Drill and Tap Size Chart: Complete Guide for Accurate Threading

In CNC machining, one of the most important tasks is creating threaded holes that fit perfectly with bolts, screws, or other components. To achieve this, machinists must first drill a hole with the correct size drill bit and then cut threads using the right tap. Using the wrong size can result in stripped threads, poor fit, or part failure. Understanding the relationship between drill bit size and tap size is essential for accurate and durable connections.

This guide explains how to choose the correct drill sizes for tapping, provides practical examples such as m8 drill size or what size drill bit for 7/16 tap, and covers thread engagement, tap hole sizes, and industry standards.

Understanding Drill Bits and Taps

A drill bit is a cutting tool designed to make precise holes in metal, plastic, or other materials. Drill bits are available in both metric and imperial sizes, and selecting the right size is critical when tapping threads.

A tap is a tool used to cut internal threads into a pre-drilled hole. For example, a 5/16 tap size requires a hole drilled with a specific bit to ensure proper thread engagement. If the hole is too small, the tap can break. If it is too large, the threads will be shallow and weak.

When making threads, it is necessary to first drill a suitable hole with a drill bit, and then use a tap of the corresponding specifications to make deep cuts to form a threaded structure. This is why machinists use drill bit to tap size charts or a tap and die drill bit size chart to ensure accuracy.

How Drill and Tap Sizes Work

To create threads, you must first drill a hole slightly smaller than the tap’s outer diameter. This leaves enough material for the tap to cut threads. The required drill size depends on the thread pitch and type of thread.

For example:

• An M8 tap (metric thread) requires a 6.8 mm drill bit. This is the standard m8 drill size used in most CNC operations.
• For a 5/16-18 UNC tap, the correct drill is 17/64 inch (0.257 in). Many machinists look up “drill size for a 5/16-18 tap” when working with imperial fasteners.
• A 7/16-14 UNC tap requires a 23/64 inch drill bit. This answers the common query: what drill bit size for 7/16 tap.
• For fine threads, like a 1/4-28 UNF tap, the drill size is #3 (0.213 in). Machinists often ask what size drill bit for a 1/4-28 tap when working with aircraft or automotive parts.

These examples highlight why machinists always refer to a drill bit and tap size chart before beginning.

Tap Hole Sizes and Thread Engagement

The tap hole size is the drilled diameter that allows a tap to form proper threads. The amount of thread engagement (percentage of thread depth) depends on this hole size.

• A smaller hole increases thread engagement but risks breaking the tap.
• A larger hole makes tapping easier but weakens the thread.

Most industry standards recommend 70–80% thread engagement for strength and ease of tapping.

For example:

• A 1/2-13 UNC tap typically uses a 27/64 inch drill bit. Machinists asking “what size hole to drill for 1/2 tap” or “drill size for 1/2-13” are referring to this standard.
• A 3/8-16 UNC tap requires a 5/16 inch drill bit. This answers the frequent question: what size drill bit for 3/8 tap.
• For small screws like 6-32 UNC, the correct drill size is #36 (0.1065 in). This is often searched as what size drill for 6-32 tap or 6-32 form tap drill size.

Understanding tap hole sizes ensures accurate threads that match standard bolts.

Using Drill Bit to Tap Size Charts

A drill bit to tap size chart or tap and die chart drill size is a reference tool used in every machine shop. It lists the correct drill bit sizes for both metric and imperial taps.

Instead of memorizing, machinists consult the chart whenever working with new thread sizes. For example:

• M16 tapping drill size is 14.0 mm, so machinists asking “drill bit size for m16 tap” will always find this value in a chart.
• A 9/16-12 UNC tap requires a 31/64 drill bit, so the query “what size drill for 9/16 tap” is solved by the chart.
• Fine threads like 7/16-20 UNF require a 25/64 drill bit, often searched as drill size for 7/16-20 tap.

Charts also include special cases such as form tap drill charts, tapping standards chart, or thread drill dimensions, which differ slightly from cutting taps.

By using these references, machinists avoid costly mistakes in threading.

Drill and Tap Specification Chart

The following is a table of drill and tap specifications to make it easier to determine the matching relationship between drills and taps. When selecting the correct tap specification, please choose the appropriate drill specification according to the size of the threads to be made to ensure a proper matching relationship between the threads and the drill.

1. Tap specification table

Thread Specifications

2. Drill and Tap Matching Chart

Metric Coarse Thread (M)

Unified National Coarse Thread (UNC)

Unified National Fine Thread (UNF)

Common Examples in Practice

Let’s go through some common tap and drill combinations frequently used in workshops:

• What drill size for a 5/16 tap?
  The answer depends on the thread type. For 5/16-18 UNC, use 17/64 in. For 5/16-24 UNF, use letter F drill (0.257 in).
• What size drill bit for 1/4-20 tap?
  The correct drill size is a #7 drill (0.201 in). This is one of the most common tapped holes in machinery.
• What is #7 drill bit size?
  A #7 drill equals 0.201 inches in diameter, often used for 1/4-20 UNC taps.
• What size drill bit for half inch tap?
  For a 1/2-13 UNC tap, use 27/64 in. For 1/2-20 UNF, use 29/64 in.
• What size drill for 3/8 tap?
  For 3/8-16 UNC, the correct size is 5/16 in. For 3/8-24 UNF, use 21/64 in.
• What size drill bit for 7/16 tap?
  For 7/16-14 UNC, the drill size is 23/64 in. For 7/16-20 UNF, use 25/64 in.
• What size drill bit for M4 tap?
  The correct drill is 3.3 mm. Similarly, an M8 drill size is 6.8 mm, and an M16 tap drill size is 14 mm.

These examples show how critical it is to consult a drill size chart for taps or a tap drill size chart for accurate results.

Why Choose Richconn for Machining Services?

Getting the correct drill size for tap chart is only the first step. Precision machining also requires high-quality equipment, proper cutting speeds, and expert handling. Richconn offers professional CNC machining and tapping services that guarantee accuracy, durability, and efficiency.

Whether you need prototypes or mass production, Richconn ensures every tapped hole dimension matches the design specification. With expertise in both metric tap drill sizes and imperial standards, their team provides solutions tailored to industries such as automotive, aerospace, and manufacturing.

By partnering with Richconn, you gain access to skilled engineers, advanced machines, and reliable quality control.

Conclusion

Drill and tap sizes are essential for creating accurate threaded holes in CNC machining. From basic examples like the m8 drill size to complex queries like what size drill bit for a 1/2-20 tap, understanding the relationship between drill bits and taps is crucial. Using the right tap hole size ensures proper thread engagement, strength, and performance.

While charts and guides provide answers, the best results often come from working with professionals. For precise machining, threading, and tapping, trusted experts like Richconn deliver accuracy and consistency across all projects. By following correct drill-to-tap standards, you can achieve reliable connections that improve product quality and extend part life.

FAQ

What is a drill bit size for 5/16 tap?
For a 5/16-18 UNC tap, use a 17/64 drill. For a 5/16-24 UNF tap, use an F drill.

What size hole to drill for 1/4-20 tap?
The hole size should be 0.201 inches, which is a #7 drill bit.

What size drill bit for a 3/8 tap?
For 3/8-16 UNC, use 5/16 in. For 3/8-24 UNF, use 21/64 in.

What size drill bit for a 1/2 tap?
For 1/2-13 UNC, use 27/64 in. For 1/2-20 UNF, use 29/64 in.

What size drill bit for M16 tap?
The correct m16 tapping drill size is 14.0 mm.

What is a tap hole size?
A tap hole size is the drilled diameter needed before cutting threads with a tap. It must match tap standards for proper thread engagement.

What size drill bit for 7/16 tap?
For 7/16-14 UNC, use 23/64 in. For 7/16-20 UNF, use 25/64 in.

What size drill for 6-32 tap?
Use a #36 drill bit (0.1065 in). This is often searched as 6-32 form tap drill size.

What is 7 drill diameter in fraction?
A #7 drill is 0.201 inches, often used for 1/4-20 UNC taps.

Where can I find a full drill bit and tap size chart?
A complete drill bit and tap size chart or standard drill and tap chart is available in every machining handbook. For custom projects, you can also consult machining service providers like Richconn.