What Size Of Drill Bit Should Be Used For A 5/16″ 18 Tap?

So you very enthusiastically started a DIY project at your home. Still, it’s getting quite difficult to drag across the finish line. You thought knowing how to use the drilling machine and screws would be enough. But now that you’re halfway through, you realize the importance of tapping instead of drilling. And how if you’d want your project to be sturdier and last longer, you’d need to figure out all the different drill bit sizes that match your specific tap. To land on this article specifically, you’re probably looking for a drill bit size for a 5/16” 18 tap.

Lucky for you, we’ve already done the calculations for you. To understand what drill bit sizes work best, you must familiarize yourself with the following few terms.


Some terms for reference:

Threads are referred to as these indents or raised spikes on a screw or bolt that appear like zig-zag lines if the bolt was laid down on the table.

The major diameter is referred to as the diameter measured from the top of those spikes.

The minor diameter is referred to as the diameter measured from where the spike protrudes (the innermost external diameter of the tap drill).

What Size Of Drill Bit Should Be Used For A 5/16″ 18 Tap?

Just so you don’t have to go and buy the entire drill bit kit and test each one, we’ve figured out what size you’ll need for all 5/16 taps. Just refer to the table below, and buy as needed:

Drill BitTap SizeBit Size
F5/16” 1817/64”
I5/16” 2417/64”
9/32”5/16” 329/32”
  • For the tap size 5/16” 18, consider buying a 17/64” which is also called the F drill bit. In decimals, this is 0.257
  • For the tap size, 5/16” 24, consider buying a 17/64” corresponding to the I drill bit.
  • For the tap, size 5/16”32, consider buying the 9/32”.

Best products

Just to make your job even easier, we’ve come up with a few best amazon recommendations. You don’t even have to step out of your home to buy the drill bits. Instead, go through these and pick the one that best fulfills your needs:

Owl Tools masonry (pack of 10)

This Owl Tools Masonry set is a set of 10 carbide-tipped drill bits that are great to have around the house in the long term. Additionally, for that low price, you get 10 drill bits that are all carbide-tipped. Carbide is as strong as a drill bit gets. They may not be fully made of carbide, only tipped with it. Nonetheless, the carbide tip does make it easier to tap or drill through some very hard materials. Carbide is usually a material used in production-level drilling. However, the major component is still stainless steel. So, it is functional for home usage, drilling through:

  • cement
  • tile
  • cement
  • brick
  • and more!

Becollo Cobalt set (pack of 15)

Cobalt is another tough material that lands right in the middle of stainless steel and carbide. This Becollo 5/16 inch drill set is cobalt, making it suitable to go through tougher metals and steel grades. It’s a 135° split design compared to a regular 118° standard design, which can cut metal faster than others. The best part, it is just as inexpensive as the Owl Tools carbide-tipped stainless steel drill bits.

Amoolo (pack of 2)

Suppose your home improvement project involves specifically working with wood. In that case, this Amoolo 5/16” and 3/8” drill bit is a greater starter pack to get. Its high-speed steel material is made especially to cut through the wood from all angles. At the same time, the parabolic double flute provides a smooth finish while doing so. You get two very useful drill bits to work on your project, other combinations include 3, 4, or 9 drill bits, and you can choose from them.

LepoHome Hex Shank Combination set (pack of 6)

It is another cost-efficient set you can get your hands on from LepoHome, a combination of several pieces you’ll find handy around the house. It not only fulfills your tapping needs but also cuts and deburring needs. With its self-centering design, you won’t need a center drilling or punching piece to start the tapping. The Center drilling piece is a tool you will need to punch the beginning of the hole so there is a guide to follow and the drill doesn’t wander elsewhere.

Bosch IMC09 Impact MultiConstruction

Suppose you just want to get your current job done because you’ve realized you’re not a DIY person. In that case, you just want a 5/16” drill bit and nothing else. Then, pick up this single-piece Bosch Multi Construction set which also happens to be Amazon’s choice. The carbide-tipped tool will efficiently tap into various materials with ease, and its steep flutes will also be quick to remove the material from the hole that will be made.

What is tap drill size?

Tapping is the process of creating a hole, often referred to as a clearance hole, that has threads.

These threads are created by a drill bit. The tapped hole makes it easier for a bolt to be screwed or threaded in. A good tap drill size is roughly 75% to 90% of the diameter of the threads. It ensures that the bolt is screwed in tighter and closer in the end.

Now you may think, why do you need a tap drill to make the hole? You may think of manually using the screw to tap the hole. However, that would result in tapping a hole that is equal to the size of the screw. Inevitably, the screw will be loosely held in the wood/metal you’re working with. As a rule of thumb, it is why a good tap drill is 85% of the major diameter when dealing with larger threads. However, one should keep the tap drill size 90% of the major diameter when it comes to fine drills.

Benefits of using the correct one

There’s a reason why tapping is preferred over drilling to yield a stronger hold. Here are the benefits of using the correct size to tap a hole:

  • It results in tight joints that last a lot longer.
  • Prevents the screws from moving around and keeps them lined up.
  • Screws also don’t break, and neither do their threads.
  • Wood pieces don’t split when the correct tap drill size is used. It usually happens when dealing with softer wood.
  • With drilling, the screw is usually either buried too deep or the head is too visible from the top of your project. Tapping makes it easier to maneuver, so it sits where it needs to be.

What To Consider

man using a drill

To achieve a product with a high degree of finishing, tapping and drilling both need patience and practice. We’d like you to consider a few things before you begin!

  • It is advised to grab a spare piece of metal, such as tin, and practice drilling and tapping. That way, there’s a lesser chance of you making mistakes on a bigger more important project.
  • The drill bit is not all that you’ll need. If you’re tapping manually, you’ll need tools to rotate the tap. You can use a T-style handle or ratchet to rotate it.
  • You should also consider getting cutting fluid to lubricate the process of tapping.
  • A center punch and chamfer tool will help you start and finish the tapping process easily.

How To Use Them

Using 5/16” taps is the same as using any other tap. The drilling and tapping process is the same for all kinds of taps.

The 5/16” high-speed steel taper taps usually make new threads or fix an old damaged hole by re-threading. The process is described below:

How To Drill

drill bit

Drilling a hole where you’d want the screw to go, run the tap through it, clean it and eventually insert the bolt. It all sounds easy, but are the steps that simple? Not really. Let us take you through it, one by one.

Step 1: Use a center punch to mark where you want the hole to be. The center punch has an in-built spring. All you have to do is point it where you’d want the hole to be and apply some pressure there. Once the spring goes off, a tiny hole should be created. It’s small, but it’ll be enough to hold the drill bit in place.

Step 2: Use the drilling machine to make the hole and then a chamfer tool to clean the start of the hole. It helps the tapping to begin smoothly.

Step 3: Start digging the tap in. We need to break the chip for the tap to go in slowly and prevent anything from breaking. The chip is essentially the metal bits that are being removed by the tap to create threads that need to be broken entirely. To do so, you need to turn the tap half-turn forward and then a quarter-turn back. Be sure to use a tool that makes it easy to rotate the tap, like a T-style handle or ratchet.

Step 4: You can thread your screw in once the tapping is done. 

5/16 vs. 1/4

The difference is simpler than it seems. 5/16 is 0.312 of an inch ( 7.93mm), while ¼ is 0.25 (6.35mm) of an inch.

The 5/16” drill bit is slightly bigger than the ¼.”


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