What Size Drill Bit for a 3/8″ NPT Tap? [Answered]

As a seasoned DIY hobbyist, I understand that accuracy is paramount, especially when drilling. If you’re working with a 3/8″ NPT Tap, choosing the right drill bit can be challenging, yet it’s essential for a successful project. In this article, I’ll cut through the confusion and guide you to the precise drill bit size you need for your task. Let’s get started.

Here are some other common closely related sizes and the drill bits they require in case you are looking for information on more than one.  

As you can see from the chart, every tap size has a different drill size. If you do not use the correct drill size, you run the risk of making holes that are too large or too small which can ruin the project.  

Tap sizing depends on many different variables, so you need to make sure you have all the information about your materials before you start to make sure you get the correct sizing for everything. The tap size always depends on the tap’s diameter, the full thread percentage, and the drilled hole size.  

If you have a 3/8” inch tap, you might have ones with different thread pitches depending on which one you have bought. You will be able to find the thread pitch on the packing information or on the tap itself. You will find it as the number that is after the hyphen if there is one. For example, 3/8”-16 or 3/8”-24.  

Even though the numbers at the end are different, the taps still have the same diameter. They don’t have the same thread pitch though as you can see from the differing numbers. The thread pitch doesn’t matter too much though because it really depends on your desired outcome.  

Thread percentages range from 70 to 85 percent, however, most people who have been drilling for a long time recommend a pitch somewhere in the middle such as 80 percent.  

How to Use 3/8” NPT Tap

Getting the measurement of your tap drill size is not very complicated as you just need to do some simple math and you need to know what size tap you are using.

First, since you wanted to know what drill bit was needed for 3/8”, this is probably the size tap you are using.  

Next, you need to find the diameter of the tap. You can measure this yourself or you can look at the first number on the tap’s specifications which is a lot easier. For example, a 3/8” diameter is 0.375. The diameter is the same for all 3/8” taps regardless of what the number after the hyphen is.  

Next, you can then use the diameter figure and put it into the formula DH=DMB-.0130(% of full thread desired/Ni). It might look a bit complicated, but the formula is pretty easy once you have all the numbers because all you have to do is plug them in.  

The DMB is the diameter which you already know is 0.375. The full thread percentage is up to you as it depends on what you want it to be usually between 70 and 85. The example in this article is going to use 80. The last part of the equation is the Ni, which is the number that comes after the hyphen. So, if you are using the 3/8”-16 then the Ni is 16.  

You can insert the numbers into the formula and it will look like this:  

DH= 0.375-0.130(80/16)  

If you solve for DH you will get .275 and then you round it to the nearest fraction to get the most relative drill size. This is a great formula to know if you are having trouble knowing which drill bit size you should get.

When to Use the 3/8” NPT Tap?  

You can use a 3/8” NPT Tap for many different home improvement projects around the house or at work if you need to make something for a presentation or your office. If you have a carbon steel pipe of this size, you can use it to clean or re-thread damaged or jammed pipe threads. They are also used for threading a wide variety of materials.  

Once you have the right drill bit size, you are ready to use the NPT tap for virtually any project and threading you want to do. You can also use to make pilot holes to make sure your bolts and screws go into the material easily.

Other Considerations for 3/8” NPT Tap

You might be wondering what the differences are between the 16 and 24 sizes. They have the same diameter but the number of threads per inch is different. This is also called the thread pitch. The number of threads per inch will influence the length of the tap. The 16-inch tap will not be able to produce as many threads, so it will be shorter compared to the 24 sizes.  

Since the lengths have major differences, they will need different drill bit sizes even though they have the same diameter. If you ignore the thread pitch, you will not get a drillbit that fits and you will be stuck with a project that does not turn out well because the holes will not be the right size.  

Final Thoughts

Now that you have all the information you need about 3/8” taps, you will be able to get started on your next project with the drill bit that fits perfectly. Having the right drill bit ensures you have a perfect project.