What Size Drill Bit for 5/16 Tap? Your Comprehensive Guide to Drill Bit Pairing

When it comes to the efficiency of a drilling procedure, finding the right drill bit size for the job is crucial. Handling a 5/16 tap? Let’s decipher what size drill bit brilliantly pairs with a 5/16 tap and optimize your drilling operations.

A Quick Look at the Tap and the Drill Bit

Before discerning the perfect drill bit size for your 5/16 tap let’s break down some basics. A tap is a tool used to cut threads on the interior surface of a hole, enabling a fastener with matching threads to be screwed into it. One of our focal points in this guide, the 5/16 tap, comes in two measurement variations: an 18 tap and a 24 tap, with differing pitch.

Drill bits on the other hand, are cutting tools that remove material to create holes in varying sizes and shapes. The type of drill bit used is contingent on the material, hole size and depth, and the type of hole to be made. Drill bits come in myriad varieties with the twist drill bit being one of the most common types, best suited for general use.

Drill Bit Sizes for a 5/16″ Tap

Depending on the pitch and thread of your 5/16″ tap, you would need a different size of drill bit to ensure the threads are cut correctly. So, which drill bit size pairs best with 5/16″ tap?

For a 5/16”-18 UNC (Unified National Coarse), you’d need an F or 17/64″ drill bit. For a 5/16”-24 UNF (Unified National Fine), you are better off with an I or 9/32″ drill bit.

Why Do the 18″ and 24″ Taps Require Different Drill Bit Sizes?

Both taps may be 5/16″ in diameter, but the difference lies in the number of threads per inch, hence, a different thread pitch. An 18″ tap, having National Coarse threads (NC), and a 24″ tap with National Fine Threads (NF), correlate to a different thread pitch diameter. Accordingly, an 18″ has a thread pitch diameter of .2570″, while a 24″ possesses a thread pitch of .2720″. Thus, they necessitate different drill bit sizes.


Q: What’s a 5/16 Tap?

A: A 5/16 Tap is designed to cut threads on an already drilled hole. The tap size refers to its diameter, with “5/16 inch” being its size. It comes in two variations – 18 and 24-inch taps, signifying the number of threads per inch.

Q: Why is it vital to use the right drill bit size for a tap?

A: It’s crucial to use the correct drill bit size since it determines adequately cutting threads on the interior of a hole. A drill bit that’s either too large or too small interferes with the tapping process and ultimately compromises your work’s quality.

Q: Can I use a 5/16 Tap with any drill bit type?

A: The type of drill bit used should be compatible with the material being drilled and the type of hole. For general-purpose drilling, a high-speed steel (HSS) twist drill bit would perfectly suit your 5/16 tap.


Whether pursuing a DIY project or engaged in a professional job, preparing for a drilling task involves understanding what size drill bit for 5/16 tap best serves your needs. Taking note of your tap’s specifications, particularly its thread count, will guide you to choose the most appropriate drill bit size. Remember, using the right tool goes a long way in ensuring effective, efficient, and safe drilling. Have fun with your drilling operations!

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