Dimensions - Pipe Threads

PIPE / PIPE THREAD DIMENSIONS

Nominal Pipe sizes do not match any of its actual physical dimensions
The OD of pipe / fitting must be measured & compared to the table for identificationFor example, a 3/4" NPT pipe thread has an outside diameter of 1.050 inches and 14 threads per inch. Each thread size has a defined number of threads per inch (TPI), but several sizes have the same TPI; because of this, both the TPI and OD of the thread are required for positive identification of thread size.

Male threads:
Measure the outside diameter of the large portion of the thread at "A"; Find figure nearest this dimension in column 1 or 2 of chart. The dimension in column 3 will be your nominal pipe thread size.

Female Threads:
Measure top diameter of thread at "B"; Find figure nearest this dimension in column 1 or 2 of chart. The dimension in column 3 will be your nominal pipe thread size.

Column 1

Column 2

Column 3

Column 4

Column 5

OD
Fraction Inch
(only for quick reference )

Actual

Decimal Inch

Pipe Size

Nominal

TPI

Threads per Inch

Normal Engagement
for a tight joint (dimension "C")

5/16"

0.3125"

1/16"

0.2611

13/32"

0.405"

1/8"

0.2639

35/64"

0.540"

1/4"

0.4018

43/64"

0.675"

3/8"

0.4078

27/32"

0.840"

1/2"

0.5337

1-3/64"

1.050"

3/4"

0.5457

1-5/16"

1.315"

11-1/2

0.6828

1-21/32"

1.660"

1-1/4"

11-1/2

0.7068

1-29/32"

1.900"

1-1/2"

11-1/2

0.7235

2-3/8"

2.375"

11-1/2

0.7565

2-7/8"

2.875"

2-1/2"

1.1375

3-1/2"

3.500"

1.2000

4.000"

3-1/2"

1.2500

4-1/2

4.500"

1.3000

Two common pipe thread sizes exist, the tapered National Pipe Thread (NPT) and the straight National Standard Free-Fitting Straight Mechanical Pipe Thread (NPSM).
The tapered threads are for joining and sealing, the straight threads are only for joining.
Dry-seal thread (NPTF) allows for joining without sealants.
Female NPT threads may be designated as "FPT"
Male NPT threads may be designated as "MPT."
The pipe size is often written such as this 1/8"-27, with a hyphen between size & thread per inch, and pronounced as "one eighth twenty-seven"

Three less common threads exist, the Garden Hose Thread (GHT),
Fire Hose Coupling (NST) and British Standard Taper Pipe Thread (BSPT).

The NPT and NPTF threads are interchangeable with sealants such as PTFE tape.
None of the other thread standards are interchangeable.