What Size Drill Bit for 3/8 Bolt: A Comprehensive DIY Guide

Navigating the complex avenues of DIY can sometimes feel overwhelming. Yet, when it comes to drilling, you’re not alone. In this insightful guide, we tackle a frequently asked question: What size drill bit is needed for a 3/8” bolt? Drawing from extensive DIY experience, we’ll not just reveal the ideal drill bit size, but also delve into variations and vital considerations before you get drilling. Eager to know more? Let’s drill down into the specifics!

The Drill Bit Size for a 3/8″ Bolt

For a bolt with the dimensions of 3/8″, precision is key. When dealing with a ⅜” UNC bolt with 16 threads per inch (TPI), the math is straightforward: subtract the thread depth of 1/16″ from the bolt size of ⅜”. This gives you a precise 5/16″, the drill bit size you need. Alternatively, an ‘O’-sized drill bit, being a tad larger, can also be a good fit.

Working with a ⅜” UNF bolt with 24 TPI? Aim for a drill bit that’s about 0.3333″. In standard drill bit dimensions, a 21/64″ bit is a slight undershoot, while an 11/32″ bit overshoots a touch.

Bolt Size vs. Drill Bit Size:

  • ⅜“ Bolt = 5/16” Drill Bit
  • ⅜” 16 Tap = 5/16” Drill Bit
  • ⅜” 24 Tap = 21/64” Drill Bit or Q Letter Drill Bit

With this information at hand, you’re geared up to drill a hole perfectly suited for a ⅜” bolt.

When and How to Use a 3/8″ Bolt

Bolts, especially the 3/8″ size, are indispensable when reinforcing or binding materials like wood, drywall, metal, and concrete. The sheer holding power of bolts makes them an invaluable asset for specific projects. Here are instances when a 3/8″ bolt comes to the rescue:

  • Securing table legs: For long-lasting stability in furniture like tables, a 3/8″ bolt offers robust reinforcement.
  • Countertop installation: For heavyweight materials like granite, the 3/8″ bolt ensures everything remains firmly in place.

Usage Tips:

  1. Differentiate between screws and bolts. While both serve as fasteners, screws have threads for binding materials, while bolts, larger in size, necessitate holes slightly bigger than their threads.
  2. For 3/8″ bolts, a 5/16″ bit is ideal. Drill deep enough to accommodate the bolt’s threads, then secure your bolt in place.
  3. Tools with torque wrench functionality make the process seamless and precise.

Other Key Considerations for Using a 3/8″ Bolt

Ready to use a 3/8″ bolt for your next endeavour? These tools are lauded for their remarkable strength. Here’s a quick checklist to optimize their usage:

  • Tool Selection: A conventional corded drill usually does the trick. However, multi-function power tools that combine drilling and torque wrench features are excellent.
  • Additional Reinforcements: Pair your 3/8″ bolts with nuts and washers for an added layer of support. A flat washer measuring either 1.000 (USS) or 0.406 (ASE) complements a 3/8″ bolt perfectly.
  • Weight Bearing Capacity: A 3/8″ bolt can bear impressive loads. With granite, it supports up to 7,000 pounds, while sandstone can hold up to 4,000 pounds.
  • Precise Drilling: Before installing your bolt, ensure the drilled hole is of the correct size. Use markers for placement, and consider washers for additional reinforcement.
  • Post-Drill Clean-Up: Clear away dust and debris post-drilling for a smoother fastening procedure.
  • Torque Considerations: Gauge if your project necessitates a torque wrench. Take care of materials susceptible to damage from excessive force.


Can a 3/8″ bolt be used for all materials?

While versatile, consider the material’s strength and the bolt’s size before usage.

How do I measure the diameter of my bolt?

The bolt’s packaging often includes its diameter. If not, measure the bolt’s widest section.

Is a 3/8″ bolt stronger than screws?

Bolts generally offer superior holding power compared to screws, making them ideal for heavy-duty tasks.


The journey of DIY brings along a plethora of questions, and with the right guidance, the answers become clear. Understanding the intricacies of bolts, especially the 3/8″ size and their corresponding drill bit sizes, can elevate your DIY projects. With the wisdom shared here, you’re well on your way to masterful drilling endeavours. As always, preparation and the right tools are the cornerstones of any successful project.

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