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 of the threaded section. Higher TPI means finer threads. Metric threads specify pitch (mm per thread revolution) instead of TPI.

How do I select a tap drill size?

The tap drill size is the drill bit diameter to use before threading so the tap cuts the correct thread depth. The values in this chart follow the 75% thread engagement standard common for pipe threads.

Can I use metric threads on US plumbing pipe?

Metric pipe threads (ISO 7-1 parallel or tapered) are common in European and Asian equipment. They are not interchangeable with NPT or BSPT without adapters. Always verify the standard before connecting dissimilar fittings.

What is the standard for pipe threads?

In the United States the governing standard is ANSI/ASME B1.20.1 for NPT (tapered) and NPSM (straight) threads, both cut at a 60° flank angle. Internationally, BSPT and BSPP follow ISO 7-1 and ISO 228 with a 55° Whitworth form, and metric pipe threads follow ISO 7-1. This chart lists dimensions for each so you can match the correct standard.

Pipe Thread Reference Chart

Name: Pipe Thread Reference Chart
Availability: InStock
Author: Nham Vu

Look up NPT, BSPT, and metric pipe thread specs: TPI, pitch, major diameter, minor diameter, and tap drill size for each standard size.

Nominal Size	TPI	Major Dia.	Minor Dia.	Tap Drill

Selected Size

Click any row to see details.

Notes

Diameters in inches for NPT/BSPT, millimeters for Metric.

TPI = threads per inch (NPT/BSPT). Pitch = mm/rev (Metric).

Tap drill sizes follow 75% thread engagement.

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

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

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