What Size Drill for 10-32 Tap? (Simple Guide with Personal Tips)

When I first started working with metal and threaded holes, the term “10-32 tap” completely stumped me. I remember standing at my workbench, drill in one hand, tap in the other, and thinking, “What Size Drill for 10-32 Tap?” It felt like a puzzle with no instructions.

Let me walk you through exactly what I’ve learned—step by step. Whether you’re new to threading or just need a refresher, this guide will make it easy to find the right drill size for a 10-32 tap.

What Is a 10-32 Tap, Really?

Think of a 10-32 tap as a threading tool for small screws—like those you’d find on electrical boxes, electronics, or automotive trim.

• “10” is the screw size.
• “32” means 32 threads per inch (TPI).
• It’s a Unified National Fine (UNF) thread.

These are common in both DIY and professional metalwork. But the real trick is getting the drill hole size just right before you thread it with the tap.

What Size Drill for a 10-32 Tap?

• Drill Size: #21 drill bit
• Decimal Equivalent: 0.159 inches (4.04 mm)

I always keep a #21 drill bit labeled just for 10-32 taps in my toolbox. It saves me from guessing every time.

Why Drill Size Matters (And How I Learned the Hard Way)

One time, I used a slightly smaller bit—just a hair off. But that hair? It made the tap seize, and I ended up breaking it in the hole. Not only was it frustrating, but I had to redo the entire piece.

Here’s why size matters:

• Too small = Tap can break
• Too large = Threads won’t hold

Finding that sweet spot is essential.

Drill Size Chart for Taps (Including 10-32)

Here’s a table I keep printed on my garage wall:

Tap Size	Thread Pitch	Drill Size	Decimal (Inch)	Notes
10-32	32 TPI	#21	0.159″	Best for full threads
10-24	24 TPI	#25	0.1495″	Coarser thread
8-32	32 TPI	#29	0.136″	Common in switchplates
6-32	32 TPI	#36	0.1065″	Electrical use

What If You Don’t Have a #21 Drill Bit?

Here’s a quick hack I used when my #21 went missing:

• Use a 5/32″ drill bit (0.15625″) as a substitute.
• It’s slightly smaller but works in a pinch.

Tip: Go slow and use cutting oil if you’re substituting. It helps prevent tap binding.

How to Drill for a 10-32 Tap (Step-by-Step)

Here’s how I do it every time now:

1. Gather Your Tools

• Tap handle
• 10-32 tap
• #21 drill bit
• Drill (preferably with a slow speed setting)
• Center punch
• Cutting fluid or oil

2. Mark Your Spot

Use a center punch to make a small dent where you want the hole. This keeps your drill bit from wandering.

3. Drill the Hole

• Use the #21 bit
• Keep the drill at a 90-degree angle
• Use cutting oil if drilling into metal

4. Tap the Threads

• Start slowly
• Turn clockwise 1/2 turn, then reverse slightly
• Repeat until the tap bottoms out

I learned to never force it. Tapping is all about rhythm and patience.

Common Mistakes to Avoid

1. Skipping the Center Punch

Your bit will wander, and the hole won’t line up. I’ve ruined brackets this way.

2. Drilling Too Fast

High speeds heat the bit and dull it. Go slow and steady.

3. Forgetting Lubrication

Metal-on-metal needs oil. It reduces friction and heat.

4. Using the Wrong Bit

If the drill bit isn’t the right size, the tap won’t cut properly, and your threads may fail.

Personal Tip: Keep a Tap and Drill Index

Over the years, I built a small laminated chart with my most-used taps and bits. I keep it right on my toolbox lid. No more second-guessing.

Bonus tip: Color-code your drill bits if you use them for specific tap sizes. I wrapped my #21 in red tape.

Safety First (It’s Not Just a Warning Label)

Here’s what I’ve learned:

• Wear safety glasses. Broken taps or flying shavings hurt.
• Clamp your workpiece. You don’t want it spinning into your hand.
• Use proper tools. Don’t substitute a screwdriver handle for a tap wrench—ask me how I know.

What Size Drill Bit Is Needed for a 10-32 Tap?

Let me tell you, I’ve made this mistake before—grabbing the wrong drill bit size for a tap and ending up with threads that just won’t hold. If you’re working with a 10-32 tap, you need the right bit or it’s game over.

So, what size drill for 10 32 tap?

👉 The correct drill bit size is a #21 drill bit.

I keep this bit labeled in my toolbox because it’s so easy to forget. That #21 drill bit gives just the right hole diameter to cut clean, tight threads for a 10-32 screw. Too small, and you risk snapping your tap. Too big, and your threads won’t grip.

Here’s what you need to know:

• Thread size: 10-32 (fine thread)
• Drill bit needed: #21
• Hole size: About 0.159 inches
• Use case: Great for machine screws and fine-thread bolts

I usually double-check with a drill chart before I start. Honestly, it saves time and stops me from wrecking parts. I also learned to use cutting oil when tapping—it keeps the tap cooler and makes threading smoother.

💡 Tip: If you don’t have a #21 bit, don’t guess with a close size. Go grab the right one. It really makes a difference.

What Size Hole Do I Need to Drill for a 10mm Tap?

Now, switching gears a bit—let’s talk about metric taps. When I first worked with a 10mm tap, I was confused. Metric sizing felt like a new language. But once I figured out the rule, it clicked.

For a 10mm tap, here’s the hole you’ll need:

👉 Drill a hole that’s 8.5mm in diameter.

That’s right. For a standard M10 x 1.5 tap (which is the most common), the correct drill size is 8.5mm. This gives your tap enough material to cut solid threads without too much friction.

Quick breakdown:

Tap Size	Thread Pitch	Drill Size Needed
M10 x 1.5	1.5mm	8.5mm
M10 x 1.25	1.25mm	8.8mm

I learned this from trial and error. I once used a 9mm bit for an M10 tap. The result? The threads were too loose, and the bolt kept backing out. That’s how I found out that accuracy matters.

🔧 Pro Tip: Always check the thread pitch of your tap. Not all 10mm taps are the same!

So now, whether I’m working with inch taps like the 10-32, or metric ones like M10, I always stop and check the right drill size first. Saves me a headache—and broken taps.

FAQs About 10-32 Tap Drill Size

What size is a #21 drill bit in mm?

• 4.04 mm (0.159″)

Can I use 5/32 instead of #21?

• Yes, but it’s a tiny bit smaller. Use with caution and oil.

What’s the difference between 10-32 and 10-24?

• Both are size 10, but 10-32 has finer threads (32 vs 24 TPI).

Can I use a hand drill for tapping?

• It’s not ideal. Always use a tap handle for control.

Do I need to use oil when tapping?

• Yes! Especially with metal, oil reduces friction and wear.

Final Thoughts: Drilling for 10-32 Shouldn’t Be a Guessing Game

I used to feel lost when it came to matching taps with drill bits. But once I got the hang of it—and made a few mistakes along the way—I found a rhythm.

Now, when someone asks me, “What size drill for 10-32 tap?” I just smile and say, “#21, every time.”

So whether you’re working on a bike bracket, electronics case, or metal sign, I hope this guide saves you the confusion I had. Drill smart, tap easy.

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