What is the difference between NPT and BSPT?

Both are tapered pipe thread standards, but they use different thread angles and are not interchangeable. NPT (National Pipe Taper, ANSI/ASME B1.20.1) has a 60° thread angle, while BSPT (British Standard Pipe Taper, ISO 7/1) uses a 55° Whitworth thread form. Mixing them can cause leaks or cross-threading even if they seem to engage initially.

Are NPT and BSP threads compatible?

No. While some sizes appear to thread together due to similar pitch, the different thread angle (60° NPT vs 55° BSP) means they do not seal reliably and should never be used together in pressurized systems.

What does nominal pipe size mean?

Nominal pipe size (NPS) is a designator, not an actual measurement. A 1/2" NPT fitting does not have a 0.5" thread diameter — the major thread diameter is approximately 0.840". The nominal size roughly corresponds to the bore of the pipe the fitting connects to.

TPI stands for threads per inch — the number of complete thread crests counted along one inch. This chart displays BSP parallel Rp pitch as millimeters per thread revolution.

How do I select a tap drill size?

The tap drill is a pre-drill recommendation for an internal thread. Do not compare it directly with the external-thread minor diameter shown in the adjacent column: internal and external thread limits are different. Confirm the tap maker's chart, material, engagement target, and applicable standard before fabrication.

Are BSP parallel Rp threads metric threads?

No. Rp is a parallel 55° Whitworth-form pipe thread in the BSP family. Its dimensions may be listed in millimeters, but it is not an ISO 261 M-profile metric thread and is not interchangeable with NPT.

What standards does this chart cover?

NPT follows ANSI/ASME B1.20.1. BSP tapered threads follow ISO 7-1, while BSP parallel thread dimensions are associated with the ISO 7-1/ISO 228 BSP family. Verify the exact fitting designation and applicable edition before fabrication.

Pipe Thread Reference Chart

Look up NPT, BSPT, and BSP parallel (Rp) thread pitch, external basic diameters, and separately recommended tap-drill sizes.

Nominal Size	TPI	External Major Dia.	External Minor Dia.	Tap Drill

Selected Size

Click any row to see details.

Notes

Diameters in inches for NPT/BSPT, millimeters for BSP Parallel (Rp).

TPI = threads per inch (NPT/BSPT). Pitch = mm/rev (BSP Parallel Rp).

External diameters and internal tap-drill recommendations use different thread limits; they are not direct geometric checks against each other.

Tap drills are starting recommendations only—verify the tap manufacturer and governing standard.

NPT: ANSI/ASME B1.20.1 — 60° thread angle.

BSPT: ISO 7/1 — 55° Whitworth thread angle.

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Summary

A lookup table for NPT (National Pipe Taper), BSPT (British Standard Pipe Taper), and BSP parallel Rp threads commonly specified with metric dimensions. The Rp rows follow the BSP family, not ISO 261 M-profile metric threads.

How it works

Select a thread standard (NPT, BSPT, or BSP Parallel Rp) using the tab buttons.
Use the size filter to jump directly to a nominal pipe size.
The table shows TPI or pitch, external-thread basic major/minor diameters, and a separate internal-thread tap-drill recommendation.
Click any row to highlight it and copy its values to the detail panel.
Use the search box to filter rows by size designation.

Use cases

Selecting the correct tap drill size when threading pipe fittings.
Verifying thread compatibility between NPT and BSPT connections.
Specifying pipe thread dimensions on fabrication drawings.
Identifying an unknown fitting by measuring its major diameter.
Cross-referencing metric and imperial pipe thread standards on imported equipment.
Checking TPI before ordering threading dies for field repair.

Frequently Asked Questions

Last updated: 2026-06-15 · Reviewed by Nham Vu