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This post was last edited by wang202 on 2011-2-10 at 15:55. Having seen many people in the forum asking questions about symbols and threads related to pipes, I have gathered some information and would like to share it with everyone! In the metric system, it’s the pitch that matters; in the imperial system, it’s the number of threads per inch. NPT, PT, and G are all pipe threads. NPT is the abbreviation for National (American) Pipe Thread; it represents a 60-degree tapered pipe thread standard used in North America. **The relevant standard can be found in GB/T12716-1991. PT is the abbreviation for Pipe Thread; it refers to a sealing tapered pipe thread with a 55-degree angle. It belongs to the Whitworth thread family and is widely used in Europe and the Commonwealth countries. It is commonly employed in the water and gas piping industries, with a taper ratio of 1:16. The relevant standard can be found in GB/T7306-2000. G is a 55-degree unthreaded sealing pipe thread, originally part of the Whitworth thread family. The designation G denotes a cylindrical thread. **The standard can be found in GB/T7307-2001. Additionally, the markings of 1/4, 1/2, and 1/8 on threads refer to the diameter of the thread size, measured in inches. Industry insiders usually refer to thread sizes in “points”; one inch equals 8 points. Thus, 1/4 inch is equivalent to 2 points, and so on. G seems to be the general term for pipe threads; the classifications of 55 and 60 degrees are based on functional considerations. It is also commonly referred to as round pipe threads, meaning that the threads are formed by machining a cylindrical surface. ZG is commonly known as a tube taper, meaning that the thread is formed by a conical surface; most ordinary pipe fittings use this design, and it is denoted as Rc under the old national standards. Metric threads are expressed in terms of pitch, while imperial and British threads are expressed in terms of the number of threads per inch; this is their biggest difference. Metric threads have an equilateral thread profile at 60 degrees, British threads have an isosceles thread profile at 55 degrees, and American threads also have a 60-degree profile. Metric threads use metric units, while US/Imperial threads use imperial units. Pipe threads are primarily used for connecting pipes; their internal and external threads fit tightly together, and there are two types: straight pipes and tapered pipes. The nominal diameter refers to the diameter of the pipe being connected; obviously, the major diameter of the thread is larger than the nominal diameter. 1/4, 1/2, 1/8 are the nominal diameters of imperial threads, measured in inches. The size code for pipe threads is the diameter of the pipe with external threads, expressed in inches. Based on the size code, the major diameter, minor diameter, and pitch of the thread can be found in the relevant table. Note: The size designation of pipe threads does not represent the actual size of the threads. It is neither the nominal diameter nor the measurement of any specific diameter of the pipe threads themselves. The specific dimensions such as the major diameter, pitch diameter, minor diameter, and pitch of pipe threads must be determined by referring to the relevant **standards. A size 4 corresponds to 1/2 inch; the outer diameter is approximately 21.3 mm, while the inner diameter is roughly DN15. NPT 4/8 is also a code. The thread code is G1/4; the basic dimension is 1/4”, the major diameter (d=D, mm) is 13.157, the pitch (p, mm) is 1.337, the threads per inch (tpi) is 19, the minor diameter (d2=D2, mm) is 12.301, the minor outer diameter (d3) is 11.445, the thread height (H1) is 0.856, and the size of the bottom hole is 11.6 mm. 1/4\"NPT–1/4 refers to the nominal pipe diameter, which is a dimensionless value that has replaced the term \"nominal size (inches)\\" used in traditional units. The standard specifies that its outer diameter is 0.54 in (13.7 mm). An example of the designation is NPT3/4-LH: 3/4 refers to the pitch; LH indicates a left-handed thread. The base diameter of the 1/4” thread is 13.572 mm, the distance from the end of the tube to the base is 5.08 mm, the effective length of the thread is 9.5 mm, there are 18 threads per inch, the pitch is 1.411 mm, and the taper angle is given by 2tgα=1:16. M20*1.5, with a pitch of 1.5mm ； G1/2 has 14 teeth per inch, with a pitch of 1.814 mm. BSP is the general designation for imperial pipe threads, but it is divided into three types: BSPP, BSPT, and BSPF. BSPP (British Standard Pipe Parallel (thread)) is a designation for British imperial cylindrical pipe threads; it involves only internal threads. It is equivalent to China’s 55-degree cylindrical (parallel) pipe threads, namely G. BSPT (British Standard Taper Pipe Thread) is a standard for British imperial tapered pipe threads; its thread profile angle is 55 degrees, and the thread has a taper ratio of 1:16. This corresponds to China’s 55-degree tapered pipe threads, or ZG. Lastly, BSPF (British Standard Pipe Fine Hand Taps) refers to British imperial fine-pitch pipe threads ； The first two are used for sealing joints, while the latter one is used for general joining purposes. DN stands for nominal diameter in metric units; this value is not the outer or inner diameter of the pipe, but rather a value selected from a specific series. It is close to the inner diameter. The nominal diameter in imperial units is inches, 3”, 4” and so on. Φ14 refers to the outer diameter; Φ14x2 means an outer diameter of 14 with a wall thickness of 2, resulting in an inner diameter of 10 mm. When working on a project, the piping department should provide a piping classification that specifies the outer diameter, inner diameter, wall thickness, etc., for pipes corresponding to DN80 or 3\" and 4\". G3/8 and M20x1.5 are ways of indicating threads. G3/8 is the Chinese representation of the imperial standard; G stands for pipe, with a thread size of 3/8". ZG refers to tapered pipe threads. M20x1.5 is the metric notation. The imperial thread notation is 1/2” NPT(F)-14, which denotes a 1/2” tapered female thread; -14 or -18 seems to refer to the pitch of the threads. The nominal diameter can be converted to the outer diameter and wall thickness, but the others cannot be converted to each other.