Free Pipe Thread Size Converter
Instantly convert nominal pipe sizes between BSP, NPT, and metric — check the real diameter and TPI before you order.
Your Fitting
Step 1Conversion Result
ISO 228 · ANSI B1.20.1Full Thread Size Table
| Nominal Size | BSP Outer Ø (mm) | BSP TPI | NPT Outer Ø (mm) | NPT TPI | Common Metric Equivalent* |
|---|---|---|---|---|---|
| 1/8" | 9.728 | 28 | 10.287 | 27 | M10×1 |
| 1/4" | 13.157 | 19 | 13.716 | 18 | M14×1.5 |
| 3/8" | 16.662 | 19 | 17.145 | 18 | M16×1.5 |
| 1/2" | 20.955 | 14 | 21.336 | 14 | M20×1.5 |
| 3/4" | 26.441 | 14 | 26.670 | 14 | M26×1.5 |
| 1" | 33.249 | 11 | 33.401 | 11.5 | M33×2 |
| 1-1/4" | 41.910 | 11 | 42.164 | 11.5 | M42×2 |
| 1-1/2" | 47.803 | 11 | 48.260 | 11.5 | M48×2 |
| 2" | 59.614 | 11 | 60.325 | 11.5 | M60×2 |
*Metric figures are the sizes commonly seen on compression/hydraulic fittings near that nominal size — not an official pipe-thread equivalent, and a different fitting system (straight thread + O-ring, not a tapered pipe joint).
Governing standards: ISO 228-1:2000 defines BSP (parallel) thread dimensions and tolerances; ASME B1.20.1-2013 defines NPT dimensions and gauging. Both are paywalled primary documents, so the numeric table above is cross-checked against two independent published engineering references that reproduce the same standard figures: The Engineering ToolBox (ISO 228) and Engineers Edge (NPT).
Learn More
A technical walkthrough of NPT and BSP thread geometry. Source: Machining Doctor. Background reading: National pipe thread · British Standard Pipe. On fenilo.com: brass vs plastic pipe fittings.
"Same Size" Isn't the Same Thread
NPT cuts at a 60° angle with flat crests — a U.S. standard, default across North America. BSP cuts at 55° with rounded crests — a British/international standard, default across Europe, the Middle East, and most of Asia.
Threads-per-inch also differs at several sizes: close enough that a BSP and NPT fitting of the "same" nominal size will often start to thread together by hand — not close enough to seal reliably, or to survive forcing.
What You Need to Know
BSPT vs BSPP
BSPT (tapered) seals by wedging together as it's tightened, like NPT. BSPP (parallel) is straight and needs a bonded washer or O-ring against a flat face to seal.
Identifying an Unmarked Fitting
- Check the region of origin first
- Look for taper vs. a straight, flat-faced thread
- Count threads per inch against the table above
- When unsure, ask the supplier for a datasheet
Connecting the Two Standards
Use a proper NPT–BSP transition adapter rather than forcing a direct thread engagement — forcing mismatched threads together risks damaging both parts.
Where Metric Threads Fit In
M-series straight threads show up on compression and hydraulic fittings near certain nominal sizes, but they're a different sealing system entirely — not a drop-in substitute for BSP or NPT.
On the Products You're Actually Ordering
Frequently Asked Questions
Are BSP and NPT threads the same size if the nominal label matches?
No. A 1/2" BSP fitting and a 1/2" NPT fitting share the same nominal name but have different outer diameters, thread pitch, and thread angle, so they are not directly interchangeable.
What is the difference between BSPP and BSPT?
BSPP (parallel) threads are straight and require a bonded seal or O-ring to seal, while BSPT (tapered) threads seal by wedging together as they are tightened, similar in principle to NPT.
Can I use an adapter to connect NPT to BSP?
Yes, NPT-to-BSP adapters are commonly available and are the recommended way to connect the two standards, rather than attempting to force a direct thread engagement.
How can I tell which thread standard a fitting already uses?
Check the region of origin first, then check for taper and count threads per inch against a reference table — visual inspection alone is unreliable on a worn or unmarked fitting.
Ordering fittings or valves for a specific market?
Fenilo manufactures brass thread fittings, inserts, and PPR valves in both BSP and NPT. Tell us your destination market and we'll confirm the right thread standard before you order.
Confirm Thread Standard