What Size Drill Bit for a 3/8 Tap? Complete Tap Drill Size Guide

For a 3/8-16 UNC tap, use a 5/16-inch drill bit. For a 3/8-24 UNF tap, use a Letter Q drill bit. For a 3/8-18 NPT tap, use a 37/64-inch drill bit.

A lot of people search for “3/8 tap” and expect one answer. That is where the confusion starts. A 3/8 tap can mean coarse, fine, or pipe thread. I’ll make it simple and show you the exact drill size, chart, formula, and steps so you can tap the hole right the first time.

Quick Answer: What Size Drill Bit Do You Need for a 3/8 Tap?

The right drill bit depends on the exact 3/8 tap you have. In most home shop and repair work, these are the common answers:

• 3/8-16 UNC = 5/16-inch drill bit
• 3/8-24 UNF = Letter Q drill bit
• 3/8-18 NPT = 37/64-inch drill bit

Before you drill, always check the tap marking. The number after the dash matters just as much as the 3/8 size.

Quick Reference Table

Why “3/8 Tap” Alone Is Not Enough

When I help beginners in the shop, this is the first thing I explain: 3/8 does not tell the whole story. It only gives you the nominal thread diameter. You still need the thread standard and pitch.

That means you must know if the tap is:

• 3/8-16 UNC
• 3/8-24 UNF
• 3/8-18 NPT

Each one uses a different hole size. If you drill the wrong size, the tap may bind, cut weak threads, or break.

What 3/8-16 Means

3/8-16 means the thread has a major diameter of about 3/8 inch and 16 threads per inch. This is a coarse thread in the Unified National Coarse system.

It is common in brackets, frames, repair work, and general metal fabrication because it is strong and easy to start. If you want a solid overview of UNC and UNF thread dimensions, Engineering Toolbox is a useful reference.

What 3/8-24 Means

3/8-24 still has a 3/8-inch major diameter, but now it has 24 threads per inch. That makes it a fine thread.

Fine threads are common in automotive and machinery work. They give you more threads in the same length, which can help with adjustment and holding power in some setups.

What 3/8-18 NPT Means

3/8-18 NPT is different from UNC and UNF. It is a tapered pipe thread, not a straight machine screw thread.

NPT taps are used for air lines, plumbing, fuel systems, and shop fittings. Because the thread is tapered, the drill size is much larger than most people expect. That is why 37/64 inch is normal for a 3/8-18 NPT tap.

3/8 Tap Drill Chart (USA Standard Sizes)

Below is the quick chart I use for standard U.S. workshop sizing. I always cross-check odd taps with a trusted chart or handbook before I drill. If you want another reference, Engineering Toolbox also has a handy tap drill size chart.

Standard Tap Drill Chart Table

Fractional, Letter, and Decimal Drill Size Comparison Table

How to Choose the Right Drill Bit for a 3/8 Tap

If you want clean threads and less risk of breaking a tap, choose the drill size in this order: read the tap, check the standard, then match the chart.

Check the Tap Marking First

Look at the shank of the tap. You will usually see markings like:

• 3/8-16 NC
• 3/8-24 NF
• 3/8-18 NPT

This stamp tells you what drill size to use. Never assume that every 3/8 tap uses the same bit.

Match the Drill Size to the Thread Standard

Once you know the marking, match it to the right drill:

• UNC threads are coarse and usually use a smaller tap drill than the final thread diameter.
• UNF threads are finer and usually use a slightly larger drill than the coarse version.
• NPT taps use a tapered thread system, so the drill size is completely different.

If you do a lot of tapping, a standard reference like Machinery’s Handbook is still one of the best shop books to keep around.

Consider the Material You’re Tapping

The drill size usually stays the same for the thread standard, but the material changes how easy the tap cuts.

• Aluminum: easy to tap, but use lubricant so threads stay clean.
• Mild steel: very common and usually forgiving.
• Stainless steel: tougher to tap and easier to work-harden.
• Brass: often taps clean, but still go slow.
• Plastic: easy to overcut or strip, so don’t rush.

Thread Percentage and Why It Matters

Many people think more thread is always better. In real shop work, that is not always true.

A very high thread percentage means:

• more cutting force
• more heat
• more risk of a broken tap

In many cases, 60% to 75% thread is a smart target. It gives strong threads without making the job harder than it needs to be.

Tap Drill Formula Explained

If you want a quick estimate, the basic imperial tap drill formula is:

Tap drill size = major diameter − thread pitch

For inch threads, thread pitch is 1 divided by TPI. This formula works well for common cutting taps, but I still use a chart for final confirmation.

Example for 3/8-16 UNC

For a 3/8-16 UNC tap:

• Major diameter = 0.375
• Pitch = 1 ÷ 16 = 0.0625
• 0.375 − 0.0625 = 0.3125

The answer is 0.3125, which is exactly 5/16 inch.

Example for 3/8-24 UNF

For a 3/8-24 UNF tap:

• Major diameter = 0.375
• Pitch = 1 ÷ 24 = 0.0417
• 0.375 − 0.0417 = 0.3333

The nearest common drill is Letter Q, which is 0.3320.

When the Formula Is Useful vs When to Use a Tap Chart

The formula is great when you want to understand why the size works. It is also handy if you are standing at the bench and want to estimate a drill quickly.

Still, I use a tap chart when:

• the tap is NPT
• the tap is a form tap
• I need the closest real drill size
• I want to avoid guesswork

Step-by-Step: How to Drill and Tap a 3/8 Hole Correctly

This is the simple process I use in the garage and shop when I want clean threads and fewer problems.

Tools You’ll Need

• 3/8 tap
• correct drill bit
• drill or drill press
• tap wrench
• cutting oil
• center punch
• vise or clamps
• deburring tool
• safety glasses

Step 1: Confirm the Tap Size and Thread Type

Read the marking on the tap. Make sure it says 3/8-16, 3/8-24, or 3/8-18 NPT. This step saves you from drilling the wrong hole.

Step 2: Mark and Center Punch the Hole

Measure the spot, mark it, and use a center punch. This keeps the drill bit from walking across the metal when you start.

Step 3: Drill the Correct Tap Hole

Use the correct bit for the tap. Keep the drill as straight as possible. If you have a drill press, use it. A straighter hole makes tapping much easier.

For best results:

• clamp the workpiece
• use the right speed for the material
• apply cutting fluid when needed

Step 4: Deburr the Hole

After drilling, lightly deburr the top edge. This helps the tap start clean and reduces chip buildup at the opening.

Step 5: Lubricate the Tap

Add the right fluid before you start tapping. This lowers friction and helps the threads come out smoother.

• Steel: use cutting oil
• Aluminum: use a suitable aluminum-safe fluid
• Stainless steel: use a high-quality tapping fluid

Step 6: Start the Tap Straight

This part matters more than most beginners think. Hold the tap square to the hole and apply light pressure as it starts. If it begins crooked, the threads will be crooked too.

Step 7: Turn Slowly and Break Chips

Turn the tap forward, then back it up a little to break chips. I do this often, especially in steel. It lowers torque and helps stop tap breakage.

Step 8: Clean and Test the Threads

Blow out or brush out the chips. Then test the hole with the correct bolt or fitting. The fastener should start clean and thread in smoothly.

Common Mistakes When Choosing a Drill Bit for a 3/8 Tap

Most tapping problems happen before the tap even touches the metal. These are the mistakes I see the most.

Mistake 1: Using 5/16 for Every 3/8 Tap

5/16 inch is correct for a 3/8-16 UNC tap. It is not correct for every 3/8 tap. It is wrong for 3/8-24 UNF and 3/8-18 NPT.

Mistake 2: Ignoring Thread Pitch

A 3/8-16 and a 3/8-24 have the same major diameter, but the pitch is different. That changes the correct hole size.

Mistake 3: Confusing Straight Threads with Pipe Threads

UNC and UNF are straight machine threads. NPT is a tapered pipe thread. They are not interchangeable, and neither are the drill sizes.

Mistake 4: Drilling Too Small

A hole that is too small makes the tap work too hard. That can cause:

• high torque
• rough threads
• broken taps

Mistake 5: Drilling Too Large

A hole that is too large gives you weak threads. The fastener may fit loose or strip out more easily.

Mistake 6: Tapping Dry

Dry tapping creates more friction and heat. It can ruin the finish and raise the chance of snapping the tap.

Mistake 7: Starting the Tap Crooked

If the tap starts at an angle, the threads will be off. That leads to cross-threading, poor fit, and ugly results.

Pro Tips for Better 3/8 Tapped Holes

Once you know the right drill size, these small habits make the job cleaner and safer.

Use a Drill Press for Straighter Holes

A drill press helps you drill square to the work. That makes tapping easier, especially if you are new to thread cutting.

Use the Right Tap Style

Different taps do different jobs:

• Taper tap: easy to start
• Plug tap: best general-purpose choice
• Bottoming tap: useful for blind holes

Adjust for Material Type

Softer materials can be more forgiving. Harder materials need better alignment, slower cutting, and good lubrication. Stainless steel is where patience really pays off.

Don’t Chase Maximum Thread Percentage

Many strong threads do not need maximum engagement. A slightly larger hole can make tapping much easier while still giving excellent holding power.

Use Cutting Fluid Properly

Do not flood the work. Just use enough to lubricate the cut and clear chips. Reapply as needed, especially in steel and stainless.

Clear Chips Often

This is extra important in blind holes. Back the tap out, clean chips, and keep going. Chips packed in the bottom are a common reason taps break.

Drill Bit and Tap Size Table by Application

These examples make it easier to match the tap type to the job you are doing.

Common Uses for 3/8-16 UNC

Common Uses for 3/8-24 UNF

Common Uses for 3/8-18 NPT

3/8 Tap Drill Size FAQ

What size drill bit for a 3/8-16 tap?

Use a 5/16-inch drill bit for a standard 3/8-16 UNC tap.

What size drill bit for a 3/8-24 tap?

Use a Letter Q drill bit for a standard 3/8-24 UNF tap.

What size drill bit for a 3/8 NPT tap?

For a 3/8-18 NPT tap, use a 37/64-inch drill bit.

Can I use a 5/16 drill bit for all 3/8 taps?

No. A 5/16-inch bit is correct for 3/8-16 UNC, but not for 3/8-24 UNF or 3/8-18 NPT.

How do I know if my 3/8 tap is UNC or UNF?

Check the marking on the tap. If it says 3/8-16, it is usually UNC. If it says 3/8-24, it is usually UNF.

What happens if the tap drill hole is too small?

The tap gets harder to turn, cuts hotter, and is more likely to break.

Do I need a different drill size for form taps?

Usually yes. Form taps often need a different hole size than cutting taps, so always check the maker’s chart.

Final Answer: Which Drill Bit Fits a 3/8 Tap?

The short answer is simple: 3/8-16 UNC uses 5/16 inch, 3/8-24 UNF uses Letter Q, and 3/8-18 NPT uses 37/64 inch. The key is to identify the exact tap before you drill. If you keep that one rule in mind, you will get cleaner threads and avoid most tapping mistakes.

If you are setting up your toolbox, keep the right drill bits, a good tap wrench, and proper cutting fluid close by. It makes every tapping job easier.