# NPT and BSP Pipe Thread TPI and Outside Diameter Reference

> Source: https://netray.co/tools/npt-and-bsp-pipe-thread-tpi-and-diameter-reference
> Publisher: Netray (netray.co), AI and integration engineering for ERP and manufacturing systems.
> Section: Free Tools

## Short answer

Pipe thread sizes with TPI, pitch in mm and outside diameter for NPT and BSP (G) threads, so you stop mixing them up.

## Category

manufacturing

## Industry

Machining

## Audience

- CNC machinist
- Hydraulics fitter
- Purchasing

## Tool

**Kind:** worksheet

**Mode:** reference

**Columns:**

#### Nominal size

**Key:** size

**Type:** text

#### Thread type

**Key:** standard

**Type:** text

#### Threads per inch

**Key:** tpi

**Type:** number

**Format:** decimal

#### Pitch (mm)

**Key:** pitch

**Type:** number

**Format:** decimal

#### Major OD (in)

**Key:** od_in

**Type:** number

**Format:** decimal

#### Major OD (mm)

**Key:** od_mm

**Type:** number

**Format:** decimal

#### Form

**Key:** form

**Type:** text

**Rows:**

**Size:** 1/16

**Standard:** NPT

**Tpi:** 27

**Pitch:** 0.941

**Od in:** 0.3125

**Od mm:** 7.94

**Form:** Tapered 1:16, 60 deg

**Size:** 1/8

**Standard:** NPT

**Tpi:** 27

**Pitch:** 0.941

**Od in:** 0.405

**Od mm:** 10.29

**Form:** Tapered 1:16, 60 deg

**Size:** 1/4

**Standard:** NPT

**Tpi:** 18

**Pitch:** 1.411

**Od in:** 0.54

**Od mm:** 13.72

**Form:** Tapered 1:16, 60 deg

**Size:** 3/8

**Standard:** NPT

**Tpi:** 18

**Pitch:** 1.411

**Od in:** 0.675

**Od mm:** 17.15

**Form:** Tapered 1:16, 60 deg

**Size:** 1/2

**Standard:** NPT

**Tpi:** 14

**Pitch:** 1.814

**Od in:** 0.84

**Od mm:** 21.34

**Form:** Tapered 1:16, 60 deg

**Size:** 3/4

**Standard:** NPT

**Tpi:** 14

**Pitch:** 1.814

**Od in:** 1.05

**Od mm:** 26.67

**Form:** Tapered 1:16, 60 deg

**Size:** 1

**Standard:** NPT

**Tpi:** 11.5

**Pitch:** 2.209

**Od in:** 1.315

**Od mm:** 33.4

**Form:** Tapered 1:16, 60 deg

**Size:** 1-1/4

**Standard:** NPT

**Tpi:** 11.5

**Pitch:** 2.209

**Od in:** 1.66

**Od mm:** 42.16

**Form:** Tapered 1:16, 60 deg

**Size:** 1-1/2

**Standard:** NPT

**Tpi:** 11.5

**Pitch:** 2.209

**Od in:** 1.9

**Od mm:** 48.26

**Form:** Tapered 1:16, 60 deg

**Size:** 2

**Standard:** NPT

**Tpi:** 11.5

**Pitch:** 2.209

**Od in:** 2.375

**Od mm:** 60.33

**Form:** Tapered 1:16, 60 deg

**Size:** G 1/8

**Standard:** BSPP (ISO 228)

**Tpi:** 28

**Pitch:** 0.907

**Od in:** 0.383

**Od mm:** 9.728

**Form:** Parallel, 55 deg Whitworth

**Size:** G 1/4

**Standard:** BSPP (ISO 228)

**Tpi:** 19

**Pitch:** 1.337

**Od in:** 0.518

**Od mm:** 13.157

**Form:** Parallel, 55 deg Whitworth

**Size:** G 3/8

**Standard:** BSPP (ISO 228)

**Tpi:** 19

**Pitch:** 1.337

**Od in:** 0.656

**Od mm:** 16.662

**Form:** Parallel, 55 deg Whitworth

**Size:** G 1/2

**Standard:** BSPP (ISO 228)

**Tpi:** 14

**Pitch:** 1.814

**Od in:** 0.825

**Od mm:** 20.955

**Form:** Parallel, 55 deg Whitworth

**Size:** G 3/4

**Standard:** BSPP (ISO 228)

**Tpi:** 14

**Pitch:** 1.814

**Od in:** 1.041

**Od mm:** 26.441

**Form:** Parallel, 55 deg Whitworth

**Size:** G 1

**Standard:** BSPP (ISO 228)

**Tpi:** 11

**Pitch:** 2.309

**Od in:** 1.309

**Od mm:** 33.249

**Form:** Parallel, 55 deg Whitworth

**Size:** G 1-1/4

**Standard:** BSPP (ISO 228)

**Tpi:** 11

**Pitch:** 2.309

**Od in:** 1.65

**Od mm:** 41.91

**Form:** Parallel, 55 deg Whitworth

**Size:** G 1-1/2

**Standard:** BSPP (ISO 228)

**Tpi:** 11

**Pitch:** 2.309

**Od in:** 1.882

**Od mm:** 47.803

**Form:** Parallel, 55 deg Whitworth

**Size:** G 2

**Standard:** BSPP (ISO 228)

**Tpi:** 11

**Pitch:** 2.309

**Od in:** 2.347

**Od mm:** 59.614

**Form:** Parallel, 55 deg Whitworth

**Notes:** Pipe thread nominal sizes refer to the pipe bore, not the thread diameter, which is why 1/2 NPT has an OD of 0.840 in. NPT outside diameter is the pipe OD; thread major diameter varies along the taper. BSPT (R and Rc) is tapered like NPT but has the 55 degree form and uses the same TPI values as the BSPP rows shown. NPT and BSP do not seal together reliably even when the TPI matches.

## Content

**Intro:** NPT and BSP threads look similar and are routinely confused at the counter. This table lists TPI, pitch and outside diameter side by side, so you can identify a fitting or set up a threading cycle correctly.

**Sections:**

#### How to use the table

**Body:** Search for the nominal size, for example 1/2, and compare the NPT row with the G 1/2 BSPP row. Both use 14 threads per inch, but the NPT form is a 60 degree thread on a 1:16 taper, while BSPP is a 55 degree Whitworth thread that is parallel. The OD columns tell you which one you are holding with a caliper: 0.840 in for 1/2 NPT versus 20.955 mm for G 1/2. Export to Excel to build a fitting identification card for the shop.

**Bullets:**

- Count threads over one inch with a thread pitch gauge.
- Measure the major OD with calipers on a clean male thread.
- Check whether the thread tapers by feel and by gauge.

#### The standards behind the numbers

**Body:** NPT is defined by ASME B1.20.1, with a 1 in 16 taper on diameter, which is 3/4 in per foot, and a thread angle of 60 degrees. BSPP threads are defined by ISO 228-1 and have a parallel form at 55 degrees. BSPT (taper) is defined in ISO 7-1. NPTF dry-seal threads (ASME B1.20.3) have a different crest and root truncation to seal without tape. Always confirm the thread standard on the drawing before choosing the tap or insert.

#### Programming and cutting notes

**Body:** For turning, a pipe thread is a normal threading cycle with a taper value, so you need the TPI converted to lead (25.4 divided by TPI). For a 1/2 NPT lead is 1.814 mm, and the taper is 1:16 on diameter, or about 1.79 degrees per side. Tapered holes are normally cut with a pipe tap, and a tap for NPT will not cut a good G thread. Use the correct thread mill or insert profile, 60 degree for NPT and 55 degree for BSP.

#### Common mistakes

**Body:** The classic mistake is screwing an NPT male into a BSPP female. At 1/2 in the TPI matches, the thread angles differ by five degrees, and the fitting seems to start but will leak or damage the female thread. Another mistake is quoting a pipe thread by the thread diameter instead of the nominal pipe size. In an ERP item master, always keep the standard (NPT, BSPP, BSPT, NPTF) as a separate attribute from the size.

**Cta:** If fittings and ports are buried in item descriptions in your ERP, Netray can help structure thread type and size attributes so purchasing stops ordering the wrong standard.

**Faq:**

#### How many threads per inch is 1/2 NPT?

**Answer:** Half inch NPT has 14 threads per inch, a pitch of 1.814 mm. The outside diameter of the pipe it fits is 0.840 inches. 3/4 NPT also uses 14 TPI.

#### Can I screw NPT into BSP?

**Answer:** Not reliably. Even where TPI matches, such as 1/2 inch at 14 TPI, the thread angle is 60 degrees for NPT and 55 degrees for BSP, and NPT tapers while BSPP does not. It may start and then leak or gall.

#### What is the taper on NPT?

**Answer:** NPT tapers at 1 in 16 measured on the diameter, which is 3/4 inch per foot. That is about 1.79 degrees per side. The taper is what creates the sealing interference as the threads tighten.

#### Why is 1/2 NPT not 0.5 inch in diameter?

**Answer:** Nominal pipe size refers to the original inside diameter of standard pipe. The outside diameter of 1/2 inch pipe is 0.840 in, and the thread major diameter is based on that, not on the nominal size.

## Search terms

npt thread size chart, bsp thread chart, npt tpi chart, pipe thread od chart, npt vs bsp difference, g 1/4 thread dimensions, 1/2 npt threads per inch, pipe thread dimensions mm

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