10 mm hole, 20 mm deep at full diameter, drilled with a standard 118 degree jobber drill.

• Diameter D = 10 mm, point angle = 118 degrees, half angle = 59 degrees
• Tip length = (10 / 2) / tan(59 degrees) = 5 / 1.6643 = 3.004 mm
• Tip length in inches = 3.004 / 25.4 = 0.1183 in
• Cutting edge = (10 / 2) / sin(59 degrees) = 5 / 0.8572 = 5.833 mm
• Total drill travel for 20 mm of full-diameter depth = 20 + 3.004 = 23.004 mm
• Result guidance for mild steel: use cutting oil and moderate speed, standard angle, general purpose

Swap the same drill for a 135 degree split point and the tip shortens to 2.071 mm, so total travel drops to 22.071 mm. Nearly a millimetre of difference on a blind hole is the difference between a floor that cleans up and one that does not. When the hole is a clearance hole for a fastener rather than a finished bore, pair the depth with the shank allowance from the bolt length calculator so grip length and hole depth are planned together.

Using Tip Length for Blind Holes, Through Holes and CNC Depth

For a blind hole, the drawing depth almost always means the cylindrical, full-diameter portion. Set the drill to travel drawing depth plus tip length, then confirm the floor with a test piece if the bottom must be flat, because a pointed drill always leaves a conical floor. If a flat floor is required, drill with the twist drill first, leaving stock, then finish with an end mill or a flat bottom drill.

For a through hole, the tip must fully exit the far side before the hole is at diameter all the way through. Program travel as material thickness plus tip length plus a small breakthrough allowance, typically 0.5 to 1 mm, so the point clears without the flutes striking the fixture below. On a CNC, this tip compensation is exactly what the Z depth in the drilling cycle needs: canned cycles position the drill tip, not the shoulder, so the number from this calculator belongs inside every G81 or G83 depth you write. The same discipline applies to stock preparation before drilling, where the cut length calculator helps plan blanks so thickness, and therefore breakthrough travel, stays consistent across a batch.

What Other Drill Tip Calculators Get Wrong

The most visible error in competing tools is a worked example that does not survive its own formula. One widely ranked calculator states that a 10 mm drill at 118 degrees has a tip length of 5.95 mm. Run its own geometry and the answer is 3.00 mm. The 5.95 mm figure is close to the cutting edge length, not the tip length, and anyone who programmed Z depth from it would plunge almost 3 mm too deep on every hole.

Two quieter problems matter as well. Some tools fold a user-entered safety margin into the reported tip length, which mixes a machine setup allowance into a geometric fact and makes the number impossible to reuse in a different setup. Others accept diameter in millimetres only while their readers measure drills in fractional inches, with no conversion shown. This page keeps the geometry clean, prints the inch result beside the metric one, and keeps setup allowances where they belong, in your depth plan rather than in the point length itself. Tooling makers publish depth and point guidance the same way, separating geometry from setup, as in the Seco Tools drilling and holemaking guidance.

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