Pipe Thread Standards

• National Pipe Thread (NPT) - a tapered thread
• National Standard Free-Fitting Straight Mechanical Pipe Thread (NPSM) - a straight thread

• Garden Hose Thread (GHT)
• Fire Hose Coupling (NST)
• British Standard Taper Pipe Thread (BSPT)

Female NPT threads can be designated as "FPT" and male NPT threads can be designated as "MPT." I have also seen female NPT threads designated as "FNPT" and male as "MNPT".

Straight Versus Tapered Threads

Straight threads only provide a mechanical junction. They don't really seal.

Tapered threads both join and seal. The deeper you drive them, the tighter the seal.

NPT is a tapered thread.

Pipe Thread Tape

In addition to sealing, pipe thread tape also lubricates the joint, which:

• makes it easier to tighten it
• makes it easier to disassemble the joint in the future
• reduces/eliminates thread galling [Thread galling is when threads weld themselves together. This is more common with pipes and fasteners made from alloys that protect themselves from corrosion by developing their own oxide surface film, like aluminum and stainless steel.]

Standard Pipe Sizes

Historical note:

• The sizes derive from the days when pipes were made of iron, pipe walls were rather thick, and the internal diameter (ID) was standardized. At that time, if you specified a 1/2" pipe, it had an internal diameter of 1/2", and an outer diameter that depended on the thickness of the wall.
• Nowadays, the standards measure the threaded part of the pipe, the outer diameter (OD). This lets thin-wall and thick-wall pipe use the same fittings.
• But we kept the same names, so when you say 1/2" pipe, you get a pipe whose outer diameter is the same as that of an old-fashioned iron pipe whose inner diameter was 1/2".

In order to get the right size, you should measure the outer diameter (OD) of the pipe, and look it up in a table to find out what it is officially called. It's also a good idea to measure the threads per inch (TPI). The combination of OD and TPI provide positive identification of standard pipe sizes.

Pipe Thread Size	Threads per Inch	OD Fraction (approximate)	OD Decimal (actual)	Normal depth for tight joint
1/16"	27	5/16"	0.3125"	0.2611"
1/8"	27	13/32"	0.405"	0.2639"
1/4"	18	35/64"	0.540"	0.4018"
3/8"	18	43/64"	0.675"	0.4078"
1/2"	14	27/32"	0.840"	0.5337"
3/4"	14	1-3/64	1.050"	0.5457"
1"	11-1/2	1-5/16"	1.315"	0.6828"
1-1/4"	11-1/2	1-21/32"	1.660"	0.7068"
1-1/2"	11-1/2	1-29/32"	1.900"	0.7235"
2"	11-1/2	2-3/8"	2.375"	0.7565"
2-1/2"	8	2-7/8"	2.875"	1.1375"
3"	8	3-1/2"	3.5"	1.2000"
3-1/2"	8	4"	4.0"	1.2500"
4"	8	4-1/2"	4.5"	1.3000"

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