Drill Tip Length Calculator: Formula & Examples

This calculator works out the tip length of a drill bit from its point angle and diameter, giving you the length of the pointed cutting end in millimeters or inches. You enter the drill diameter and the point angle (the standard is 118° for general-purpose bits), and it returns the tip length, plus the total length added by the point if you also enter the flute length. The result is the axial distance from the tip’s apex to the point where the full diameter begins.

Formula

The tip length of a drill bit is derived from basic trigonometry on the cone shape of the point. The formula is:

Tip length = (Diameter / 2) / tan(Point angle / 2)

Where:

• Diameter – the nominal drill bit diameter, in mm or inches (same unit as the result).
• Point angle – the included angle at the tip, in degrees. Standard twist drills use 118°; harder materials often use 135°.
• Tip length – the axial distance from the sharp point to the start of the full cylindrical body, in the same unit as the diameter.

The formula assumes a perfectly conical point with straight cutting lips. Real drills have a chisel edge at the very tip, which shortens the effective point slightly, but for most layout and depth calculations the conical approximation is accurate to within a few tenths of a millimeter.

Worked Example

Using the default values from the calculator above: a 10 mm drill bit with a 118° point angle.

1. Divide the diameter by 2: 10 / 2 = 5 mm (this is the radius).
2. Divide the point angle by 2: 118 / 2 = 59°.
3. Find the tangent of 59°: tan(59°) ≈ 1.6643.
4. Divide the radius by the tangent: 5 / 1.6643 ≈ 3.00 mm.

The tip length is approximately 3.00 mm. This matches the calculator’s output of 3.00 mm when rounded to two decimals.

Why Tip Length Matters

Knowing the tip length is essential for accurate hole depth. When you drill a blind hole (one that does not go all the way through), the flat bottom of the hole is not at the depth you read on the drill bit’s side. The bit’s point extends beyond the cylindrical body, so the actual flat-bottomed depth is the measured depth minus the tip length. For example, if you need a blind hole 15 mm deep with a 10 mm drill, you must advance the bit until the cylindrical part has penetrated 15 mm – the point will have cut about 3 mm deeper, giving a total pointed hole of 18 mm. If you stop at 15 mm total depth, the flat bottom will only be about 12 mm deep.

This matters for:

• Counterbored holes where a screw head must sit flush.
• Holes for dowel pins or alignment pins that need a specific engagement depth.
• Any hole that must not break through the opposite side.
• Estimating the extra travel needed when drilling through material – the point adds length that must pass through before the full diameter exits.

How to Choose the Point Angle

The point angle is not arbitrary; it is chosen based on the material being drilled. The most common angles are:

• 118° – the general-purpose angle for steel, aluminum, wood, and most plastics. It offers a good balance between cutting efficiency and tip strength.
• 135° – used for harder materials like stainless steel and high-strength alloys. The flatter angle makes the tip stronger and reduces walking, but it increases the tip length for the same diameter.
• 90° – common for soft materials like acrylic or for spot drilling where a shallow center is needed.
• 60° – used for center drills and some specialized applications where a very deep, sharp point is required.

If you do not know the angle of your drill bit, measure it with a protractor or point gauge. Many drill bits are stamped with the angle, but when in doubt, 118° is the safest assumption for general-purpose bits.

Common Mistakes and Misconceptions

One frequent error is confusing the tip length with the total length added to the hole. The tip length is the axial distance from the point to the full diameter, but the actual hole depth when drilling through is the material thickness plus the tip length – because the point must exit the far side before the full diameter clears. For blind holes, the usable flat depth is the drill depth minus the tip length.

Another mistake is using the full diameter in the formula without halving it. The tangent relationship applies to the radius, not the diameter. Using the full diameter would give double the correct tip length.

A third issue is unit mixing. If you enter the diameter in inches and the angle in degrees, the result is in inches. Do not mix millimeters with inches without converting. The calculator assumes consistent units for the diameter and output; the angle is always in degrees.

Finally, the formula assumes a sharp, symmetrical point. A worn or reground bit may have a different effective angle or a chisel edge that makes the actual tip length slightly shorter. For precision work, measure the bit directly rather than relying solely on the nominal angle.

Limitations of the Formula

The conical model is an idealization. Real drill bits have a chisel edge at the center – a flat or slightly curved area where the two cutting lips meet. This chisel edge can be 0.1 to 0.5 mm wide depending on the bit size and grind, and it effectively reduces the point length by a small amount. For most practical purposes (hole depth calculations, countersink depth, spot drilling), the error is negligible, but for high-precision work you should measure the actual tip with a depth gauge or optical comparator.

The formula also assumes the drill is held perfectly on-axis. In a hand drill, slight wobble can change the effective point geometry. For critical holes, use a drill press or guide.

Finally, the formula gives the tip length for a single point. If you are using a step drill or a bit with a split point, the geometry is different and the simple conical formula does not apply.

Practical Tips for Using the Result

When you have the tip length, you can apply it in several ways:

• Blind hole depth: To drill a flat-bottomed hole of depth D, advance the bit until the cylindrical body has penetrated D. The total drill travel will be D + tip length.
• Through hole: To ensure the full diameter exits the far side, the drill must travel at least material thickness + tip length.
• Countersink depth: A countersink is essentially a drill with a very large point angle. The same formula applies – the depth of the conical recess is (diameter/2) / tan(angle/2).
• Spot drilling: A spot drill with a 90° point creates a shallow cone. The depth needed to create a certain diameter spot is given by the same formula.

Always add a small allowance for the chisel edge and for measurement error. For most workshop tasks, adding 0.1–0.2 mm to the calculated tip length is sufficient.

Frequently Asked Questions

What is the standard drill point angle?

The most common point angle for general-purpose twist drills is 118°. This angle works well for steel, aluminum, wood, and most plastics. For harder materials like stainless steel, a 135° point is often used because it provides a stronger tip that is less likely to chip or wander.

How do I measure the point angle of a drill bit?

You can measure the point angle with a protractor, a drill point gauge, or by placing the bit in a V-block and using a bevel protractor. If you do not have a gauge, you can compare the bit to a known standard – most general-purpose bits are 118°, and most split-point bits are 135°.

Does the tip length affect the hole diameter?

No, the tip length does not affect the final hole diameter. The hole diameter is determined by the cylindrical body of the drill. The tip length only affects the depth of the pointed part of the hole, which matters for blind holes and through-hole clearance.

Can I use this formula for a center drill?

Center drills have a 60° point angle, so you can use the same formula with the angle set to 60°. However, center drills have a pilot section that changes the effective geometry. For a standard center drill, the 60° point creates a countersunk hole, and the formula gives the depth of that conical portion.

Why does a 135° drill bit have a longer tip than a 118° bit of the same diameter?

Because the tangent of half the angle is smaller for a larger angle. For a 10 mm bit, the tip length at 118° is about 3.00 mm, while at 135° it is about 3.70 mm. The flatter angle means the cone is longer for the same diameter, which is why 135° bits are stronger but require more travel to reach the same depth.

How accurate is this calculation for real drill bits?

For a sharp, properly ground bit, the calculation is accurate to within about 0.2 mm. The main source of error is the chisel edge, which is not accounted for in the conical model. For most workshop and engineering applications, this level of accuracy is sufficient. For precision work, measure the actual tip length with a depth gauge.

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in d | Drill diameter | mm | 10 | 0
in a | Point angle | degrees | 118 | 0
out tip | Tip length | mm | (d / 2) / tand(a / 2) | 2