What Size of Drill Bit Should be Used for a 5/16″ Tap?

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Do you hate calculating? Does figuring out what size of drill bit to use for a 5/16” tap make your head hurt? Well, you don’t have to worry about that anymore because we did all the math for you! 

Check out the chart below. I’ve compiled all the key details you’ll need, and I’ve made sure it’s straightforward. Let’s dive in.

Drill Bit Sizes for a 5/16" Tap

The best-recommended drill bit size for a 5/16′′-18 tap is a 17/64′′ drill bit, also known as an F drill bit.

Tap SizeDrill BitDrill Bit Size
5/16” 18F17/64”
5/16” 24I17/64”
5/16” 329/32”9/32”

Understanding sizing differences is the first step in determining the size of drill bit you need for a 5/16′′ tap. When discussing the 5/16′′ tap, remember that there are two measurement variations: an 18 tap and a 24 tap.

While both are 5/16′′ taps, they have different length and thread pitch measurements. Because of the varying dimensions, the taps necessitate using different-sized drill bits.

How Does Tap Sizing Work for 5/16" Taps

Tap pitch diameter limits are commonly used by American tap manufacturers. When it comes to the diameter of the taps, these tolerances range from .0005″ for sizes #0 through 1 to.001″ or more for sizes 1-1/2″ and larger.

When I’m sizing a tap, I look at a few key factors: the tap’s diameter, the desired full thread percentage, the thread pitch, and the drilled hole size. For instance, a 5/16″ tap will always have the same diameter, whether it’s 18 or 24 inches. What changes, however, is the thread pitch.

The full thread percentage is the same whether you have a 5/16′′ tap or an 8-32 inch tap because the desired outcome determines it. The whole thread percentage typically varies between 70% and 85%, but most experts recommend 80%.

drill bits

You can calculate the required drill bit size once you have the specific measurements for the one you intend to use or own.

Even though the idea of tapping threads is straightforward, the actual process can be somewhat challenging.

Taping extends far into the middle of the hole if the hole is too tiny. Additionally, the bolt threads will not connect with the drilled hole if it is too large.

Depending on the hardness of the material being drilled, you may need to use a different tap drill size than recommended.

How to Calculate the Tap Drill Size for 5/16"

You only need to understand basic math and the size tap you’re using, which is the 5/16′′ tap, to calculate your tap drill size. First, determine the diameter of the tap. You can either measure the diameter physically or refer to the first number in the tap’s specifications.

determining the size of a drill bit

For instance, both the 18 and 24 5/16” taps have a diameter of 5/16” or 0.3125. Once you determine your target diameter, you can use the calculation DH=DMB – 0.0130 (percentage of entire thread desired/Ni).

The 5/16′′-18 tap’s numbers should be entered into the equation:

DH=.3125 – .0130(75/18).

drill bits for tap

Next up, you’ll want to determine the DH value. Once you’ve done that and round to the nearest drill bit size, you’ll land on approximately .2583. From my experience, the closest drill bit size to a 5/16″-18 tap is 17/64, which lines up with an F drill bit.

You can also use basic math to determine the relative drill bit size. We concluded that it was the closest bit because 17/64 is so close to the 5/16′′-18 (DH=.2583) tap when divided.

Why Do 18" and 24" Taps Require Different Sizes?

Having an 18 or 24 tap means you’ll need a different-sized drill bit. There are 18 and 24 threads per inch in each of these taps, meaning they are both 5/16′′ in diameter [1].

A 5/16-18 tap and 5/16-24 tap require different sizes of drill bits because they have different thread pitch diameters. An 18” has a thread pitch diameter of .2570”, while a 24” has a thread pitch diameter of .2720.

Additionally, an 18” has National Coarse threads (NC), while a 24” has National Fine Threads (NF).

drilling and tapping hole

The tap’s length is greatly influenced by the number of threads per inch. With the 18-inch tap, I can’t achieve as many threads as with the 24-inch tap, making it notably shorter. While both taps might share the same diameter, I’ve found that the variance in length means selecting different drill bit sizes.

To calculate the drill bit size, the length of the threads is a factor in determining the thread pitch. To get a drill bit that will fit the 18 and 24-inch taps, you need to pay attention to the thread pitch. If you don’t, you won’t be able to get the device to work at all.


What drill bit do I need for a 5/16 36 Tap?

You need a 7.3mm drill bit for a 5/16 36 Tap. A 5/16-36 tap is usually used for making new threads or for rethreading jammed or damaged threads. They are also popular for general machine tapping; thus, it is important to use the right drill bit.


To find out what size of a drill bit to use for a 5/16” tap, you have to determine what type you have, which is either an 18′′ or a 24′′. Because of their different thread pitches, the two will require the use of different drill bits.

A 5/16′′-18 tap requires an F drill bit as it almost matches the drilled hole size measured in inches–the DH we solved for earlier. always double-check the size you’re working with, as different sizes have their own compatible drill bit sizes. Trust me, it will save you a lot of hassle.

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Robert Johnson is a passionate furniture maker & carpenter, sought after for his knowledge on the craft.
You’ve probably seen his down-to-earth wisdom in USA Today, Bobvila, Family Handyman, and The Spruce, where he has shared commentary and guidance on various woodworking topics.

Robert is the brain behind Sawinery, where he aims to share tips, tricks, and a passion for all things carpentry.

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