1/4-20 Tap Drill Size in MM – Types, Uses and Benefits

Updated: 17 Jan 2025

Have you ever seen a screw that holds things together? To make threads in a hole so the screw can fit, we use something called a tap. A tap drill size is the size of the hole we drill before we make those threads. Using the right size is very important to make sure the screw fits perfectly.

We will talk about how to find the tap drill size for a special screw called 1/4-20, and we will convert it to millimeters (mm), which is a metric unit.

Table of Content

Understanding Tap Drill Sizes
What Does 1/4-20 Mean?
Making Process of 1/4-20 Tap Drill Size in MM
Types of Drill Bits for 1/4-20 Tap Drill Size
Converting 1/4 Inch to Millimeters
Calculating the Tap Drill Size for 1/4-20 Thread
Tap Drill Size in Millimeters
Importance of Using the Correct Tap Drill Size
Benefits of 1/4-20 Tap Drill Size in MM
Drawbacks of the 1/4-20 Tap Drill Size in MM
FAQs about 1/4-20 Tap Drill Size in MM
Conclusion

1/4-20 Tap Drill Size in MM Working Process

Understanding Tap Drill Sizes

When we make a hole in metal or wood to put a screw or bolt, we need to make sure the hole is the right size. A tap drill size is the size of the hole we drill before making the threads inside it. Think of it like this: if you want to put a lid on a jar, the lid needs to fit perfectly on the jar’s opening. If the opening is too big or too small, the lid won’t work right.

In the same way, choosing the correct tap drill size helps make sure the screw or bolt fits snugly into the hole without breaking or getting stuck. Using the right tap drill size is very important for building strong and safe things.

What Does 1/4-20 Mean?

You might wonder what “1/4-20” means when people talk about drill sizes. Let’s break it down step-by-step:

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1/4 is the size of the screw or bolt’s width, which is one-fourth of an inch wide. Imagine cutting a whole inch into four equal parts one of those parts is 1/4 inch.
20 tells us how many threads (the spiral lines) are in one inch of the bolt. In this case, there are 20 spiral lines going around the bolt in every inch.

When we say 1/4-20, we are describing a bolt that is 1/4 inch wide with 20 threads per inch. This combination is very common and used in many tools and machines.

Making Process of 1/4-20 Tap Drill Size in MM

Making the right tap drill size is important for creating strong threads in metal. In this article, we’ll show you how to calculate the 1/4-20 tap drill size in millimeters, step by step.

Step 1: Understand the Meaning of 1/4-20

1/4 means the screw is 1/4 inch wide.
20 means there are 20 threads (or ridges) per inch.

Step 2: Convert the Size from Inches to Millimeters

Use this formula: 1 inch = 25.4 millimeters.
So, 1/4 inch = 25.4 ÷ 4 = 6.35 millimeters.

Step 3: Use the Tap Drill Formula

The formula to find the tap drill size is:

Tap Drill Size = Major Diameter – (1 ÷ Threads per Inch).
Major Diameter is 6.35 mm.
Threads per Inch is 20.

Step 4: Do the Math

1 ÷ 20 = 0.05 inches (or its millimeter equivalent).
Subtract this from 6.35 mm to get the tap drill size in mm.

Step 5: Find the Closest Drill Bit Size

Check standard metric drill bit sizes to choose the closest match.
Usually, a bit close to 5.2 mm works best for a 1/4-20 thread.

Types of Drill Bits for 1/4-20 Tap Drill Size

Standard Tap Drill Size:

For a 1/4-20 screw, the standard drill size is 6.5 mm.
This size works best because it gives the screw just the right amount of room to fit tightly into the hole.

For a Slightly Tighter Fit:

If you need the hole to be a little tighter, you might use a 6 mm drill bit.
This is a bit smaller, so the screw fits tightly and won’t move around easily.

For a Looser Fit:

If you want a bit more space for the screw, you could use a 7 mm drill bit.
This makes the hole slightly bigger, the screw fits, but it’s not as tight as the 6.5 mm hole.

Converting 1/4 Inch to Millimeters

Inches and millimeters are both units used to measure length.
- Inches are often used in the U.S., while millimeters are used in many other countries.
To convert inches to millimeters, you need a special formula.
- 1 inch = 25.4 millimeters.
Now, let’s convert 1/4 inch to millimeters.
- You need to divide 1 by 4 (because 1/4 is one part of a whole).
- Then, multiply the result by 25.4 millimeters (because 1 inch = 25.4 mm).
Here’s the math:
- 1 ÷ 4 = 0.25 inches
- 0.25 inches × 25.4 mm = 6.35 millimeters
- 1/4 inch is equal to 6.35 millimeters.

Calculating the Tap Drill Size for 1/4-20 Thread

Step 1: Understand the Formula

To find the right tap drill size, we use this formula:
Tap Drill Size = Major Diameter – (1 / Threads Per Inch)

Step 2: What’s the Major Diameter?

For a 1/4-20 thread, the major diameter is 1/4 inch.
1/4 inch equals 6.35 millimeters (since 1 inch = 25.4 mm).

Step 3: What’s the Threads Per Inch (TPI)?

The number “20” in 1/4-20 means there are 20 threads per inch.

Step 4: Plug the Numbers Into the Formula

Subtract (1 divided by 20) from the major diameter:
6.35 mm – (1 ÷ 20)
6.35 mm – 0.05 mm = 6.30 mm

Step 5: The Tap Drill Size

The tap drill size you need is 6.30 mm.

Tap Drill Size in Millimeters

1/4-20 means: The “1/4” part refers to the size of the bolt (in inches), and “20” means there are 20 threads per inch on the bolt.
Convert inches to millimeters: Since 1 inch = 25.4 millimeters, we convert the 1/4 inch diameter into millimeters. So, 1/4 inch = 6.35 mm.
Tap Drill Size Formula: To calculate the tap drill size, we use this formula:
- Tap Drill Size = Major Diameter – (1 / Threads per Inch).
- For 1/4-20, it becomes: Tap Drill Size = 6.35 mm – (1 / 20).
- That gives us 6.35 mm – 0.05 mm = 6.30 mm.
Recommended Drill Size: The best drill size to use is 6.30 mm for a 1/4-20 thread.

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Importance of Using the Correct Tap Drill Size

Better Thread Quality: Using the right drill size helps create smooth and strong threads. This means the parts will fit together perfectly!
Avoids Breaking the Tap: If the hole is too small or too big, the tool (called a tap) used to make threads can break. Using the right size keeps your tap safe!
Saves Time and Effort: When you use the correct drill size, you don’t have to redo the job. It makes the whole process faster and easier.
Prevents Wobbly Joints: If the hole is too big or too small, the parts won’t fit right. The right tap drill size makes sure everything lines up neatly.
Helps in Stronger Fittings: When the threads are made correctly, they help parts stay tight and strong, so they don’t loosen over time.
Better Safety: If the drill size is wrong, it can cause problems in your work, leading to accidents or parts not working well. Correct sizing keeps you safe.

Benefits of 1/4-20 Tap Drill Size in MM

Benefits
Perfect Fit for Threads: Ensures the right fit for creating clean and strong threads in materials. Easy to Use: The 1/4-20 tap drill size is a common and simple size, making it easy for beginners. Less Risk of Damage: Using the correct size helps prevent damaging the tool or the material. Accurate Threads: It creates smooth, accurate threads for bolts and screws to fit properly. Stronger Joints: Helps make stronger connections between parts, reducing the chance of loosening. Improves Efficiency: Saves time and effort when working with threaded materials because it’s the right size. Good for Many Projects: Ideal for many projects like building or fixing machines, tools, or equipment.

Benefits

Perfect Fit for Threads: Ensures the right fit for creating clean and strong threads in materials.
Easy to Use: The 1/4-20 tap drill size is a common and simple size, making it easy for beginners.
Less Risk of Damage: Using the correct size helps prevent damaging the tool or the material.
Accurate Threads: It creates smooth, accurate threads for bolts and screws to fit properly.
Stronger Joints: Helps make stronger connections between parts, reducing the chance of loosening.
Improves Efficiency: Saves time and effort when working with threaded materials because it’s the right size.
Good for Many Projects: Ideal for many projects like building or fixing machines, tools, or equipment.

Drawbacks of the 1/4-20 Tap Drill Size in MM

Drawback
Not Always Perfect Fit: Sometimes, the drill size may not match exactly with the thread you want, causing a loose or tight fit. Requires Accurate Conversion: Converting between inches and millimeters might confuse some people, leading to mistakes. Harder to Find: The exact drill bit size in millimeters might be difficult to find in some stores. May Cause Damage: Using the wrong size drill bit can damage the material you’re working with or even break the tap. Need for Special Tools: Some tools or equipment might be required to get the right tap drill size, making it tricky for beginners.

Drawback

Not Always Perfect Fit: Sometimes, the drill size may not match exactly with the thread you want, causing a loose or tight fit.
Requires Accurate Conversion: Converting between inches and millimeters might confuse some people, leading to mistakes.
Harder to Find: The exact drill bit size in millimeters might be difficult to find in some stores.
May Cause Damage: Using the wrong size drill bit can damage the material you’re working with or even break the tap.
Need for Special Tools: Some tools or equipment might be required to get the right tap drill size, making it tricky for beginners.

FAQs about 1/4-20 Tap Drill Size in MM

What is a tap drill size?

A tap drill size is the size of the hole you need to make before adding threads to it. These threads help screws or bolts fit perfectly in the hole. It’s like getting the right size for a key to fit into a lock!

What does 1/4-20 mean?

1/4-20 is a way to describe a specific type of thread used for screws and bolts. “1/4” means the width of the screw is 1/4 inch, and “20” means there are 20 threads in each inch of the screw. It helps people know which screw fits in which hole.

How do I convert 1/4 inch to millimeters?

To change inches to millimeters, you can use this rule:

1 inch = 25.4 millimeters.
1/4 inch = 6.35 millimeters. It’s like using a special measuring tool that changes inches into millimeters!

What is the correct tap drill size for a 1/4-20 thread in millimeters?

The correct tap drill size for a 1/4-20 thread is around 5.3 mm. This size is perfect for making the hole so that the screw can fit tightly with the threads.

Why is it important to use the right tap drill size?

Using the wrong size can cause problems. If the hole is too small, the screw won’t fit. If it’s too big, the screw won’t stay tight. It’s important to measure carefully to make sure everything works right.

Can I use any drill bit to make the hole?

No, you can’t use any drill bit. You need a drill bit that is the exact size for the tap drill. It’s like using the right tool to build something strong.

How do I measure a tap drill size?

To measure a tap drill size, you can look up the size you need (like 5.3 mm for a 1/4-20 thread). Then, use a ruler or caliper to find the drill bit that matches that size.

What happens if I use the wrong tap drill size?

If you use the wrong size, the screw might not fit properly. This can cause the threads to be weak or even break. Always check your size carefully to make sure everything works smoothly

Conclusion

The 1/4-20 tap drill size and how to convert it to millimeters is important when you want to make sure your threads fit perfectly. We learned that the 1/4-20 size means a 1/4-inch diameter and 20 threads per inch. By converting the size into millimeters, we can use the right drill bit to make sure the threads are clean and strong. Using the correct tap drill size helps your project go smoothly, and the wrong size could cause problems like bad threads or breaking tools. Always double-check your measurements, and you’ll be able to complete your threading projects with ease.