How to Calculate Drill Point Depth for Blind Holes (Free Calculator)

Every machinist has been there. You need a blind hole exactly 0.500" deep at full diameter, so you program Z-0.500" and hit cycle start. The hole comes out short. What happened?

The drill point happened.

That conical tip at the end of your drill doesn't cut a flat bottom. It cuts a cone. And if you don't account for it, every blind hole you drill will be shallower than you intended.

This guide covers everything you need to know about drill point geometry, the math behind it, and includes a free calculator so you never screw this up again.

Why Drill Point Depth Matters

When you drill a through hole, who cares about the point? It pokes out the other side and you're done.

But blind holes are different. Common situations where point depth is critical:

Get it wrong and you're either scrapping parts or making a second operation to fix it.

The Geometry: What's Actually Happening

A standard jobber drill has a 118° included angle at the tip. That means each cutting edge is angled 59° from the centerline.

When you plunge the drill into material, you get:

The depth of that cone depends on two things:

  1. The drill diameter
  2. The point angle

Bigger diameter = deeper point. Sharper angle = deeper point.

The Formula

Here's the math:

Point Length = Diameter ÷ (2 × tan(Angle ÷ 2))

Or if you prefer it written out:

P = D / (2 × tan(θ/2))

Where:

Example Calculation

Let's say you have a 1/2" drill with a 118° point angle:

P = 0.500 / (2 × tan(59°))
P = 0.500 / (2 × 1.6643)
P = 0.500 / 3.3286
P = 0.150"

So a 1/2" drill has about 0.150" of point length.

If you need 0.500" of full-diameter depth, you need to drill:

Total Depth = 0.500" + 0.150" = 0.650"

Program Z-0.650" and you'll get your 0.500" of usable depth.

Quick Reference: Common Point Lengths

Rather than calculate every time, here's a cheat sheet for 118° drills:

Drill Diameter Point Length
1/8" (0.125")0.038"
3/16" (0.1875")0.056"
1/4" (0.250")0.075"
5/16" (0.3125")0.094"
3/8" (0.375")0.113"
1/2" (0.500")0.150"
5/8" (0.625")0.188"
3/4" (0.750")0.225"
1" (1.000")0.300"

For 135° split-point drills, the point is about 25% shorter. That's one reason machinists like them for CNC work — less wasted depth.

Use Our Free Calculator

Tired of doing trig every time? We built a calculator that handles this instantly:

→ Drill Point Depth Calculator

Just enter:

It spits out the answer plus a visual diagram showing exactly what's happening.

Different Point Angles Explained

Not all drills are 118°. Here's when you'd use different angles:

118° Standard

The default for general-purpose jobber drills. Works fine for most materials. You'll find this on 90% of the drill bits in any shop.

135° Split Point

Self-centering, so it won't walk on the surface. Preferred for:

The shallower angle also means less point length, so you waste less depth on blind holes.

90° Spot Drill

Not really for drilling holes — it's for creating a starting dimple so your actual drill doesn't walk. Also used for chamfering hole edges and countersinks for flat-head screws.

60° Center Drill

Used on lathes to create center holes for tailstock support. The 60° angle matches standard live centers.

Pro Tips for Blind Hole Accuracy

1. Always Spot Drill First

Even with split-point drills, a spot drill gives you a more accurate start location. Use a 90° carbide spot drill and just kiss the surface — you don't need to go deep.

Recommended: Accusize 90° Carbide Spot Drill Set — Good quality, reasonable price.

2. Use Depth Stops for Consistency

If you're drilling multiple holes on a drill press or by hand, depth stops save time and ensure every hole is identical.

Recommended: Anytime Tools Drill Stop Set — Covers most common sizes.

3. Account for Peck Cycles

If you're peck drilling, the point may not fully form on the first peck. Make sure your final depth accounts for the full point length, not some partial amount.

4. Measure What You Actually Have

Cheap drills often have sloppy point angles. If you're doing precision work, measure your actual drill geometry rather than trusting the label.

What About Flat-Bottom Holes?

If you truly need a flat bottom, a regular twist drill won't cut it. Options:

  1. End mill — Plunge with a center-cutting end mill (slow, but works)
  2. Flat-bottom drill — Specialty tool, expensive
  3. Drill then bore — Drill undersized, then use a boring bar to flatten the bottom
  4. Drill then counterbore — Similar approach, use a counterbore tool

For most applications, you don't actually need a flat bottom — you just need to know how deep the flat portion is.

Tapped Hole Depth: A Related Problem

When you're drilling for a tap, the question becomes: "How deep do I drill to get X threads of engagement?"

The calculation:

Drill Depth = Thread Depth + Point Length + (2-3 threads clearance for tap)

Most taps need 2-3 threads of clearance at the bottom to fully form the last good thread. So if you need 1/2" of thread engagement with a 1/4-20 tap:

The Reference: Machinery's Handbook

All these formulas come from the same place every professional machinist keeps on their bench: the Machinery's Handbook.

It's been the bible of metalworking for over 100 years. The current 32nd edition has:

If you're serious about machining, you need a copy.

→ Compare Machinery's Handbook Editions

Available in Toolbox (compact), Large Print, and Digital editions.

Summary

  1. The point isn't "free" — Always add point length to your desired full-diameter depth
  2. Formula: Point Length = D / (2 × tan(angle/2))
  3. For 118° drills: Point length ≈ 30% of diameter
  4. For 135° drills: Point length ≈ 23% of diameter
  5. Use the calculator — It's faster than doing trig every time

→ Open the Drill Point Calculator

Formulas verified against Machinery's Handbook, 32nd Edition. Calculator results are for reference — always verify critical dimensions.