Drill Tip Length Calculator
Enter diameter and point angle, then choose your output unit.
Formula
L = (D / 2) / tan(A / 2)
Waiting for input...
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Complete Guide: How to Calculate Drill Tip Length Accurately
When machining blind holes, one of the most important geometry checks is the drill tip length. If this value is ignored, parts can fail dimensional inspection because the flat-bottom equivalent depth was assumed incorrectly. In CNC workflows, drill tip length directly affects how deep the tool must travel to ensure the required cylindrical depth exists in the finished feature. This page gives you a practical calculator, the exact formula, real examples, and process notes so you can plan depth confidently.
Table of Contents
What Is Drill Tip Length?
Drill tip length is the axial length of the conical point at the end of a twist drill. Geometrically, it is the distance from the apex of the drill point to the location where the drill reaches full diameter. In a through hole, this dimension may be less critical. In a blind hole, it is essential because part of your drilled depth is consumed by the cone, not by full-diameter cylindrical cutting.
In production environments, this is often the hidden reason behind depth nonconformance. A print may call for a specific depth with no explicit note on drill point geometry, while setup documents assume a default 118° or 135° point. If the tool angle differs from assumptions, the effective full-diameter depth changes.
Drill Tip Length Formula
The standard formula for included point angle is:
L = (D / 2) / tan(A / 2)
Where:
- L = drill tip length
- D = drill diameter
- A = included point angle in degrees
The formula comes from right-triangle trigonometry on the half-angle profile of the drill point. The radius is D/2, and the opposite/adjacent relation gives the tangent term.
Worked Examples
Example 1: 10 mm drill, 118° point angle.
L = (10/2) / tan(118/2) = 5 / tan(59°) ≈ 3.00 mm
Example 2: 0.500 in drill, 135° point angle.
L = (0.500/2) / tan(67.5°) = 0.250 / 2.4142 ≈ 0.1036 in
Example 3: 6 mm drill, 90° point angle.
L = (6/2) / tan(45°) = 3 / 1 = 3 mm
These examples show why steeper point angles (such as 135°) typically produce shorter tip lengths than sharper profiles at smaller included angles.
How Point Angle Affects Tip Length
As included point angle increases, half-angle increases, tan(A/2) increases, and calculated tip length decreases for the same diameter. Practically:
- Smaller included angle → longer tip length.
- Larger included angle → shorter tip length.
| Included Point Angle | Half Angle | Length Factor (L/D) |
|---|---|---|
| 90° | 45° | 0.5000 |
| 100° | 50° | 0.4195 |
| 118° | 59° | 0.3004 |
| 120° | 60° | 0.2887 |
| 130° | 65° | 0.2332 |
| 135° | 67.5° | 0.2071 |
| 140° | 70° | 0.1820 |
Factor relationship: L = Factor × D
Blind-Hole Depth Planning
For blind holes, you usually care about full-diameter usable depth. Since the drill tip creates a cone at the bottom, commanded drill depth must include this cone length. A common planning equation is:
Commanded Depth = Required Cylindrical Depth + Tip Length + Process Allowance
Allowance may include breakthrough margin against tool wear, machine repeatability, spindle growth, workholding variation, and material behavior. If a mating fastener or pin has strict engagement needs, always verify with process capability data rather than a single nominal add-on.
CNC Programming Considerations
In CNC drilling cycles, the Z endpoint may be interpreted differently based on programming convention and control logic. To avoid ambiguity:
- Document whether Z depth is to drill tip apex or to full-diameter depth target.
- Tie setup sheets to actual tool point angle from tool library, not assumption.
- Standardize point angle by process family when possible.
- Re-validate after tool vendor or grind changes.
When peck drilling deep holes, chip packing and deflection can alter achieved geometry; measuring first-off parts for actual depth is best practice. If bottom condition matters (for example, seating or O-ring land relations), follow drilling with finishing operations such as flat-bottom interpolation, counterboring, or end milling.
Inspection and Tolerance Strategy
Depth callouts should clearly state whether they reference full diameter, point-to-surface, or a derived functional depth. Metrology teams should use inspection methods matched to the requirement. A common pitfall is mixing apex depth measurements with functional cylindrical depth acceptance criteria.
For critical features, include process notes in the control plan:
- Nominal drill angle and tolerance
- Tool life stage when first-off and last-off checks occur
- Depth gauge method and datum strategy
- Compensation policy for wear
Common Mistakes to Avoid
- Using drill diameter without checking actual point angle.
- Forgetting unit conversion between inch and metric workflows.
- Assuming all 118° tools are exactly 118° after regrind.
- Programming blind-hole depth equal to print depth without adding tip length.
- Ignoring tolerance stack from machine, holder runout, and material spring effects.
The calculator above helps prevent these issues by giving quick, geometry-based tip length output from first principles.
Frequently Asked Questions
Is drill tip length the same as web thickness?
No. Web thickness is a core feature of drill cross-section. Tip length is an axial geometric result of diameter and point angle.
Can I use this for center drills or spot drills?
The same cone geometry concept applies if the angle and effective diameter are known, but tool-specific geometry may require additional checks.
Does material type change the formula?
No. The formula is purely geometric. Material affects process parameters, tool selection, and allowance decisions, not cone geometry math.
What if angle is given as half-angle?
This calculator expects included angle. If you have half-angle directly, use L = (D/2)/tan(half-angle).
Summary
To calculate drill tip length accurately, use the diameter and included point angle in the formula L = (D/2)/tan(A/2). This value is essential for blind-hole depth planning, CNC program consistency, and dimensional reliability. Standardizing assumptions and verifying actual tool geometry will reduce scrap risk and improve first-pass yield.