Drilling and tapping holes is a critical process in various DIY projects and professional tasks. Selecting the correct drill bit size for a tap ensures a perfect fit and prevents damage to your materials. In this article, we will focus on the 1/4 tap and guide you through choosing the right drill bit size to achieve precise and reliable results.

What Size Drill Bit for 1/4 Tap

Understanding Taps and Drill Bit Sizes

What is a Tap?

A tap is a tool used to cut threads inside a hole, allowing screws or bolts to be securely inserted. Taps come in various sizes, matching the diameter and thread count of the screw or bolt they are intended to work with.

Why the Drill Bit Size Matters

Using the correct drill bit size is crucial when preparing a hole for tapping. If the hole is too small, the tap may break or the threads may be poorly formed. Conversely, if the hole is too large, the threads may be too shallow and weak to hold a screw or bolt properly.

Determining the Correct Drill Bit Size for a 1/4 Tap

Types of 1/4 Taps

  • 1/4-20 UNC (Unified National Coarse): This is a common tap size with 20 threads per inch.
  • 1/4-28 UNF (Unified National Fine): This tap size has 28 threads per inch and is used for finer threading.

How to Choose the Drill Bit Size

To determine the correct drill bit size for a 1/4 tap, you need to consider the specific tap type and thread count.

For 1/4-20 UNC Tap:

  • The recommended drill bit size is a #7 drill bit.
  • This drill bit has a diameter of 0.201 inches or approximately 5.1 mm.
  • This size ensures enough material is left for forming strong threads.

For 1/4-28 UNF Tap:

  • The recommended drill bit size is a #3 drill bit.
  • This drill bit has a diameter of 0.213 inches or approximately 5.4 mm.
  • It provides a slightly larger hole for finer threads, ensuring they are well-defined and secure.

Drill Bit Size Chart for Taps

To assist in selecting the correct drill bit size for other taps, here’s a quick reference chart:

Tap SizeThread CountDrill Bit SizeDrill Diameter (in)Drill Diameter (mm)
1/4-20 UNC20#70.2015.1
1/4-28 UNF28#30.2135.4
1/4-18 NPT187/160.43811.1
1/4-19 BSP197/160.43811.1
Drill Bit Size Chart for Taps

Tips for Drilling and Tapping

  1. Mark the Drilling Spot: Use a center punch to mark the spot where you intend to drill. This prevents the drill bit from wandering and ensures precision.
  2. Use Lubrication: Applying cutting oil or a suitable lubricant reduces friction and heat, making the drilling and tapping process smoother and prolonging the life of your tools.
  3. Drill Steadily: Apply consistent pressure and maintain a steady speed while drilling to avoid damaging the material or the drill bit.
  4. Tapping Technique: After drilling, use the tap with a tap wrench. Turn the tap slowly and back it out occasionally to clear chips and prevent binding.
  5. Safety First: Always wear safety goggles and gloves to protect yourself from metal shavings and sharp edges.


Choosing the right drill bit size for a 1/4 tap is crucial for creating precise and durable threads. Whether you’re working with 1/4-20 UNC or 1/4-28 UNF taps, following the recommended drill bit sizes will ensure successful results. Always remember to use the proper drilling and tapping techniques for safety and efficiency.

For more detailed information on other tap sizes and drill bit recommendations, refer to the provided drill bit size chart. Happy drilling and tapping!

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