What Size of Drill Bit Should Be Used for a 1/8″ NPT Tap?

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Having spent countless hours drilling, and I know firsthand how frustrating it can be to damage materials. Nobody wants to waste time or money, right? That’s why choosing the right tools is crucial. So, dive into my guide below, and I’ll show you the perfect drill bit size for a 1/8″ NPT tap.

Drill Bit Size for a 1/8" NPT Tap

The R drill bit is an ideal size for a 1/8′′ NPT tap. Straight or tapered NPT taps can be used for various applications, depending on the task.

Tap SizeDrill Bit Size
1/16” NPT TapD
1/8” NPT TapR
1/4” NPT Tap7/16”

How to Use 1/8" NPT Taps

Step #1: Check the Shank

The shank of the drill bit is marked above the thread with a set of numbers.

drill bits

The drill bit’s diameter is stamped right next to the markings. Infractions are the most common way to express the diameter.

The screw’s diameter is determined by measuring it from one side’s outer thread to the other.

Step #2: Use Calipers

Calipers resemble a small flat handle with two upper jaws. To secure the screw, place the head of the screw between the jaws and press firmly with your fingers. The screw’s body should not be placed between the jaws.

black caliper

Step #3: Test the Hole

Make holes in a thin metallic sheet to ensure the drill bit size is perfect. Measure the depth of the holes and compare it to the diameter of the drill.

To be sure, insert the bit into the sample hole and see if it’s a good fit. You’re good to go if the fit is snug enough.

National Piper Thread (NPT Series)


Tapered pipes offer various applications, including ensuring the safe transportation of liquids and gases.

tapered pipe

When pressed against the flanks, the tapered nature of NPT threads makes them ideal for use as a seal to generate torque.


Issues with Leakages

  1. Using pipe threads in high-pressure systems is not recommended because they are the leakiest type of connection.
  2. Before inserting the NPT, apply a sealant to the threads. Proper measurement is needed to ensure a leak-free seal.
  3. NPT threads are commonly found in hydraulic systems.
applying sealant to pipe threads

How to Get the Right Measurement for No Leaks

  1. Holding the fitting up to the light will reveal gaps between the gauge and the thread. The deeps in male-to-male connections are more pronounced.
  2. Measure the thread diameter with a caliper [1]. For male threads, use an OD caliper and an ID caliper for female threads.
  3. Calibrate the threads using the same method for male and female threads, but at an angle to get a more accurate reading. 
  4. Use a seat angle gauge on a flared surface to determine the male thread’s seal angle.
  5. Hold the gauge up to the ceiling to ensure it’s at the proper angle. Both the centerlines and the gauge must be parallel.
  6. Once you’ve inserted the gauge’s female connection, place it against the ceiling. The gauge must be parallel to the centerlines.

1/8" Drill Bit Size in Summary

  1. The general rule for finding out the size of a drill bit is to subtract the thread pitch from its diameter.
  2. An R-shaped drill bit is required for a 1/8″ NPT tap. The 8.61 mm diameter of the R drill bit is 0.339 inches.


What size drill bit do you use for a 1/8 27 tap?

The drill bit you should use for a 1/8 27 tap is a Q drill bit. A Q drill bit is 8.433 mm or 0.3320” in diameter and is perfect for ferrous and non-ferrous materials.

What will happen if the drill bit size is imperfect?

A drill bit that’s too big or too small results in a loose drill bit. An imperfect drill can cause jerking when drilling. Injuries may result, as well as flaws in the drilling material.

Is it difficult to search for the correct bit size?

The bit size is usually specified in the correct drill model’s datasheets. Alternatively, if your drill is an antique and you’ve lost the model number, you can use a sample cut to find the proper drill bit diameter.


After trying various drill bits for a 1/8″ NPT Tap, I’ve discovered that knowing your drill’s model and practicing with sample holes makes all the difference. 

By doing so, you’ll not only pinpoint the perfect drill bit size, but you’ll also enjoy a smoother drilling experience. Trust me, a little prep work goes a long way!

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.
Robert Johnson

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