Drill Point Calculator: Find the Right Angle

A drill point calculator works out the tip length and the depth of the angled point on a twist drill bit, based on the bit diameter and the point angle you enter. You get the point length (the cone-shaped tip) and the total drill depth needed to cut through a given material thickness. This page provides a simple calculator and explains the geometry behind it, so you can set your drill press stop or CNC toolpath correctly.

Formula

The point of a twist drill is a cone. The point length (the axial distance from the tip to the start of the full diameter) is found using the right triangle formed by half the drill diameter and the half-angle of the point:

Point length = (d / 2) / tan(α / 2)

Where:

• d = drill diameter (same unit as the result, e.g., mm or inches)
• α = point angle (degrees, typically 118° for general-purpose drills, 135° for harder materials)
• tan = tangent function (make sure your calculator is in degree mode)

To get the total drill depth needed to go through a material of thickness t, add the point length to the material thickness:

Total drill depth = point length + material thickness

This total is the distance from the tip of the drill to the point where the full diameter has fully exited the bottom of the workpiece.

Worked Example

Using the default values from the calculator above:

• Drill diameter (d) = 10 mm
• Point angle (α) = 118°
• Material thickness (t) = 5 mm

First, compute the half-angle: 118° / 2 = 59°.

Convert to radians for the tangent function (if needed): 59° × π / 180 ≈ 1.0297 radians. tan(59°) ≈ 1.6643.

Now compute the point length:

Point length = (10 / 2) / 1.6643 = 5 / 1.6643 ≈ 3.00 mm.

Add the material thickness: 3.00 + 5 = 8.00 mm.

The calculator shows a point length of 3.00 mm and a total drill depth of 8.00 mm. That means if you are drilling through a 5 mm plate with a 10 mm drill, the tip must travel 8 mm past the top surface to ensure the full diameter has cleared the bottom.

Why the Point Angle Matters

The point angle is the angle between the two cutting lips, measured at the tip. Common angles are:

• 118° – the standard for general-purpose drilling in steel, aluminum, and wood.
• 135° – often used for harder materials like stainless steel; the flatter point reduces walking and requires less thrust.
• 90° – used for soft materials like plastics or when a flat-bottomed hole is needed (e.g., for countersinking).
• 60° – sometimes used for very soft materials or for starting holes in sheet metal.

A larger point angle (flatter tip) gives a shorter point length for the same diameter. A smaller angle (sharper tip) gives a longer point. This directly affects how deep you need to drill to achieve a through-hole.

How to Use the Calculator

Enter the drill diameter, point angle, and material thickness. The calculator returns the point length and the total depth required. Here are practical steps:

1. Measure the drill diameter with a caliper or check the bit’s label.
2. Identify the point angle. If you don’t know it, 118° is a safe default for most twist drills. Some bits have the angle stamped on the shank.
3. Measure the thickness of the material you are drilling through (use calipers for accuracy).
4. Set your drill press depth stop to the total depth value, or use it to program a CNC operation.

For a blind hole, you only need the point length to be fully inside the hole, so you can subtract the desired remaining material thickness. For example, if you want a 10 mm deep blind hole in a 20 mm block with a 10 mm drill at 118°, you need to advance the drill 10 mm + 3.00 mm = 13.00 mm from the surface.

Common Mistakes and Misconceptions

One frequent mistake is forgetting that the point length is not the same as the drill’s full tip-to-shank dimension. The point length is only the conical tip. Another error is using the full point angle in the tangent function instead of half the angle. Using the full angle (tan 118°) would give a much smaller (and incorrect) point length.

Also, be careful with units. If you mix inches and millimeters, the result will be off. The calculator uses consistent units for diameter and thickness, so if you enter inches, the point length will be in inches.

Another misconception: the point angle does not affect the hole diameter. The hole diameter is determined by the drill’s cutting edges. The point angle only affects the shape of the bottom of the hole and the required depth.

Limitations of the Formula

The formula assumes a perfectly conical point with straight cutting lips. In reality, drill points are often ground with a slight chisel edge and relief angles, which can make the actual point length slightly different. For most workshop purposes, the formula is accurate enough.

Also, the formula does not account for the drill’s web thickness at the tip, which can cause the point to be slightly longer for small drills. For very small diameters (under 1 mm), the chisel edge becomes relatively larger, and the actual point may be longer than calculated.

Finally, the material thickness you enter should be the actual thickness at the hole location. If the surface is curved or angled, the effective thickness may be different.

Alternatives to a Drill Point Calculator

If you don’t have a calculator, you can use a simple reference chart that lists point lengths for common drill sizes and angles. Many machinists’ handbooks include such tables. You can also measure the point length directly with a depth gauge on a drill that is already ground.

For CNC work, most CAM software automatically accounts for the drill point when generating toolpaths. You still need to enter the correct point angle in the tool definition.

Another alternative is to use a drill point gauge, a small tool that measures the point angle of an existing drill. This is useful when you are unsure of the angle.

Frequently Asked Questions

What is the standard point angle for a twist drill?

The most common point angle is 118°, which works well for steel, aluminum, and wood. For harder materials like stainless steel, a 135° point is often preferred because it reduces thrust and prevents work hardening.

How do I know the point angle of my drill bit?

Check the shank for a stamped number (e.g., 118 or 135). If not marked, you can measure it with a protractor or a drill point gauge. When in doubt, assume 118° for general-purpose bits.

Does a longer point length mean a stronger drill?

Not necessarily. A longer point (smaller angle) is sharper and cuts more aggressively, but it is also more fragile. A shorter point (larger angle) is stronger and less likely to chip, but requires more force to start.

Can I use this calculator for center drills or spot drills?

Center drills have a different geometry (a pilot point and a countersink section), so this simple cone formula does not apply directly. Spot drills typically have a 90° or 120° point, but the calculation for spot depth is similar if you know the spot drill’s diameter and angle.

Why does my drilled hole have a conical bottom?

That is the natural result of the drill point. The bottom of a blind hole will always be conical unless you use a flat-bottom drill or a separate flat-bottoming operation. This is why you must add the point length to the depth for through-holes.

What if I need a flat-bottomed hole?

Use a flat-bottom drill (also called a D-bit or a spade drill with a flat tip) or drill a pilot hole and then use an end mill to flatten the bottom. The formula above does not apply to flat-bottom tools.

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in d | Drill diameter | mm | 10 | 0
in a | Point angle | deg | 118 | 30
in t | Material thickness | mm | 5 | 0
out pl | Point length | mm | (d / 2) / tan((a / 2) * pi / 180) | 2
out td | Total drill depth | mm | pl + t | 2