Thread Standards

To ensure compatibility and ease of maintenance, threads are standardized. For existing and new applications, it is necessary to identify the thread type to guarantee a proper connection of the joint.

Table of Contents

Common Thread Standards

BSP – British Standard Pipe

The BSP, or Whitworth thread, is a family of thread standards that has been adopted internationally, except in the United States. This thread form is based on a 55° V-thread with rounded roots and crests, as seen in Figure 1. For a thread that conforms to BSP, the major diameter of the pipe thread is slightly smaller than the actual OD of the pipe, and the minor diameter will be very close to (smaller than) the inside diameter of the female thread. There are two types of BSP threads:

  1. BSPP: Both the male and female threads are parallel. BSPP connections are widely used in UK, Europe, Asia, Australia, New Zealand and South Africa. Sizes can be seen in Table 1.
  2. BSPT: The male threads are tapered and the female threads are commonly parallel. BSPT connections are especially popular in China and Japan. Sizes can be seen in Table 2.
A BSPP thread profile (left) and a BSPT thread profile (right)

Figure 1: A BSPP thread profile (left) and a BSPT thread profile (right)

BSP threads are identified with letters each of which represents the type of thread and their associated standards1:

  • G: external and internal parallel (ISO 228, DIN 259) - BSPP
  • R: external taper (ISO 7, EN 10226, BS 21, JIS B 0203) - BSPT
  • Rp: Internal parallel (ISO 7-1, EN 10226) - BSPT
  • Rc: internal taper (ISO 7) - BSPT
  • Rs: external parallel (BS 21) - BSPT- Obsolete

1ISO 7: Pipe threads where pressure-tight joints are made on the threads. ISO 228: Pipe threads where pressure-tight joints are not made on the threads.

Labeling Example: EN 10226 Rp 2 ½
This refers to a British Standard Pipe thread tapered (EN 10226) with an internal parallel form (Rp) and a nominal size of 2 ½.

The actual sizes of the most commonly used BSP threads are listed in Table 1 and Table 2 for BSPP and BSPT threads respectively. Table 3 provides data on pipe sizes associated with these threads.

Note: Each thread size is identified with a number which has little to do with the actual size of the thread. This discrepancy originates from amendments in industrial practices and standardisation throughout the history of standardisation of pipe threads.  Therefore, always compare measurements with actual sizes listed in the tables.

Table 1: BSPP (G) – British Standard Pipe Parallel
Nominal Thread Size Major Diameter (mm) Minor Diameter (mm) TPI (in-1)
G 1/16 7.723 6.561 28
G 1/8 9.728 8.566 28
G 1/4 13.157 11.445 19
G 3/8 16.662 14.950 19
G 1/2 20.955 18.631 14
G 3/4 26.441 24.117 14
G 1 33.249 30.291 11
G 2 59.614 56.656 11
Table 2: BSPT (R/Rp) – British Standard Pipe Tapered
Nominal Male Tapered Thread Size (inch) Nominal Female Parallel Thread Size (inch) Major Diameter (mm) Minor Female Diameter (mm) TPI (in-1)
R 1/16 RP 1/16 7.723 6.490 28
R 1/8 RP 1/8 9.728 8.495 28
R 1/4 Rp 1/4 13.157 11.341 19
R 3/8 Rp 3/8 16.662 14.846 19
R 1/2 Rp 1/2 20.955 18.489 14
R 3/4 Rp 3/4 26.441 23.975 14
R 1 Rp 1 33.249 30.111 11
R 2 Rp 2 59.614 56.476 11

Table 3: British Standard Pipe Dimensions for Standard Thread Sizes
Nominal G / R size (in) Corresponding Pipe
DN (mm) Actual OD (mm) Wall (mm)
1/16 3
1/8 6 10.2 2
1/4 8 13.5 2.3
3/8 10 17.2 2.3
1/2 15 21.3 2.6
3/4 20 26.9 2.6
1 25 33.7 3.2
2 50 60.3 3.6
NPT thread profile

Figure 2: NPT thread profile

NP – National (American) Pipe Thread

The American national pipe thread was created based on a 60° V-thread with flattened peaks and valleys (Figure 2) and is widely used in the US and Canada. There are two types of NP threads:

  1. NPS: Straight threads, meaning the male and female threads are parallel.
  2. NPT: Tapered threads, meaning the male and female threads are tapered. NPT threads are more commonly used. NPT Thread specifications are based on ANSI B1.20.1 and the threads sizes can be seen in Table 4.
Table 4: NPT – National (American) Pipe Thread Tapered
Nominal Thread Size (inch) Major Diameter (mm) TPI (in-1)
1/16 7.950 27
1/8 10.287 27
1/4 13.716 18
3/8 17.145 18
1/2 21.336 14
3/4 26.670 14
1 33.401 11.5
2 60.325 11.5

Labeling Example:
 3/8 - 18 NPT

National pipe threads are designated with their nominal size (3/8) followed by number of threads per inch (18) and the symbol for the thread series (NPT).

Note: NPS (National Pipe Straight) is not to be confused with NPS (Nominal Pipe Size) which is an American set of standards for pipes. For a given outside diameter, NPS (Nominal Pipe Size) provides multiple pipe schedules (wall thicknesses) while the pipe thread profile remains the same among them.

M – Metric Thread (ISO)

Metric Thread profile

Figure 3: Metric Thread profile

The Metric Thread is one of the first internationally agreed general-purpose thread type. The V-shaped thread form has a 60° flank angle and male and female threads are both parallel (Figure 3). Metric Threads come in different pitch sizes for a given diameter: coarse pitch and fine pitches. Coarse threads have the default pitch size whereas fine threads have smaller pitch sizes and are used less often. As a result, coarse threads are identified by diameter only while fine threads are recognized by diameter as well as pitch size.

Metric threads come in two different pitch sizes for a given diameter:

  1. Coarse Pitch: These have the default pitch size according to Table 5 and are the most commonly used. They adhere to ISO 724 (DIN 13-1).
  2. Fine Pitch: These have a smaller pitch size and in the labeling they require the diameter of the pitch size. They adhere to ISO 724 (DIN 13-2 to 11).

Labeling Example: M8
This refers to a metric coarse thread with a diameter of 8 mm (which with reference  Table 5, corresponds to a pitch size of 1 mm)

Labeling Example: M4 x 0.5
This indicates a fine thread with a diameter of 4 mm and a pitch size of 0.5 mm. 

M 10M 10

Table 5: Metric Threads (Coarse)
Thread Size (mm) Major Diameter (mm) Minor Diameter (mm) Pitch (mm)
M 3 2.98 2.459 0.5
M 4 3.978 3.242 0.7
M 5 4.976 4.134 0.8
M 6 5.976 4.917 1
M 8 7.974 6.917 1
M 10 9.968 8.376 1.5
M 12 11.97 10.106 1.75
M 16 15.96 13.835 2
M 20 19.96 17.294 2.5
M 24 23.95 20.752 3

How to identify an unknown thread type?

Pitch gauge

Figure 4: Pitch gauge

To identify an unknown thread, verify the following:

  • Gender: Male or female.
  • Taper: Parallel or tapered. This can be determined whether by visual inspection or by measuring the diameter of the thread at the beginning and the end via a caliper.
  • Thread Diameter: For male threads, measure the major diameter and for female threads measure the minor diameter. Note that this diameter will always be different from the nominal size of the thread.
  • Thread Pitch: For this purpose, a pitch gauge can be used (Figure 4). Make sure to measure multiple samples to confirm accurate pitch. If a pitch gauge is not available, count the number of threads in a 1-inch span (TPI) and the reciprocal is your thread pitch in inches. If you are skeptical that the thread is metric, count the number of threads in a 10-mm span. The reciprocal of this number is your pitch size in mm.

Check Tables 1-5 to find a matching actual diameter corresponding to a nominal thread size. Confirm thread pitch or TPI with the values corresponding to that diameter.

Thread Compatibility

Although some BSP and NPT threads might be tightened and engaged together fairly well, pitch and thread angle differences between them will allow spiral leakage. Specifically speaking, at 1/16”, 1/8”, 1/4”and 3/8” sizes, pitch difference results in misalignment of the threads. This misalignment might result in the threads to be fastened only partially. However, at 1/2” and 3/4” sizes, NPT and BSP have the same pitch which will allow them to engage well. Nevertheless, since there are differences in thread forms and thread angles, there will still be issues with this connection. For low end applications, if an appropriate thread sealant is used, these threads might be used together effectively. Table 6 shows the compatibility of threads and the required seal type to be used as a reference for your application.

Table 6: Thread Compatibility Chart
My Fitting Designation Matching Fitting Designation Seal Type Notes
BSPP Parallel Female G BSPP Parallel male G O-ring/washer Rs or NPT male threads are incorrect installations here. Rs threads are slightly smaller than G threads, and NPT threads have a different form.
BSPT taper male R Tape/pipe dope
BSPT Parallel Female Rp BSPT taper male R Tape/pipe dope G threads are incorrect installation here as they have different tolerances.
BSPT Taper Female Rc BSPT parallel male Rs Tape/pipe dope Inserting an NPT male here is an incorrect installation, as NPT threads have a different form.
BSPT taper male R Tape/pipe dope
NP Taper Female NPT NP taper male NPT Tape/pipe dope BSP threads are not to be used with NPT threads even if the threads seem to engage fairly well.
NP Parallel Female NPS NP parallel male NPS O-ring/washer NPT thread is an incorrect installation here as sealing is not possible.

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