What Size Drill Bit for a 1/8″ NPT Tap?

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Drilling is often performed in households or in industries for multiple purposes. To make the process smooth and worth the effort, you should know the right drill bit size for the operation. A wrong bit size may cause damage to the material used.

Here, you’ll see what size drill bit is needed for a 1/8″ NPT tap.

What Size Drill Bit For a 1/8″ NPT Tap?

The best drill bit size for 1/8″ NPT tap is a R drill bit. NPT taps are generally straight or tapered in shape depending on the type of job you use it for.

Screw SizeDrill Bit Size
1/16″ NPT TapD
1/8″ NPT TapR
1/4″ NPT Tap7/16″
Person selecting the right size bit for 1/8 NPT tap

Step 1: Check The Shank

There are markings on the shank of the drill bit above the thread.

You will find the drill bit diameter stamped near the markings. The diameter is usually written infractions.

The diameter of the screw is measured from the outer thread of one side to the outer thread of the other side.

Step 2: Use Calipers

They look like a flat and small handle with two jaws on the top. It’s a simple machine and an easily usable one. The bottom jaw can slide up and down.

Place the body of the screw between the jaws and then press them with your fingers to hold the screw in place. Don’t place the body of the screw between the jaws.

Calipers are easily available in any local home supply store. It is essential for measuring the drill bit’s size.

Step 3: Test The Hole

Even if you have bought a new drill bit or sized an old one using calipers, you would not want to have an imperfect drill due to improper sizing.

  1. To ensure the drill bit size is perfect, make some sample holes in a thin metallic sheet. Measure the depth of these holes and tally them with the size of the drill bit.
  2. To be sure, put the bit inside the sample hole and check the fit. If the fit is tight enough, you are good to go.
  3. Now that you know how to size a drill bit, it is good to know some details about National Piper Thread or NPT. This is the abbreviated version of American Nation Standard Pipe Thread.
Threading dies and taps in an old dusty workshop

The NPT series contains variously tapered and straight thread series and has standards for testing various parameters like rigidity or pressure-tight sealing, or both.


Tapered pipes can be used for several purposes, like effectively sealing materials used for transporting solids, liquids, or gases.

The tapered nature of NPT threads allows them to be used as a seal to generate torque when pressed on flanks. This video has more information on the uses of NPT threads.


There are certain characteristics of NPT threads that make them very unique and helpful. Unfortunately, this also means their sizing requires a lot of care. Some factors include the following:

  • The center axis and taper of the pipe have an angle of 1° 47′ 24″ (1.7800°) between them.
  • The crests and roots have a flat truncation.
  • The thread angle is exactly sixty degrees.
  • Threads per inch (TPI) is the unit used to measure the pitch.

Leakage Issues

NPT threads may have some leakage issues associated with them.

  1. NPT threads are mainly used in hydraulic systems despite being initially designed for water pipe plumbing (60 psi).
  2. Pipe threads are the leakiest style of connections and are not generally used in high-pressure systems.
  3. Use a sealant on the threads before placing the NPT for assembly. The stated leakage-related issues demand proper measurement for a leak-free seal.

Measurement For A Leak-Free Seal

Find the tightest fit by trying out different sizes of threads. Trial and error is the best way to do it. For an accurate reading, engage as many threads as possible.

  1. Look for gaps between the gauge and thread by holding the fitting against the light. The gaps are more prominent in male connections.
  2. Use a caliper to measure the thread diameter. Use OD caliper for male threads and ID calipers for female ones. 
  3. For male threads, hold the caliper at a slight angle, and for female ones’, hold the caliper perpendicular to the thread. This ensures a more accurate reading.
  4. To determine the pipe or connection seal angles of the male thread, use a seat angle gauge. Place the arrangement on a flared surface to get a proper reading.
  5. To check if the correct angle is reached, hold the gauge against the surface of the ceiling. The centerlines must be parallel with the gauge.
  6. Place the female connection against the ceiling after inserting it into the gauge. If the gauge is parallel to the centerlines, the correct angle is achieved. 
Drill bit kit

Size Of The Drill Bit

Now that you know how to measure for a leak-free seal, you need to know the size of the drill bit required for a 1/8″ NPT tap.

  1. As a general rule, the size of the drill bit required can be easily estimated by subtracting the pitch of the thread from its diameter.
  2. A 1/8″ NPT tap requires an R-shaped drill bit. The R drill bit corresponds to a size of 0.339 inches or 8.61 mm.

Frequently Asked Questions

Is It Difficult to Find the Correct Bit Size?

The bit size is usually specified in datasheets, and if you can refer to the correct drill model, getting the correct size will not be a problem. However, if your drill is old and you can’t find the model number, make some sample cuts and measure the depth of the hole to find out the size of the drill bit needed.

What Happens if the Drill Bit Size Is Imperfect?

An imperfect size of the drill bit will result in a loosely attached bit. Drilling with an imperfect drill can result in unusual jerking. This can cause injury and also imperfections on the drilling material


Choice of the drill bit size is a simple but essential step to be taken before the drilling operation. If you carefully find the model of your drill or make sample holes, you will find the perfect drill bit size and have an easy time while drilling.

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.