What Size Drill Bit for a 5/16″ Tap

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So you’ve got yourself a 5/16″ tap, now what? You need the matching drill bit. More than that, since 5/16″ taps come in two sizes, either an 18 or 24 inch, you need a drill bit that corresponds to the specific size.

How do you know what size you need and why the different size drill bits matter? Let’s get into it. Below, we’ve completed a guide covering everything you need to know about the 5/16″ tap for both sizes. We’ve covered how to figure out what size you need for your 5/16″, how to calculate it on your own, and ultimately, what drill bits to get.

Keep reading for an in-depth explanation of what size drill bit you need for a 5/16 inch tap.

What Size Drill Bit for 5/16″ Tap?

The best overall size drill bit for the 5/16″-18 tap is 17/64″ drill bit, which corresponds to an F drill bit.

Screw SizeDrill BitDrill Bit Size
5/16″ 18F7/64″
5/16″ 24I7/64″

Figuring out what size drill bit you need for a 5/16″ tap starts with understanding a bit about sizing differences. When we talk about the 5/16″ tap, keep in mind that there are two measurement variations; an 18 tap and a 24 tap.

While the two are 5/16″ taps, they have different measurements relating to their length and thread pitch. These varying measurements lead the taps to require other-sized drill bits.

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Drill bits

To figure out what size drill bit you need for your 5/16 tap, look at the tap’s specific length or thread pitch. From there, you can determine the appropriate drill bit you’ll need.

How Tap Sizing Works for 5/16″ Tap

Tap sizing depends on several variables, including the tap’s diameter, desired full thread percentage, the thread pitch, and the drilled hole size. For a 5/16″ tap, either an 18 or 24 inch, the diameter remains the same, but the thread pitch differs.

The full thread percentage doesn’t matter whether you have a 5/16″ tap or an 8-32 inch tap because it depends on your desired outcome. Typically the whole thread percentage ranges between 70 and 85 percent, but most users and professionals recommend 80%.

When you have the specific measurements for the one you plan to use or own, you can calculate the needed drill bit size

How Do You Calculate Tap Drill Size for 5/16″?

To calculate your tap drill size, you need only a basic understanding of math and what size tap you’re using, which is the 5/16″ tap. First, you need to get the diameter of the tap. You can physically measure the diameter or look to the first number in the tap’s specifications instead.

Cordless drill placed on top of a wooden pallet

For instance, the diameter for both the 18 and 24 5/16″ taps is 5/16″ or .3125. Once you have the diameter, you can insert it into the formula DH=DMB -.0130(% of full thread desired/Ni). This formula looks a bit complicated, but don’t worry.

  • The DMB is the diameter measurement you figured out above, .3125.
  • The full thread percentage depends on what size thread you want, typically between 70 and 85, but we’ll use 75 for this example.
  • The other variable, Ni, is the other part of the tap. Let’s use the 5/16″-18 tap, for instance. The Ni for that tap is 18.

Insert the numbers for the 5/16″-18 tap into the equation to look something like this:

DH=.3125 – .0130(75/18)

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Then, solve for DH. You should get around .2583, which you then round up to the nearest drill bit size. In this case, the most relative drill bit size corresponding to the 5/16″-18 tap is 17/64, which corresponds to an F drill bit.

You can find the relative drill bit size with some simple math as well. For example, we found 17/64 as the closest bit in relation to the 5/16″-18 (DH=.2583) tap because, when divided, 17/64 is only a couple of thousands away.     

Why Do 18″ and 24″ Taps Need Different Sizes?

The fact is, you’ll need a different size drill bit depending on whether you have an 18 tap or a 24 tap. So, what’s the difference between the two? They both have the same diameter, 5/16″, but the 18 and 24 relate to the tap’s thread pitch, the number of threads per inch.

The number of threads per inch influences the length of the tap. The 18 inch tap isn’t capable of producing as many threads as the 24 inch tap and is, therefore, shorter in comparison. Because of this length difference between the two taps, they will need different drill bit sizes even though they have the same size diameter.

Selecting what size drill bit for a 5/16 tap

Their lengths directly influence the calculations since thread pitch is a component of calculating the needed drill bit size. If you ignore the thread pitch component for the 18 and 24 inch taps, you won’t get a drill bit that fits, which will not only prevent the device from working correctly but won’t work at all.  

What Size Drill Bit Do I Use for 5/16″ Tap?

It depends on the type of 5/16″ tap you have, either an 18″ or 24″. Because the two have different thread pitches, they’ll require various drill bits to fit and work correctly.

To save you the trouble of going through the sizing formula, a 5/16″-18 tap, you’ll need an F drill bit because it most closely matches up to that tap’s drilled hole size measured in inches–the DH we solved for in the previous calculations.

Based-off that formula, the 5/16″-24 tap will require an I drill bit. Just like for the 5/16″ 18inch tap, the I drill bit’s measurements most closely relate to the 5/16″-24 tap’s DH.

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Get Drilling with a 5/16″ Tap

Now that you know the fundamentals behind the 5/16″ tap measurements and drill bit sizing, you can put your knowledge to work and get to work using it. Just make sure you’re aware of what specific size you have–either the 5/16-18 inch or the 5/16-24 inch–as both have different compatible drill bit sizes. Once you’ve got that squared away, go on and get to work.

An expert at home repair, remodel, and DIY projects for nearly 40 years. His first experience came in completely restoring an antique home. Completely redone from the inside out, and restored to its original form, the home is a featured design by renowned Southern California Architect Cliff May, considered to be the father of the California Ranch Home. Now Dennis spends his time on fine woodworking projects and tool comparisons.