Pipe Thread Size Chart: NPT and BSP Thread DimensionsCommon pipe thread nominal sizes, TPI, pitch, and outside diameter references

Use this pipe thread size chart to look up common NPT and BSP-family nominal sizes, threads per inch, pitch in millimeters, and approximate major or outside diameter values. Nominal pipe size is a trade size, not the measured outside diameter. NPT, BSPP, BSPT, NPS, and NPSM threads are not interchangeable by simple inch-to-mm conversion; always verify the exact standard, thread form, taper or parallel type, sealing method, material, pressure rating, and code requirements before choosing a fitting.

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NPT and BSP Lookup

Compare common nominal pipe thread rows by family, TPI, pitch, and approximate outside diameter.

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TPI to Pitch in mm

See the metric pitch equivalent calculated from threads per inch using 25.4 ÷ TPI.

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Interchange Caveats

Use the chart as a reference only; NPT, BSPP, and BSPT differ by thread form, taper, and sealing method.

Answer first: nominal pipe size is not actual outside diameter

Answer first: nominal pipe size is not the actual measured outside diameter. For example, 1/2 NPT is commonly referenced with an approximate major outside diameter of 0.840 in, or about 21.34 mm, and 14 threads per inch, which is about 1.814 mm pitch. That does not mean the pipe thread is 1/2 inch across. Pipe thread families such as NPT, BSPP, BSPT, NPS, and NPSM must be checked by standard, thread form, taper or parallel design, sealing method, pressure rating, material, and code requirements.

Reference only: do not select or substitute pipe threads by inch/mm dimensions alone. Nominal size is not measured OD, and NPT, BSPP, BSPT, NPS, and NPSM can differ by thread angle, taper, pitch, sealing method, pressure rating, material, and code requirements. This chart is not a pressure-system design or safety compliance tool.

Pipe thread dimension chart

All NPT and BSP-family rows are crawlable HTML text. Pitch mm = 25.4 / TPI. Inch basis: 1 inch = 25.4 mm.

Print table

Print both NPT and BSPP/BSPT reference tables with the safety caveats included.

NPT tapered pipe threads

Nominal size	Thread family	Type	TPI	Pitch mm	Approx. major / OD (in)	Approx. major / OD (mm)	Caveat
1/8	NPT	Tapered	27	0.941 mm	0.405 in	10.29 mm	NPT tapered thread reference; nominal size is not actual measured OD and does not define pressure rating, seal method, or code compliance.
1/4	NPT	Tapered	18	1.411 mm	0.540 in	13.72 mm	NPT tapered thread reference; nominal size is not actual measured OD and does not define pressure rating, seal method, or code compliance.
3/8	NPT	Tapered	18	1.411 mm	0.675 in	17.14 mm	NPT tapered thread reference; nominal size is not actual measured OD and does not define pressure rating, seal method, or code compliance.
1/2	NPT	Tapered	14	1.814 mm	0.840 in	21.34 mm	NPT tapered thread reference; nominal size is not actual measured OD and does not define pressure rating, seal method, or code compliance.
3/4	NPT	Tapered	14	1.814 mm	1.050 in	26.67 mm	NPT tapered thread reference; nominal size is not actual measured OD and does not define pressure rating, seal method, or code compliance.
1	NPT	Tapered	11.5	2.209 mm	1.315 in	33.40 mm	NPT tapered thread reference; nominal size is not actual measured OD and does not define pressure rating, seal method, or code compliance.
1-1/4	NPT	Tapered	11.5	2.209 mm	1.660 in	42.16 mm	NPT tapered thread reference; nominal size is not actual measured OD and does not define pressure rating, seal method, or code compliance.
1-1/2	NPT	Tapered	11.5	2.209 mm	1.900 in	48.26 mm	NPT tapered thread reference; nominal size is not actual measured OD and does not define pressure rating, seal method, or code compliance.
2	NPT	Tapered	11.5	2.209 mm	2.375 in	60.32 mm	NPT tapered thread reference; nominal size is not actual measured OD and does not define pressure rating, seal method, or code compliance.
2-1/2	NPT	Tapered	8	3.175 mm	2.875 in	73.02 mm	NPT tapered thread reference; nominal size is not actual measured OD and does not define pressure rating, seal method, or code compliance.
3	NPT	Tapered	8	3.175 mm	3.500 in	88.90 mm	NPT tapered thread reference; nominal size is not actual measured OD and does not define pressure rating, seal method, or code compliance.
3-1/2	NPT	Tapered	8	3.175 mm	4.000 in	101.60 mm	NPT tapered thread reference; nominal size is not actual measured OD and does not define pressure rating, seal method, or code compliance.
4	NPT	Tapered	8	3.175 mm	4.500 in	114.30 mm	NPT tapered thread reference; nominal size is not actual measured OD and does not define pressure rating, seal method, or code compliance.

BSPP and BSPT pipe thread references

Nominal size	Thread family	Type	TPI	Pitch mm	Approx. major / OD (in)	Approx. major / OD (mm)	Caveat
1/8	BSPP/BSPT reference	Parallel or tapered by family	28	0.907 mm	0.383 in	9.73 mm	BSPP/BSPT representative reference; verify parallel vs tapered family, thread form, sealing method, standard, and manufacturer data.
1/4	BSPP/BSPT reference	Parallel or tapered by family	19	1.337 mm	0.518 in	13.16 mm	BSPP/BSPT representative reference; verify parallel vs tapered family, thread form, sealing method, standard, and manufacturer data.
3/8	BSPP/BSPT reference	Parallel or tapered by family	19	1.337 mm	0.656 in	16.66 mm	BSPP/BSPT representative reference; verify parallel vs tapered family, thread form, sealing method, standard, and manufacturer data.
1/2	BSPP/BSPT reference	Parallel or tapered by family	14	1.814 mm	0.825 in	20.95 mm	BSPP/BSPT representative reference; verify parallel vs tapered family, thread form, sealing method, standard, and manufacturer data.
3/4	BSPP/BSPT reference	Parallel or tapered by family	14	1.814 mm	1.041 in	26.44 mm	BSPP/BSPT representative reference; verify parallel vs tapered family, thread form, sealing method, standard, and manufacturer data.
1	BSPP/BSPT reference	Parallel or tapered by family	11	2.309 mm	1.309 in	33.25 mm	BSPP/BSPT representative reference; verify parallel vs tapered family, thread form, sealing method, standard, and manufacturer data.
1-1/4	BSPP/BSPT reference	Parallel or tapered by family	11	2.309 mm	1.650 in	41.91 mm	BSPP/BSPT representative reference; verify parallel vs tapered family, thread form, sealing method, standard, and manufacturer data.
1-1/2	BSPP/BSPT reference	Parallel or tapered by family	11	2.309 mm	1.882 in	47.80 mm	BSPP/BSPT representative reference; verify parallel vs tapered family, thread form, sealing method, standard, and manufacturer data.
2	BSPP/BSPT reference	Parallel or tapered by family	11	2.309 mm	2.347 in	59.61 mm	BSPP/BSPT representative reference; verify parallel vs tapered family, thread form, sealing method, standard, and manufacturer data.
2-1/2	BSPP/BSPT reference	Parallel or tapered by family	11	2.309 mm	2.960 in	75.18 mm	BSPP/BSPT representative reference; verify parallel vs tapered family, thread form, sealing method, standard, and manufacturer data.
3	BSPP/BSPT reference	Parallel or tapered by family	11	2.309 mm	3.460 in	87.88 mm	BSPP/BSPT representative reference; verify parallel vs tapered family, thread form, sealing method, standard, and manufacturer data.
4	BSPP/BSPT reference	Parallel or tapered by family	11	2.309 mm	4.450 in	113.03 mm	BSPP/BSPT representative reference; verify parallel vs tapered family, thread form, sealing method, standard, and manufacturer data.

Use these dimensions for reference only. Do not infer pressure rating, seal compatibility, or thread interchange from OD and TPI alone.

How to read nominal size, TPI, pitch, and OD

Nominal pipe size

Nominal pipe size is a trade-size label. It does not mean the pipe thread measures that exact diameter.

Threads per inch and pitch in mm

Threads per inch is converted to approximate pitch with pitch mm = 25.4 / TPI. For example, 14 TPI is about 1.814 mm pitch.

Approximate major or outside diameter

Use approximate OD only as a lookup reference. It does not prove fit, seal, pressure safety, or code compliance.

Why NPT, BSP, and BSPT are not simple inch-mm equivalents

NPT and BSP-family threads can look similar in a quick diameter chart, but they are different thread systems. NPT is a tapered pipe thread family commonly associated with a 60-degree thread form. BSP-family threads use a different thread form, and BSPP is parallel while BSPT is tapered. Similar nominal size or outside diameter does not prove fit, seal, pressure safety, or code compliance.

Data and rounding assumptions

Inch conversion: 1 inch = 25.4 mm.
Pitch formula: pitch mm = 25.4 / TPI.
Rounding: pitch is rounded to 3 decimals; approximate OD in millimeters is rounded to 2 decimals.
Reference scope: values are representative lookup references and must not replace standards, manufacturer drawings, or inspection gauges.
Last reviewed: 2026-07-06.

Pipe thread safety note

This chart is for dimensional lookup only. Nominal pipe size is not actual measured outside diameter. NPT, BSPP, BSPT, NPS, and NPSM are not interchangeable by simple inch/mm conversion. Always verify the exact standard, thread form, taper or parallel type, sealing method, pressure rating, material, and code requirements before selecting a fitting. This page is not a pressure-system design, seal-selection, or safety compliance tool.

Optional NPS / NPSM note

NPS and NPSM are straight pipe thread families used in specific applications. They may share nominal sizing language with NPT but are not the same as NPT tapered threads. Use the relevant standard and manufacturer documentation for selection.

Pipe thread size chart FAQ

Is nominal pipe size the same as actual outside diameter?

No. Nominal pipe size is a trade-size label, not the measured outside diameter of the thread. For example, 1/2 NPT is commonly referenced around 0.840 in major outside diameter, or about 21.34 mm. The nominal size, thread family, TPI, taper, and sealing method must be read together.

Are NPT and BSP threads interchangeable?

No. NPT and BSP-family threads should not be treated as interchangeable by simple inch/mm conversion. They can differ by thread form, taper or parallel design, sealing method, pitch, and standard. Always verify the exact standard and manufacturer specification before selecting an adapter or fitting.

What is the pitch of a 14 TPI pipe thread in millimeters?

A 14 TPI thread has an approximate pitch of 1.814 mm. The formula is 25.4 ÷ TPI, so 25.4 ÷ 14 = 1.814. This is a pitch conversion only; it does not prove thread compatibility.

Why is 1/2 NPT not 1/2 inch across?

Because 1/2 NPT is a nominal pipe thread size, not a direct outside-diameter measurement. The approximate major OD for 1/2 NPT is commonly listed around 0.840 in, or 21.34 mm. That is why the chart separates nominal size from approximate outside diameter.

What is the difference between BSPP and BSPT?

BSPP is a parallel pipe thread family, while BSPT is a tapered pipe thread family. They can share nominal size labels but differ in how sealing is achieved and how the thread is specified. Do not select BSPP or BSPT based only on a diameter lookup.

Can I use this chart to choose a pressure-rated fitting?

No. This chart is not a pressure-system design or safety compliance tool. Pressure rating depends on the standard, fitting design, material, seal, wall thickness, temperature, fluid, installation, and applicable code. Use the relevant standard and manufacturer documentation.

What does TPI mean in pipe threads?

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