What Size Drill Bit for 1/2 Tap?

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Drilling operations are very common in every household, be it to fix a photo frame on the wall or install fasteners in them. While the process seems straightforward, specific parameters need to be checked before proceeding. Here, you’ll see what size drill bit is needed for a 1/2 NPT tap.

What Size Drill Bit for 1/2 NPT Tap?

Before seeing what drill size is needed for a 1/2 NPT tap, it is useful to see how to size a drill bit.

Step 1: Find the Diameter

Drill bit diameters, written in fractions, are stamped near the markings present on the bit’s shank. It is usually above the thread of the bit.

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

A drill surrounded by measuring tools, a floor plan and screws

Step 2: Take Measurements

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

Calipers are small flat handles with two jaws on the top. It’s a simple machine which is easy to use. The bottom jaw can slide up and down.

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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 thread of the screw between the jaws.

Step 3: Make a Test Hole

The wrong size of the drill bit can result in an imperfect hole, even if your drill is a brand new one. Therefore, you should always make a test hole for better results.

  1. Measure the depth of the sample holes and tally them with the size of the drill bit. This is done to check the usability of the bit.
  2. As a precaution, you can also 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, the abbreviated version of American Nation Standard Pipe Thread.

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 useful. Unfortunately, this also means their sizing requires a lot of care. Some characteristics include the following:

  • The taper and the center axis of the pipe are at an angle of 1° 47′ 24″ (1.7800°) with each other.
  • The roots and crests have a flat truncation.
  • The thread angle is exactly sixty degrees.
  • Thread per inch (TPI) is the unit for measuring the pitch of the threads.
Small drill bits lined up on display

Leakage Issues

NPT threads may have some leakage issues associated with them.

  • NPT threads were initially used in hydraulic systems despite being designed for water pipe plumbing (60 psi).
  • Pipe threads leak the most compared to other kinds of connections. It is not recommended for high-pressure applications.
  • Sealants need to be placed on NPT before assembly. The stated leakage-related issues demand proper measurement for a leak-free seal.

Measurement for a Leak-Free Seal

You need to engage as many threads as possible to get an accurate reading. Use different thread gauges to find the tightest fit; trial and error is the way to go.

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  1. Check for gaps between the thread and gauge. Hold it against some light source for easier detection. The gaps in a male connection are easy to detect.
  2. Use OD caliper to measure male thread diameter and ID caliper to measure female thread diameter.
  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. 

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 ½ NPT tap.

  • 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.
  • A ½ tap has several types and depending on that, and you need to choose the size of the bit.
  • A ½-13 NC tap requires a 27/64″ size, ½-20 NF, and ½-24 NS requires a 29/64″ size.

To know more about what drill bit size to use for 1/4″ taps, 5/16″ taps, or more, this article has more information on NPT threads.

A dril with inserted drill bitl on a wooden table with screws

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, sharpen it, 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. 


The purpose of using a drilling machine is to ensure you get a hole of perfect dimension in the material you desire. To get this done, you have to ensure your drill bit is of the perfect size. If you are patient enough during the process and take all the necessary steps, you will drill like an expert.

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.