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Thread Type Identification Guide: NPT, BSP, JIS, SAE, Metric

TecTalk blog

So you’ve found the right product at a great price. You’re ready to checkout, but there’s just one problem. You can’t figure out what thread type you need. After diligent research on thread types you’re more confused than when you started. Fortunately, we’ve got you covered.

It is imperative to identify the specific thread type you need in order for your equipment to function properly. Pneumatic components such as air cylinders, valves, and air preparation units come equipped with specific port threads. The pneumatic components we sell in our web store are available in either NPT, PT or G. Choosing the correct thread type will ensure optimum compatibility. Different thread types may look the same at a distance, but there are small differences that make them incompatible with one another. For example, G threads are not compatible with M threads nor NPT threads because of their differing angles, shapes, and thread pitches (threads per inch).

We have compiled a simple step-by-step guide to help you identify your thread type. We’ll also cover some of the most common port thread types including: NPT/NPTF, BSPP (also known as G), BSPT, PT, Metric (M) and SAE. You’ll need a couple tools on hand to make the process easier, but a straight steel ruler may work as well.


Thread Identifying Tools You Need

Caliper – Used to measure the outside diameter of a male thread and inside diameter of a female thread.


Pitch Gauge – Used to measure the threads per inch. For metric threads, this tool measures the distance between the threads.Pitch Gauge

Step 1 – Male Threads  v. Female Threads

To begin ask yourself is the thread male or female? Take a look at where the threads are placed. If they’re on the outside of the thread, they’re male. If they’re on the inside of the thread, they’re female.

Male thread type

Male Thread

Female Thread Type

Female Thread


Step 2 – Tapered Threads v. Parallel Threads

Next, determine if the thread is tapered or parallel. Tapered threads become narrower as they extend outward while parallel threads remain the same diameter. Sometimes this characteristic can be determined through visual inspection, but if not a caliper can come in handy. Use the caliper to measure the first, fourth, and final full thread. If the measurements are all equal, then it’s parallel. If the measurements decrease/increase in size, then it’s tapered.

NPT/NPTF, BSPT, and Metric Tapered are tapered threads.

Straight Versus Tapered Threads

Step 3 – Pitch Size

The next step in identifying your thread type is to determine the pitch size. A thread’s pitch size is the number of threads per inch or the distance between threads on metric thread types. While a ruler could be used to calculate the pitch size, a pitch gauge if highly recommended as pitch sizes can be very similar. Measure a couple different sizes to find the best match.

How to use a pitch gauge

Measuring pitch size with a pitch gauge


Step 4 – Thread Diameter

After you’ve figured out the pitch size, you’ll need to determine the thread diameter. Using the caliper again, measure the outside diameter on a male thread and inside diameter on a female thread. If you find that your measurements don’t exactly line up with the measurements in the tables below, this is okay. There will inevitably be small variations due to different manufacturers.

Step 5 – Thread Type Standard

The final step in identifying your thread type is to identify the thread type standard. Gather all the information from the previous steps and compare it with the measurements in the tables below.



Note: Due to variations in manufacturing the thread diameter may not match exactly with the measurements below.

NPT/NPTF Thread Type – National Pipe Tapered Fuel

This thread type is most commonly used in North America. You’ll recognize it by its tapered outer and inner diameter which is self-sealing. When torqued the flanks of the threads compress against each other forming a leak-tight seal. Yet, it is still recommended to use PTFE tape or another sealant compound to guarantee a completely leak-tight seal.

A semi-compatible variant to NPT is NPTF (National Pipe Taper Fuel). It ensures an even more leak-free seal. However, it is important to note that using these variants together diminishes their leak-free characteristics. NPT threads should be burr-free and lubricated using lubricating paste or tape. Doing so limits corrosion on the threads which otherwise can make future disassembly nearly impossible.

Dash Size

(Nominal Size)

Thread PitchMale Thread O.D. mmMale Thread O.D. inchesFemale Thread I.D. mmFemale Thread I.D. inches
-02 (1/8)2710.30.419.40.37
-04 (1/4)1813.70.5412.40.49
-06 (3/8)1817.30.6815.70.62
-08 (1/2)1421.30.8419.30.76
-10 (5/8)1422.90.9021.10.83
-12 (3/4)1426.91.0624.90.98
-16 (1)11½33.31.3131.51.24
-20 (1 ¼)11½42.21.6640.11.58
-24 (1 ½)11½48.31.9046.21.82
-32 (2)11½60.42.3857.92.29

*O.D. = Outside Diameter  I.D. = Inside Diameter


BSP Thread Type – British Standard Pipe

This is a standard thread type that has been adopted internationally for interconnecting and sealing pipe ends. You’ll find it used all throughout Europe. Two types of BSP threads exist: BSPP and BSPT. BSPP refers to parallel or straight threads. Meanwhile, BSPT refers to tapered threads. Sometimes, BSPP threads are referred to as G threads and BSPT threads as R threads.

Note: JIS tapered pipe thread (PT thread) is interchangeable with BSPT thread.

Dash Size

(Nominal Size)

Thread PitchMale Thread O.D. mmMale Thread O.D. inchesFemale Thread I.D. mmFemale Thread I.D. inches
-02 (1/8)289.70.388.90.35
-04 (1/4)1913.20.5211.90.47
-06 (3/8)1916.50.6515.20.60
-08 (1/2)1420.80.8219.10.75
-10 (5/8)1422.40.8820.30.80
-12 (3/4)1426.41.0424.60.97
-16 (1)1133.01.3031.01.22
-20 (1 ¼)1141.91.6539.61.56
-24 (1 ½)1147.81.8845.51.79
-32 (2)1159.72.3557.42.26

*O.D. = Outside Diameter  I.D. = Inside Diameter


JIS Tapered Pipe Thread Type – PT

PT thread type is identical and interchangeable with BSPT thread type. However, since the male PT thread does not have a 30-degree flare, it will not mate with the BSPP female swivel with conical seat. Also, we recommended using thread sealant with PT threads to ensure a leak-free seal.

Dash Size

(Nominal Size)

Thread PitchMale Thread O.D. mmMale Thread O.D. inchesFemale Thread I.D. mmFemale Thread I.D. inches
-02 (1/8)289.40.378.10.32
-04 (1/4)1913.70.5312.40.49
-06 (3/8)1917.20.68160.62
-08 (1/2)1421.50.8419.80.77
-10 (5/8)1423.10.9120.60.81
-12 (3/4)1426.91.0625.41
-16 (1)11341.3431.81.25
-20 (1 ¼)1142.61.6840.41.59
-24 (1 ½)1148.51.946.21.81
-32 (2)1160.42.3758.22.29

*O.D. = Outside Diameter  I.D. = Inside Diameter


SAE Thread Type – Straight Thread O-Ring Boss

SAE straight threads are able to seal because of the 90-durometer Buna-N “O” Ring. This is a highly reliable and reusable thread type. Some thread types require the threads of the male and female end to crush together to form a seal, while the O-Ring on this thread type prevents that.

Dash Size

(Nominal Size)

Thread PitchMale Thread O.D. mmMale Thread O.D. inchesFemale Thread I.D. mmFemale Thread I.D. inches
-02 (1/8)243.90.316.90.27
-03 (3/16)249.60.388.60.34
-20(1 ¼)1241.41.6339.11.54
-24(1 ½)1247.71.8845.51.79

*O.D. = Outside Diameter  I.D. = Inside Diameter


Metric Tapered/Parallel Thread Type

Metric thread type is most common in Europe. It has a cylindrical inner and outer diameter precise in millimeters. The fine taper of metric tapered thread allows for the best possible force transmission. In writing, you can identify metric threads by a capital “M” plus an indication of their nominal outside diameter (ex. M22 x 1.5). Lastly, when measuring pitch size make sure you’re using a metric pitch gauge.

SI Metric Port Size mmThread Pitch mmMale Thread O.D. mmMale Thread O.D. inches
M5 × 0,8.850.1968
M8 × 1,0180.3150
M10 × 1,01100.3937
M12 × 1,51.5120.4724
M14 × 1,51.5140.5512
M16 × 1,51.5160.6299
M18 × 1,51.5180.7087
M22 × 1,51.5220.8661
M27 × 2,02271.063
M33 × 2,02331.299
M42 × 2,02421.654
M50 × 2,02501.969
M60 × 2,02602.362

*O.D. = Outside Diameter


Thread types can be a little overwhelming to grasp at first. There’s a lot to take in. The types we’ve covered in this guide barely scratch the surface of the many different kinds available. However, having the right tools can a make a world of a difference. We hope this guide is useful and insightful. If you have any questions, feel free to contact us or simply leave a comment below.


2 Responses to Thread Type Identification Guide: NPT, BSP, JIS, SAE, Metric

  1. James Bergman December 15, 2016 at 3:02 pm #

    When I am trying to get a tight seal on a pipe or line, is it best to use female thread connectors or male connectors? My gut is telling me that the female threads will work better. I can see fluid traveling through a pipe hitting the sides of the male connectors and being forced out, something that wouldn’t happen in a female connector.

    • Trimantec December 16, 2016 at 9:00 am #

      Hi James, thanks for reading our blog on identifying thread types. The male and female designation simply refers to whether the threads are on the outside or inside of the connector. If you’re experiencing problems with getting a tight seal, you may need to check if you have the correct thread size/type. It could also indicate the right amount of torque hasn’t been applied. In addition, using a thread sealant will further ensure a leak-free connection.
      We hope this helps to answer your question!